Biology Unit 5

Several scientists contributed to the discovery of the structure of DNA. James Watson and Francis Crick stand by their 3-D, tin-and-wire model of DNA. James Watson wrote a book about his experience called The Double Helix: A Personal Account of the Discovery of the Structure of DNA.

American James Watson and Englishman Francis Crick used Chargaff's conclusions, Franklin's X-ray images, and their own understanding of chemistry to fully discover the 3-D structure of DNA. They used wires and tin parts to make a large model of the molecule, which helped them visualize how all the parts fit together. They won the Nobel Prize in Physiology or Medicine 1962.

DNA replication results in an exact copy of the original DNA strand.

Billions of times a day, cells of your body, and of the bacteria living within you, divide. Before this can happen, the entire DNA code has to be replicated each and every time. Replication occurs when DNA's double helix opens. Each strand becomes a template for a new strand. New nitrogenous bases are brought in, and two new, identical strands are produced. Many proteins are involved in this process.

DNA is transcribed into RNA, which is translated into proteins.

DNA makes up Genes, which are transcribed into RNA, which are translated into Proteins

Base Pairing

Don't forget the base-pairing rule. Not just any tRNA can drop in. It has to have the correct anticodon to bond to the mRNA codon. The rule ensures that the proper amino acid is brought in at the correct spot. Remember that in RNA, A pairs with U, and G pairs with C.

Practice using the genetic code. Use the chart in the previous slide to practice.

In this activity, use the table to find the amino acids that match the mRNA strand. Don't forget: All proteins begin with the codon AUG, which codes for the amino acid methionine. You must start building an amino acid chain with methionine. Keep building the chain until you come to a stop codon. Remember that stop codons do not have matching amino acids.

Genes in DNA are transcribed into RNA, and RNA is translated into various types of proteins.

Like workers in a company, different types of proteins carry out most of the enormous numbers of functions accomplished in living things, both in and out of cells. In yeast, there are 6,000 different proteins; in humans, about 100,000 proteins. Where do proteins come from? What makes them different? In this lesson, the pathway from DNA to RNA to proteins is the big picture.

An Illustration shows the nucleus and DNA of a cell. The oval shaped cell contains an oval shaped nucleus inside that takes up a large portion of the cell. An R N A strand which is twisted with only one strand and has different lines coming off it moves from the nucleus to the cytoplasm of the cell. An arrow from the nucleus shows this movement.label at the boundary between gray rock and white snow cover RNA label at the boundary between gray rock and white snow cover cytopla

RNA travels from the nucleus into the cytoplasm.

Codons

The DNA code is also organized into codons. Even though the mRNA is complementary to the DNA code, its triplet nucleotides are still called codons.

The D in DNA

The sugar in DNA is called deoxyribose.

Four codons are different. They code for start and stop signals in translation.

There are specific codes to start and stop translation. The start codon for of any protein is always AUG, and AUG codes for the amino acid methionine (met in the table). Therefore, every protein starts with the amino acid methionine. To stop translation, one of three stop codons is used—UAA, UAG, and UGA. No amino acid corresponds to the stop codons.

enzyme

a protein that is a catalyst for chemical reactions in organisms; it increases the rate of the reaction without being used up or changed

DNA

deoxyribonucleic (dee-AHK-sih-riy-boh-nyoo-KLEE-ihk) acid; this is the molecule, unique to each individual, carrying the genetic information to be found in every cell; all the information an organism needs to live and reproduce is contained in its DNA

All components of translation migrate into the cytoplasm.

mRNA leaves the nucleus, as does the rRNA, for the ribosomes. The amino acids are already there, and they bond to tRNA molecules that have also left the nucleus. Everything that is needed for translation to take place comes together in the cytoplasm of the cell. It is here that the actual protein products are assembled, packaged, and sent off to wherever they need to go.

mRNA

messenger RNA; a linear RNA molecule that contains codons

RNA

ribonucleic (RIY-boh-nyoo-KLEE-ihk) acid; nucleic acid that uses the instructions stored in DNA to build proteins

rRNA

ribosomal RNA; a type of RNA molecule that makes up the main part of the structure of a ribosome

ribosimes

small organelles, made of RNA and protein; sites of protein synthesis

replication

the process in which DNA duplicates, or makes a copy of, itself

genetic code

the relationship in which each codon determines each amino acid used for making a protein

tRNA

transfer RNA; a type of RNA molecule that carries amino acids to the site of protein synthesis at ribosomes

Several scientists contributed to the discovery of the structure of DNA. Biography of Rosalind Elsie Franklin http://www.sdsc.edu/ScienceWomen/franklin.html Rosalind Franklin took X-ray photographs of DNA molecules. Franklin's X-ray photos helped others discover the structure of DNA.

A few years later, in the early 1950s, Rosalind Franklin, a scientist working in London, took X-ray photographs of a DNA molecule. Her photos gave strong clues to DNA's double-helix shape, but she was not quite able to make the connection before being diagnosed with cancer caused by exposure to X rays. She died in 1958 at the age of 37.

Translation involves many components. Keratin in a mustang's hoof is just one of the thousands of proteins that make a horse a horse. Keratin: One of Thousands Keep in mind that keratin is being used as an example protein. DNA holds the instructions for tens of thousands of other proteins, many of which perform jobs directly in the cell.

A herd of wild mustangs explodes over the ridge, galloping in unison, tearing through the expansive Wyoming landscape. Their hooves—like the condor's beak and your hair—are made of the protein keratin. The previous lesson showed how DNA makes mRNA and gives it the blueprints for making a specific protein. This lesson goes one step further in showing how proteins are made by reviewing all components in the process of translation. Since there are so many, it is easy to get them confused.

Transfer RNA (tRNA) is shaped like a cross. tRNA is a t or cross-shaped molecule that identifies and collects the amino acids coded for in mRNA.

A molecule of tRNA Opens in modal popup window has a different shape than a molecule of mRNA. tRNA is often described as a cross or even the letter t (that makes it easy to remember). Its shape is a long, single-stranded piece of RNA. In some places, the bases pair up and make regions of double strands. Like all RNA, tRNA is made in the cell's nucleus from DNA. A molecule of tRNA can be between 70 to 90 nucleotide sequences long, and its job is to identify which amino acids the mRNA is coding for and bring them in. To do the work of making proteins, all of these different kinds of RNA migrate out of the nucleus. Protein synthesis takes place in the cytoplasm.

The basic unit of the DNA molecule is a nucleotide.

A single strand of DNA has three distinct columns of repeating parts. Starting from the outer edge, there are phosphates, sugars, and nitrogenous bases. Scientists define these units as nucleotides. A nucleotide Opens in modal popup window consists of one of each of a phosphate, a sugar, and a nitrogenous base. Phosphate and sugar make up the backbone of the molecule. Learn more about the physical makeup of a DNA molecule. You may also turn to page 92 of your reference book to explore two different views of a DNA molecule.

The Start Codon

A special codon called a start codon always signals the ribosome where an amino acid chain should begin. On an mRNA strand, the start codon is always AUG. Its tRNA anticodon is UAC, and that tRNA always carries the amino acid methionine. start codon = AUG = met = methionine

DNA in a cell can replicate in a surprisingly short time. DNA is normally wound up within a cell. For this photograph, scientists have manipulated and released the DNA from an E. coli cell.

After eating a nice meal, you sit back and let your body digest it, right? Well, sort of. Your body doesn't do all of the work on its own. Millions of bacteria call your intestines home, where they help digest your food. A common bacterium is called Escherichia coli, otherwise known as E. coli. Maybe you've heard of E. coli in the news. When E. coli gets out of your intestines and into other parts of your body, it can make you sick. There are various strains of E. coli—some are helpful, while others are harmful. For E. coli to reproduce, its DNA has to be replicated. A bacterium only has one circular chromosome. To replicate its entire DNA strand takes about 42 minutes. All of the DNA in a typical human cell takes about 8 hours to replicate because it is so much longer. In a human, 25 million new cells are made every second, or 2 trillion new cells per day.

When transcription is done, the RNA strand breaks away from its DNA template and DNA rewinds. Once the transcription sequence has been finished, mRNA leaves the nucleus, and DNA winds back up into a double helix.

After making the new strand of mRNA, transcription ends. The RNA polymerase detaches from the DNA, and the DNA winds back up. mRNA then leaves the nucleus to complete the second part of making proteins—translation. Why is it important for the gene to rewind after it is transcribed? Answer By rewinding after transcription, the nitrogenous bases and, therefore, the integrity of the code are protected. All of the types of RNA, mRNA, tRNA, and rRNA are transcribed in this way. They all are transcribed in the nucleus and they all migrate to the cytoplasm later. Turn to pages 98-99 of your reference book to read about transcription.

DNA consists of four types of nitrogenous bases. Purines have two rings of carbon (C) and nitrogen (N), while pyrimidines have only one ring of carbon and nitrogen. Nitrogen These bases are called nitrogen bases or nitrogenous bases because in addition to containing the common carbon, hydrogen, and oxygen elements, they also contain nitrogen.

DNA consists of four types of nitrogenous bases. DNA consists of only four different types of nitrogenous bases, and they are organized into two groups. The purines, adenine Opens in modal popup window and guanine Opens in modal popup window , have two rings in their chemical structure. The pyrimidines, thymine Opens in modal popup window and cytosine Opens in modal popup window , are smaller with only one ring in their structure. Each base is commonly abbreviated as the first letter of its name: adenine (A) guanine (G) thymine (T) cytosine (C) To make all living things on earth, one might expect to find a code made up of thousands of different parts. Yet, the entire DNA code comes down to four bases.

An Illustration shows the nucleus and DNA of a cell. The oval shaped cell contains an oval shaped nucleus inside that takes up a large portion of the cell. The nucleus contains the D N A strands. The DNA strands appear as two twisted strands that spiral around each other and are intersected with many lines from the top to the bottom of the strands. The lines across are half one color and half another color as they are two separate lines linked together in the middle connecting the two strands.label at the boundary between gray rock and white snow cover nuclues label at the boundary between gray rock and white snow cover DNA label at the boundary between gray rock and white snow cover

DNA is located in the nucleus.

RNA is a nucleic acid found in a cell that is involved in protein production. There are many different types of RNA.

DNA is often called the brain of the cell and receives much attention and praise. It is RNA Opens in modal popup window , however, that brings the all-important DNA to life by manufacturing the thousands of different proteins an organism uses to live. Like DNA, RNA is a nucleic acid. Unlike DNA, there is more than one type of RNA in the cell. Each type has a unique function or role, and the structure of each type of RNA helps it accomplish its function. Fundamentally, however, every RNA molecule has a similar overall nucleic acid chemical structure—a phosphate-sugar backbone attached to nitrogenous bases. In RNA, the sugar is ribose Opens in modal popup window , while in DNA it is deoxyribose.

Translation stops when the ribosome detects a stop codon. Stop Codon Ends Translation Positioned in the center of the space is a representation of an mRNA-ribosome complex. The complex consists of mRNA and a ribosome. The ribosome is made up of two ribosomal subunits. One unit is a small oval shape that is above but connected to the other larger unit. The larger unit is almost circular in shape. The mRNA is horizontal across the upper part of the ribosome. It looks like a ribbon with four different small flag-like shapes hanging down from it. The flag-like shapes are nucleotides. Each nucleotide is identified by letter. The letters are A C G and U. Each A nucleotide is the same shape. Each C nucleotide is the same shape. And so forth. The nucleotides fit together like jigsaw puzzle pieces. Nucleotides A and U fit together. Similarly, nucleotides C and G fit together. Three mRNA nucleotides, A A and G, are paired with a tRNA molecule with the anticodon U U C. These nucleotides fit together like jigsaw puzzle pieces. An amino acid chain, or polypeptide, is connected to the tRNA molecule. The amino acids are represented by balls. The chain is as follows. Lysine is the amino acid connected directly to the tRNA molecule. Linked on the left to lysine is arginine. Linked on the left to arginine is leucine. Linked on the left to leucine is glutamine. Linked on the left to glutatime is methionine. Positioned directly to the right of the mRNA nucleotides A A G are nucleotides labeled U G A. In this example, U G A is the stop codon because it does not pair with any tRNA molecule. A bracket is placed around these nucleotides. The nucleotides are labeled, stop codon.

Eventually, the ribosome comes to the end of the mRNA code for a particular protein. Just as a special start codon was used to begin translation, a certain stop codon is recognized at the end of the mRNA. The stop codon-in this case UGA-does not pair with any tRNA molecule, so translation ends.

A table with 2 columns and 8 rows. Protein Types Enzymes . Transport Storage Contractile Structural Defensive Regulatory

Functions Are catalysts that aid in chemical reactions Bind and carry molecules from one place to Hold reserves of elements and compounds for use in cell functions Contract, change shape, and aid in movement. Provide support and make up hair, feathers, and shells. Provide protection, often in the form of antibodies, from viruses and bacteria. Regulate metabolic processes.

The enzymes of these very different organisms are built using the same language: the genetic code. Regardless of what the organism is, these sets of codons always match up with the same amino acid. The chart on your lessons shows a column of ser, val, and thr. mRNA codons under Ser are UCU, UCC, UCA, UCG, AGU, and AGC. mRNA codons under val are GUU, GUC, GUA, and GUG. mRNA codons under thr are ACU, ACC, ACA, and ACG.

It would seem highly unlikely that the enzymes made by white-rot fungi, leafy sea dragons, and pitcher plants would have anything in common. Not only are all the enzymes produced through translation, but they are also all based on the same exact genetic code Opens in modal popup window . The only difference between the enzymes is the order of their amino acids. The order is determined by the order that tRNA molecules bind to the mRNA in a ribosome. Each codon on an mRNA molecule is the basis for which amino acid is next in the chain; thus, a sequence of codons is the key to making enzymes. Codons match up with specific amino acids, creating the genetic code.

5.05 Transcription Transcription occurs when a specific segment of DNA is used as a template to make a molecule of mRNA. Transcription occurs in the nucleus when DNA unwinds and RNA is formed as a complementary copy of DNA

It's time for the details. You've got a grasp on the DNA to RNA to proteins pathway, so now you're ready to dive into exactly what happens during these processes. In this lesson, you'll explore the first part, which is transcription (DNA to RNA). When a cell expresses the information in a certain gene, how does it do it? It all starts in the nucleus.

5.03 Structures of RNA In protein production, the cell uses three different types of RNA. Each type has a structure that relates to its function. mRNA, tRNA, and rRNA are types of RNA involved in making proteins.

Just as a factory uses many machines to produce a product, the cell uses many parts to produce proteins. Three types of RNA, a nucleic acid, are used in this process. Before learning the details of protein production, you must become familiar with the machinery—the structure of the various RNA molecules.

A scientist's work on transcription earned him the Nobel Prize in Chemistry 2006. Arthur Kornberg, Nobel Laureate in 1959, congratulates son Roger Kornberg, who received the Nobel Prize in Chemistry 2006 for his DNA studies. The Nobel Prize The Nobel Prize is awarded in the areas of chemistry, physics, literature, physiology or medicine, and peace. From all countries of the world, individuals who have achieved outstanding accomplishments or discoveries in these fields are eligible as recipients of the award. Who knows, maybe in another 47 years, it'll be your name chosen for a Nobel Prize. Money for the award comes from the legacy of Alfred Nobel, a Swede who made millions from the invention and production of the explosive TNT.

In 1959, a 12-year-old Roger Kornberg watched proudly as his father Arthur received the Nobel Prize for Medicine or Physiology for his studies of how DNA replication takes place. Arthur Kornberg is the scientist who first discovered the action of DNA polymerase. He has produced very detailed pictures of DNA replication in a cell, including clear images of mRNA molecules being made. Little did father or son realize that 47 years later, in October 2006, Roger would also become a Nobel Laureate. Roger Kornberg contributed to the study of how the DNA-to-RNA-to-proteins pathway occurs.

5.08 DNA makes RNA The process of translation occurs through the cooperation of several different components, primarily RNA, and results in proteins. Translation requires enzymes, mRNA, amino acids, tRNA, and ribosomes to make proteins.

Have you ever had a part in a play or musical? So much goes into each production. Many people and props work together to make it all happen. Similarly, the process of translation (RNA to proteins) involves the cooperation of many different components. Before studying the script, this lesson will introduce you to the actors in the classical performance of translation.

The structure of DNA is two complementary strands twisted into a double helix.

Here's a list of features that all DNA molecules share: two complementary strands sugar-phosphate backbones on each strand hydrogen bonding in the middle of the strands between the nitrogenous bases strict base pairing rules: adenine with thymine and guanine with cytosine an overall shape of a twisted double helix

How many is Enough?

How many amino acids are in a protein? There may be more than 3,000 amino acids in a protein. Each protein has its own specific length and 3-D shape, and the number of amino acids varies.

The flow of information from DNA to RNA to proteins is a central principle in biology.

If you opened a book on the main themes in biology, you would find that one of them is the relationship between DNA, RNA, and proteins. In living things, certain segments of DNA, or genes, are essentially directions for building proteins. You can think of those DNA segments as blueprints. RNA uses those DNA segments (genes) to build proteins, as a construction contractor uses blueprints for building structures. What does RNA build? Proteins. Once completed, like workers in a company, each type of protein carries out a specific type of job.

The leafy sea dragon produces enzymes that digest fish larvae and sea lice. The enzymes produced in sea dragons are proteins. Sea dragons use the enzymes to digest their food. More about dragons Leafy sea dragons are closely related to sea horses. Sea dragons are camouflage experts that can grow up to 18 in. long and can be found in marine environments along the southern and western coasts of Australia. They have no known predators, but they are endangered due to pollution and scuba divers collecting specimens.

If you're planning a scuba trip to Australia, you'd better be on the lookout for dragons. These dragons aren't the kind that can fly and breathe fire—rather, they are leafy sea dragons (Phycodurus eques). These fish (which hardly look like fish at all) are covered in appendages that look exactly like leaves of kelp. They move very slowly and live in large masses of kelp. Known as one of the best-camouflaged creatures on the planet, leafy sea dragons blend perfectly into their habitat. Leafy sea dragons feed on fish larvae and other tiny creatures called sea lice. To digest the morsels, they use specific enzymes. And how do they produce these protein compounds? The same way the fungus does—through translation.

The genetic code connects codons from an mRNA strand to an amino acid. Shorthand for amino acids Phe = phenylalanine Leu = leucine Ile = isoleucine Met = methionine Val = valine Ser = serine Pro = proline Thr = threonine Ala = alanine Tyr = tyrosine His = histidine Gln = glutamine Asn = asparagine Lys = lysine Asp = aspartic acid Glu = glutamic acid Cys = cysteine Trp = tryptophan Arg = arginine Gly = glycine

Let's examine a table of the genetic code. As a reference, you may download a copy of the table, listed as The Genetic Code under Materials in Lesson Resources, or turn to page 103 of your reference book. The table is organized to show the possible base combinations in mRNA. Each codon codes for an amino acid. There is a specific way to read the genetic code table. The steps are illustrated for you in the online activity The Genetic Code

Double-stranded RNA (dsRNA) does not play a role in protein production. American scientists Andrew Z. Fire and Craig C. Mello received the Nobel Prize for Physiology or Medicine 2006 for their discovery of dsRNA.

In October 2006, American scientists Andrew Z. Fire and Craig C. Mello won the Nobel Prize for Physiology or Medicine. They discovered another kind of RNA, one that is double stranded and keeps other kinds of RNA from performing their tasks. In the cell, dsRNA is cut into pieces and combined with proteins to make a complex called a small interfering RNA. Its function is to destroy certain mRNA strands. If an mRNA molecule is destroyed, the protein it was going to make never gets made. In this case, one type of RNA destroys the mRNA, so that the gene that coded for the mRNA is silenced, or not expressed, in the organism. By destroying some mRNA strands, dsRNA plays an important role in regulating protein production. Therefore, dsRNA is important in controlling which genes are expressed and which genes are not expressed.

Compare DNA and RNA to highlight their similarities and differences.

Now that you've looked into the structures of DNA and RNA, review how they are the same and how they differ. Answers to questions: RNA is usually a single stranded structure. DNA is a double helix structure. RNA and DNA have a sugar, a phosphate, and a nitrogenous base. RNA has ribose sugar present. DNA has deoxynbose sugar present, DNA nitrogenous bases are A, C, G, and T. RNA nitrogenous bases are A, C, G, and U.

Transcription begins at a specific segment of DNA. Keratin is the protein that gives a condor's beak the strength to tear into a meal of carrion. A Series of Genes Some proteins are made from a combination of different genes. If keratin were made out of three genes, you might think of it as books in a series: Together, volumes I, II, and III complete the story.

On a breezy morning, a condor with a 10-ft wingspan soars for several hours, riding a thermal air current high above the Andes in Ecuador. Finally, using its keen eyesight, it locates a deer carcass and swoops down to gorge on the meat. Competing with flies and maggots, the condor uses it beak—which is made of keratin—to rip apart the rotting flesh. Whether keratin is being made for the condor's beak or your hair, the process is the same. In the nucleus, the DNA contains the genes for every single protein that the entire organism is capable of making. But right now, it only needs to make keratin.

Nitrogenous bases pair up in a specific way. Base Types In each pair of nitrogenous bases, there is one purine and one pyrimidine. It is because of this balance in base sizes that the overall structure of DNA is smooth and uniform.

In the center of the DNA molecule, the nitrogenous bases from the two strands line or pair up. The bases always pair up according to the following rules: adenine (A) with thymine (T), and vice versa guanine (G) with cytosine (C), and vice versa Two DNA bases that are paired up are called complementary bases. Because all bases have to be complementary, it is also said that the two DNA strands are complementary to each other. Therefore, if you know the order of bases of one strand, you can figure out what the bases are on the other.

An Illustration shows the nucleus and DNA of a cell. The oval shaped cell contains an oval shaped nucleus inside that takes up a large portion of the cell. An R N A strand which is twisted with only one strand and has different lines coming off it is now in the cytoplasm and creates new structures. One new structure looks like the top of a trumpet with lines coming off of the front of the horn. The other objects created are two rounded discs that have a crackled appearance on their surface.label at the boundary between gray rock and white snow cover RNA label at the boundary between gray rock and white snow cover callout label at the boundary between gray rock and white snow cover callout

In the cytoplasm, RNA forms new structures.

Molecules of tRNA move into the ribosome. tRNA Moves to the Ribosome Positioned in the center of the space is a representation of an m R N A-ribosome complex. The complex consists of m R N A and a ribosome. The ribosome is made up of two ribosomal subunits. One unit is a small oval shape that is above but connected to the other, larger unit. The larger unit is almost circular in shape. The m R N A is positioned horizontally across the upper part of the ribosome. It looks like a ribbon with sequences of four different small flag-like shapes hanging down from it. The flag-like shapes are nucleotides. Each nucleotide is identified by letter. The letters are A, C, G, and U. Each A nucleotide has the same shape. Each C nucleotide has the same shape, and so forth. The nucleotides fit together like jigsaw puzzle pieces. Nucleotides A and U fit together. Similarly, nucleotides C and G fit together. Among the nucleotides is the sequence A, U, G. A bracket surrounds these nucleotides. The bracket is labeled, start codon. Moving into the space from the lower left is a t R N A molecule. The molecule has the nucleotides U, A, and C. This molecule moves upward toward the m R N A molecule and matches with the nucleotides on the start codon. The nucleotides in the two groups fit together like jigsaw puzzle pieces. The t R N A molecule carries with it an amino acid, methionine, or m e t. The amino acid is represented by a ball suspended from the t R N A molecule. After the t R N A molecule and its associated amino acid pair with the m R N A sequence, a second t R N A molecule moves into the space from the lower right. This molecule has the nucleotides G, U, and U and the amino acid glutamine, or g l n. The nucleotides G, U, U pair with the nucleotide sequence C, A, A on the m R N A molecule. When this happens, the amino acids methionine and glutamine bond. The bond is represented by a horizontal line between the two circles that represent the amino acids. After the bond forms, methionine leaves its t R N A molecule and moves toward glutamine. The line between the two amino acids shortens. When that happens, the first t R N A molecule falls away from the m R N A molecule, floating down and out of the space. Under the presentation is a caption: Once the m R N A-ribosome complex has formed, the t R N A molecules bring in the amino acids. The first amino acid brought in is methionine, and in this case the second is glutamine. The t R N A anticodon must match them R N A codon precisely.

In the next step in translation, a tRNA Opens in modal popup window molecule joins the mRNA-ribosome complex. The large subunit of the ribosome has two spots for tRNA molecules; therefore, it can only hold two tRNA molecules at a time. Only a tRNA with the correct anticodon Opens in modal popup window can move in and match to the mRNA. The anticodon on the tRNA pairs up with the codon Opens in modal popup window on the mRNA. Base-pairing rules must be followed. At its other end, tRNA carries a specific amino acid.

Several scientists contributed to the discovery of the structure of DNA. Erwin Chargaff's work with nitrogenous bases helped determine the structure of DNA.

It wasn't much more than 50 years ago that the structure of DNA was unknown. Scientists the world over were using various techniques to try to uncover the mystery. The first piece of the puzzle came in 1949, after scientist Erwin Chargaff collected data on the occurrence of the four nitrogenous bases in the DNA of various organisms. He discovered that the number of guanine bases equals the number of cytosine bases and that the number of adenine bases equals the number of thymine bases. In human DNA, for example, adenine and thymine each appear about 30 percent of the time, while guanine and cytosine appear about 20 percent of the time. Chargaff's work helped lead later scientists to realize the base-pairing makeup of DNA.

Proteins that will leave the cell are manufactured on the endoplasmic reticulum.

The endoplasmic reticulum (ER)—a long, noodle-shaped organelle attached to the nucleus. Sometimes ribosomes attach to the rough ER where some translation takes place. Many of the proteins that need to be transported out of the cell are made on the ER. After being translated and produced, those proteins need to be wrapped in packages, just as you would wrap a box to be shipped. The packaging helps protect the proteins from being damaged. When ribosomes attach to the ER, it is called the rough ER due to the bumpy appearance of the ribosomes. ER without ribosomes is called the smooth ER and has other functions.

The protein product is now complete and ready to be used by the organism.

The finale is here: A molecule of keratin protein has been made. The keratin polypeptide can now leave the cell and join thousands of other keratin molecules to build the tortoise's shell. Keratin is also an important protein in your hair, a bird's beak, and a horse's hooves. Whether it is keratin to make structures, antibodies to support the immune system, or ribosomes to work in translation, thousands of different types of proteins—each with its own specific role, shape, and function—are all manufactured in the same way: First, DNA is transcribed into RNA. Then, RNA is translated into proteins.

Gene-silencing dsRNA has many potential health benefits for people. If scientists can harness gene-silencing dsRNA, they might cure cancer or many other life-threatening diseases.

Recent discoveries about the actions of gene-silencing dsRNA might lead to a cure for cancer or other maladies. Scientists have already been successful in turning off certain genes in the roundworm and fruit fly. They feel it is within their reach to turn off those genes that cause disorders in people. Also, many viruses operate using RNA. The virus injects its RNA into its host's cells, and the cells translate the viral information. However, with gene silencers, viral RNA can be targeted and destroyed before it ever reaches translation. Think of how the world would change if such a thing happens.

DNA's unique structure is formed by linking large numbers of individual nucleotides. When the two nucleotide strands of a DNA molecule are untwisted, they resemble a ladder in shape. A DNA molecule consists of two strands of individual nucleotides linked together and twisted to form a double helix. A nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

Refer to the structure of DNA in your book for your test.

5.02 Structure of DNA DNA is a nucleic acid that has two complementary strands and the shape of a double helix. DNA has 2 complementary strands consisting of backbone of phosphates and deoxyribose and nitrogenous.

Regardless of how different squids, tortoises, roses, and spiders look, all living things are made of the same chemical compounds. The DNA of all of these organisms has the same basic structure, and knowing that structure is critical for understanding its function.

Replication:Here is a section of DNA that has unwound and is ready for replication. Watch how an enzyme unzips the DNA. When the DNA is ready to replicate, it unwinds into a straight double strand. An enzyme moves along the DNA double strand and breaks the bonds between its nucleotides, resulting in two separate single strands. The region of DNA where the strands separate is called a replication fork.

Replication:The DNA polymerase enzymes have just made parts of two new DNA strands by matching the complementary bases to form new pairs. The two new DNA strands will form into two new DNA double helixes. When replication is complete, they will be separate molecules.

Polymerase enzymes make new copies of nucleic acids. What's in a suffix? Enzymes usually either put things together or take them apart. In either case, enzymes end in the suffix -ase.

Review polymerase enzymes for a minute. You probably recognize the word polymerase from DNA polymerase—the enzyme that makes new DNA strands in DNA replication. In transcription, another polymerase enzyme—RNA polymerase—is responsible for catalyzing the reactions that make RNA molecules. What does polymerase mean? As scientists, the natural thing to do is to dissect each part of the term. A polymer is a compound made out of several similar parts. Because DNA and RNA are made out of repeating parts of nucleotides, they are considered polymers. To polymerize something also means to put something together. The suffix -ase is used in biology to identify enzymes. A polymerase, therefore, is an enzyme that puts together similar parts to make a larger molecule. DNA polymerase makes DNA. RNA polymerase makes RNA.

An Illustration shows the nucleus and DNA of a cell. The oval shaped cell contains an oval shaped nucleus inside that takes up a large portion of the cell. The nucleus contains the D N A strands. The DNA strands appear as two twisted strands that spiral around each other and are intersected with many lines from the top to the bottom of the strands. The lines across are half one color and half another color as they are two separate lines linked together in the middle connecting the two strands. Parts of the D N A strand are copied. This section of the strand is separate from the D N A strand and is labeled the R N A.label at the boundary between gray rock and white snow cover DNA label at the boundary between gray rock and white snow cover RNA label at the boundary between gray rock and white snow coverOpen calloutlabel at the boundary between gray rock and white snow coverOpen callout

Sections of the DNA are copied into RNA

rRNA and proteins are assembled into ribosomes. rRNA and Proteins Assemble into Ribosomes This presentation has three parts. Part One. Positioned to fill most of the space is a cell. The interior of the cell is labeled, cytoplasm. A thin line outlines the cell. Positioned within the cell is the nucleus. The nuclear membrane is a thin line around the perimeter of the nucleus. Positioned within the nucleus is a diagram of the double-helix strand of D N A. The middle segment of the helix is pulled apart. Positioned within this segment are two copies of r R N A. The copies are labeled, r R N A subunits. Outside the nucleus of the cell are 12 small circles. Five circles are labeled, proteins. Part Two. The label, proteins, disappears. When that happens, the two copies of r R N A move down and out of the nucleus toward the bottom of the cell. Part Three. The 12 circles representing proteins move toward the two copies of r R N A. After the circles reach the r R N A, they form the two parts of a ribosome. The upper part of the ribosome is larger and has a circular shape. The lower part of the ribosome has a horizontal oval shape. A caption under the presentation is, r R N A is made of the nucleolus within the nucleus. It migrates to the cytoplasm and combines with proteins; together they form a ribosome.

The main organelle involved in translation is the ribosome. A ribosome is a combination of rRNA and proteins. Molecules of rRNA are made in the nucleolus by transcription of the appropriate gene. The protein parts of the ribosome exist as two separate subunits, one large and one small, to which the rRNA molecules are linked. rRNA moves into the cytoplasm and combines with those proteins to make a ribosome. Ribosomes are too large to fit back through the nuclear pores, so they remain in the cytoplasm. There are thousands of ribosomes in the cytoplasm. Ribosomes are like docking stations for mRNA and tRNA to come together. Each ribosome brings together two mRNA codons with two tRNA anticodons at the same time.

Translation, the making of a protein from the mRNA molecule, involves other types of RNA, as well as ribosomes.

The production of proteins is the fundamental basis for how living things exists, as it brings to life the instructions in the DNA code. Whether it is to make keratin, antibodies, or any of the thousands of types of proteins, the components are the same. mRNA carries the instructions for the protein. tRNA brings in the amino acids to be made into the protein. Ribosomes are made of rRNA and proteins, and they offer a temporary bonding site. Enzymes are necessary for many reactions to take place.

Translation takes place in the presence of a number of different kinds of enzymes.

The three types of RNA discussed in this lesson are the central actors in the play of translation—but no play can go forward without people behind the scenes. For translation to occur, a number of enzymes must act to catalyze different kinds of chemical reactions. These enzymes are part of the cytoplasm and are present when transcription takes place. An example of enzymes that play a behind-the-scenes role are those enzymes that bond one amino acid to another (in a process you'll see soon). The enzymes are called dehydrogenases because when they help bond one amino acid to another, water (hydro-) is a by-product.

tRNA has specific parts that are used in translation.

The type of RNA that is shaped like a cross or the letter t is tRNA. There are two important areas on the tRNA molecule. During translation, one end of the tRNA pairs up with the codon of mRNA. How does this happen? Because of base-pairing rules. If mRNA has a codon GAU, tRNA pairs up with the code CUA. The opposite triplet code on tRNA is the anticodon Opens in modal popup window . It is the complementary code to the codon. Remember how each codon translates into a certain amino acid? Well, it is tRNA that physically delivers the amino acids one at a time. The amino acids are connected to the other end of the tRNA molecule. Where does the tRNA pick up the amino acids? There isn't an amino acid warehouse in the cell. Instead, the amino acids are just floating around in the cytoplasm, waiting to be attached to a molecule of tRNA.

ribosomes in a cell

The image of this card shows actual ribosomes on the endoplasmic reticulum in a cell. Each tiny yellow dot in the image is a ribosome. Ribosomes are often found attached to the rough ER, but they can also be free floating in the cytoplasm. Recall from an earlier lesson that ribosomes are made out of ribosomal RNA (rRNA). rRNA is created in the nucleus at a special location called the nucleolus.

DNA is a double-stranded nucleic acid. The entire DNA molecule is twisted into a helix shape.

The information contained in the structure of DNA Opens in modal popup window defines all of what an organism is and is capable of doing. A red-headed woodpecker has all red feathers on its head because its DNA has the genes that code for it. An aspen tree loses its leaves in the fall because its genes instruct it to do so. This coded genetic information is passed along from one generation to the next. This lesson focuses on the physical structure of the DNA molecule itself. Think of DNA as a ladder. DNA has two long strands, like the uprights of a ladder. Each long strand is called a backbone. Cross-connections unite the two backbones, just as a ladder has rungs. Now, imagine that the whole ladder is twisted to form a double spiral or, as it is known more often, a double helix.

The genetic code is used to construct every single organism on earth.

Think about the genetic code. What can it mean about life on earth if each bacterium, crawling bug, leafy plant, feathery bird, scaly reptile, and furry mammal is built from the same code? It is evidence for a connection between all organisms. The genetic code is universal—that is, all organisms share it. A saguaro cactus uses the same genetic code to build itself as your body does. In the upcoming units, you will continue to explore how scientists piece together the puzzle called life, click here to read an article from BBC News. http://newsvote.bbc.co.uk/2/hi/science/nature/1164839.stm

Messenger RNA (mRNA) is a single-stranded molecule. mRNA is single-stranded molecule made through transcription from DNA to create a copy of the DNA's genetic code.

This lesson only makes slight mention of the function of each type of RNA. Their roles in the cell will be explored further in upcoming lessons. The first type of RNA is mRNA Opens in modal popup window . Its structure is relatively simple: It's a piece of single-stranded RNA. It is made directly from DNA, so it is a copy of the genetic code of DNA. A molecule of mRNA can be between 300 to 9,000 nucleotides long. The process by which mRNA is made from DNA is called transcription.

The cell uses many types of RNA. Each type has its own specific structure that relates to its function.

This lesson presented four different types of RNA: mRNA, tRNA, rRNA, and dsRNA. Each has structure that relates to the role it plays in the cell. Knowing those structures will help you understand the detailed processes that will be discussed in upcoming lessons.

An Illustration shows the nucleus and DNA of a cell. The oval shaped cell contains an oval shaped nucleus inside that takes up a large portion of the cell. Outside the nucleus is the three forms of RNA. One is twisted with only one strand and has different lines coming off it. One looks like the top of a trumpet with lines coming off of the front of the horn and the third object are two rounded disc that are connected to each other and are labeled as a ribosome that has a crackled appearance on their surface. They are now linked together the twisted line runs through the ribosome which has the horn like structure inserted into it and protein sphere are coming out of the tube portion of the trumpet shaped figure.label at the boundary between gray rock and white snow cover ribosome label at the boundary between gray rock and white snow cover protein

Three forms of RNA work together to build proteins.

Replication begins when the DNA double helix unwinds. DNA strands usually unwind at several points, creating replication bubbles. Unzipped Bubbles The double helix's unwinding is often referred to as unzipping because the bases resemble teeth on a zipper. The areas where the double helix unwinds are called bubbles.

To begin replication Opens in modal popup window , the double helix unwinds. The nitrogenous base pairs pull apart from each other at a replication fork, exposing the two strands. The unwinding of the helix does not start at one end of the molecule, as you might think. Rather, it starts at many points along the DNA stand. The new DNA is made at each of these unwound segments, eventually fusing into each other. In this way, replication can be accomplished rapidly.

Transcription is the first part of making proteins from the DNA code. Hair is made of keratin, just one of some 100,000 proteins produced in your body.

Transcription is the process in which a specific gene is the template for the production of mRNA, which then goes to a ribosome in the cytoplasm. There, translation takes place and uses the code that the mRNA is carrying as directions to build a protein. Keep in mind how many proteins can possibly be made. In humans, around 100,000 different proteins are put together at the ribosomes. So, pick a protein to use as an example in this lesson. How about keratin—the protein that builds hair, fingernails, and beaks? It has been a few weeks since your last haircut, and you notice it has already grown out some. This growth indicates that the cells that produce keratin have made more of it. How did they do that? The first stage is transcription Opens in modal popup window . The directions on making keratin lie within the gene for keratin protein, and here is where transcription begins.

Transcription makes a copy of a specific gene. The copy is then used as instructions to produce a specific protein.

Transcription is the process that converts genes in DNA into portable blueprints, or mRNA molecules. Each mRNA molecule carries a copy of the code for one gene. During transcription, the enzyme RNA polymerase brings in the correct nucleotides to build an RNA molecule. The same base-pairing rules used in DNA replication are used in transcription, with the exception of uracil (U) being used as a base in RNA instead of thymine (T). Once mRNA is complete, the DNA closes back up, and the mRNA carries its information to ribosomes. All of the types of RNA, mRNA, tRNA, and rRNA are transcribed in this way. They all are transcribed in the nucleus and they all migrate to the cytoplasm later.

More than 30 different types of proteins are involved in the replication process. Just as factory workers are involved in all stages of computer assembly, enzymes are the catalyst for each stage in protein production. Question: Which processes produce all the various proteins necessary for an organism to survive? Answer: Transcription and Translation.

Unzip. Bring in bases. Make new strands. Those steps are the basics; however, the process actually occurs through the actions of more than 30 proteins. Like workers on an assembly line, each protein has its specific job to make sure the process as a whole is accomplished. In the beginning of this unit, remember learning about organisms that need many types of proteins to live? In replication, there is a certain enzyme Opens in modal popup window that cuts one strand to start the unwinding. Another enzyme helps keep the separated strands from binding back together.

Hydrogen bonds form between the nitrogenous bases of either strand. The base pairs in the school are A-T, G-C, T-A, A-T, C-G. Sugar phosphate backbone is labeled P and S is the nucleotide.

What holds the two DNA strands together? Why don't they simply fall apart and unwind? Just like the uprights of a ladder never touch, neither do the backbones of the DNA. That leaves the bases again. It is here that the two strands are bonded to each other—specifically, hydrogen bonds are between the paired bases. As you learned in an earlier lesson, a double hydrogen bond connects the nitrogenous bases A and T, while a triple hydrogen bond links G and C.

5.10 The Genetic Code The genetic code is universal—all organisms share the same code. It shows which mRNA codons represent which amino acids. Genetic code is a table matching mRNA codons to amino acids. See Genetic Code chart on mom's computer

When you were younger, did you ever do a paint-by-number picture? The bottom of the picture has a key of which number represents which color. When you have finished painting the right colors in the right places, a hidden picture is revealed. Building proteins is similar. The key is the genetic code. Each codon is the template for a specific amino acid. After putting the amino acids together, a specific protein is made.

5.04 DNA Replication During DNA replication, the double helix unwinds, and two new backbones and a new set of bases are added. The result is two DNA strands, each identical to the original. DNA replicates by unwinding its double helix, forming new backbones and new bases yielding two new identical DNA strands.

When your DNA replicates, it makes a copy of 6 billion bases with few errors in a very short time. What characteristic about DNA makes replication so efficient?

As the double helix continues to open, new nitrogenous bases are added on both sides.

With each strand pulled apart from the other, forming a replication fork, nitrogenous bases on both sides are exposed. Using the base-pairing rules, new nucleotides come in and match up on both strands. Each single strand is a template for a new strand because of base pairing. The two helixes wind as they go, very shortly after the new bases are added.

DNA differs among organisms based on the order and number of its bases. The number of bases and their sequence in DNA are all that separate a Tyrannosaurus rex from a Galápagos tortoise from a termite.

You keep reading that DNA is each organism's unique genetic blueprint and that segments of DNA are genes. So, if all DNA has identical phosphates and sugars, the differences in DNA from organism to organism must be in the nitrogenous bases. The genetic differences among organisms are inherent in the number and the order of the nitrogenous bases in DNA. From Tyrannosaurus rex to a tortoise to a termite, the critical difference is primarily in two aspects of the organism's DNA: the order of bases how many bases there are

transcription

the synthesis of an RNA molecule using a DNA molecule for a template

nucleotide

a subunit of a nucleic acid that consists of a phosphate group, a five-carbon sugar, and a nitrogenous base

The protein or polypeptide is released, and the mRNA-ribosome complex breaks apart. Ribosome Complex Splits Apart Positioned in the center of the space is a representation of an mRNA-ribosome complex. The complex consists of mRNA and a ribosome. The ribosome is made up of two ribosomal subunits. One unit is a small oval shape that is above but also connected to the other larger unit. The larger unit is almost circular in shape. The mRNA is horizontal across the upper part of the ribosome. It looks like a ribbon with four different small flag-like shapes hanging down from it. The flag-like shapes are nucleotides. Each nucleotide is identified by letter. The letters are A C G and U. Each A nucleotide is the same shape. Each C nucleotide is the same shape. And so forth. The nucleotides fit together like jigsaw puzzle pieces. Nucleotides A and U fit together. Similarly, nucleotides C and G fit together. Three mRNA nucleotides, A A and G are paired with a tRNA molecule with the anticodon U U C. These nucleotides fit together like jigsaw puzzle pieces. An amino acid chain, or polypeptide, is connected to the tRNA molecule. The amino acids are represented by balls. The chain is as follows. Lysine is the amino acid connected directly to the tRNA molecule. Linked on the left to lysine is arginine. Linked on the left to arginine is leucine. Linked on the left to leucine is glutamine. Linked on the left to glutamine is methionine. The mRNA-ribosome complex separates. The separation takes place as follows. The two ribosomal subunits break apart from each other. The amino acid chain separates from the tRNA molecule and drifts away. The tRNA molecule separates from the mRNA. At this point, the basic protein structure is complete. The cell can begin to perform its task.

After reaching the stop codon, everything breaks apart. The amino acid chain, now a completed protein, or at the very least a polypeptide Opens in modal popup window , is released. Although the cell may make some additional changes to the polypeptide before it can perform its task, the basic protein structure is now complete. The mRNA-ribosome complex separates, including separation of the two ribosomal subunits and the polypeptide.

RNA has the nitrogenous base uracil, not thymine. Nucleotide Positioned in the upper left of the space is the title, Nucleotide. Positioned below the title are diagrams of phosphate, sugar, and a base, as follows. Positioned leftmost in the diagram is a dark circular shape labeled, phosphate. A line connects this shape to a light pentagonal shape. The pentagonal shape is to the right of the phosphate. The pentagonal shape is labeled, sugar. A line connects the phosphate shape to a medium pentagonal shape and a medium hexagonal shape on the right. These two shapes share a side. The label, base, appears above the hexagonal shape. This structure moves to the lower left of the space. As it reaches the lower left, five hexagonal shapes appear and occupy the rest of the space. One large light hexagonal shape is labeled, G, guanine. To the right of this shape is a smaller dark hexagonal shape labeled C, cytosine. Below the large light hexagon is a second large medium hexagon labeled, A, adenine. To the right of this shape is a smaller hexagon labeled, T, thymine parentheses for D N A. Below and to the right of this smaller hexagon is another small hexagon labeled, U, uracil parentheses for R N A. A caption below the presentation is, D N A and R N A nucleotides all have nitrogenous bases of guanine, G, cytosine, C, and adenine, A. In the fourth base, D N A nucleotides have thymine, T, and R N A nucleotides have uracil U.

Aside from having a different sugar molecule, RNA and DNA differ in other ways. RNA does not have the pyrimidine base thymine, which is present in DNA. In its place, RNA uses another pyrimidine base called uracil. Whenever a molecule of RNA pairs up with DNA, uracil Opens in modal popup window pairs with the purine base adenine. Of the four types of RNA you will learn about in this lesson, three have only a single strand of RNA—in that way, RNA most differs from DNA, which has two strands.

Replication produces identical copies of the DNA molecule. During replication, DNA unwinds, new base pairs are formed, and two identical copies of DNA are made.

At the end of replication, two identical copies exist of DNA. Both copies consist of one original strand plus one newly made strand. Therefore, once DNA unwinds to replicate, it only rewinds with the new strands. The two original strands are separated forever. Explore the replicating DNA molecule on-screen, and then turn to pages 94-95 of your reference book to read about DNA replication.

Each tRNA molecule carries a specific amino acid that will be used to make the protein. tRNA Molecules Recycled The entire space represents the cytoplasm of a cell. Positioned in the top third of the space is a ribosome. The ribosome is holding two tRNA molecules. The molecules are side by side. An amino acid chain is suspended from the tRNA molecule on the right. The chain includes the following five amino acids in order beginning with the amino acid touching the tRNA molecule: lysine, arginine, leucine, glutamine, and methionine. No amino acids are suspended from the tRNA molecule on the left. This tRNA molecule detaches from the ribosome and drifts off into the cytoplasm. The tRNA molecule continues to move through the cytoplasm until it connects with a specific amino acid that its anticodon matches. In this instance the amino acid is arginine. When the tRNA comes close to the amino acid, the amino acid attaches itself to the molecule of tRNA. Presumably, the tRNA molecule finds its way to another ribosome. This is how tRNA molecules are recycled.

At this point, the strand of mRNA is passed through the ribosome one codon at a time. As each new codon is held in the correct position, a tRNA molecule comes in. It binds to the codon, bringing in the correct amino acid required by the codon. The amino acids bond together to form a chain. The chain is passed to, and held by, tRNA on the left as the amino acid chain grows longer.

Some enzymes check for mistakes in replication. How DNA Is Repaired Positioned horizontally across the space is a strand of DNA. The strand is made up of pairs of nucleotides that fit together like jigsaw puzzle pieces. The pairs are labeled G and C or A and T. In the middle of the strand two nucleotide pairs break apart. When this happens, the top of the molecule bulges. When that happens, the two damaged nucleotides plus one nucleotide to the right and one nucleotide to the left move up and to the right out of the space. Caption text is, An enzyme cuts out the part of the DNA that doesn't fit. New nucleotides that pair with the codon take the place of the nucleotides that have been removed. Caption text is, A DNA polymerase enzyme fills the resulting gap with appropriate nucleotides. The replacement nucleotides are dark, indicating that they are in position, but are not yet healed in position. Another enzyme seals the repaired section. When that happens, the representation of the repaired section matches the other sections.

Despite the often extremely large numbers of nitrogenous bases copied, replication takes place with a very high level of accuracy. The precision of replication is essential to the well-being of an organism. Mistakes in replication can lead to major problems in a cell. Several proteins work together to check and repair mistakes in replication. If the wrong base is put in, the proteins will cut it out, bring in the right one, and fix the other parts of the molecule. These proteins act as a DNA repair mechanism.

White-rot fungus produces enzymes that digest compounds in plastic, weapons, and toxic waste. The enzymes produced in white-rot fungus are proteins. The enzymes are useful in breaking down plastics and toxic chemicals. The fungus among us. White-rot fungus is able to break down a long list of chemicals, including pesticides, dyes, TNT, DDT, and cyanide. Still, don't be fooled: White-rot fungus can't solve all our toxic waste problems. It only grows well in specific conditions, and scientists continue to research ways in which it may be helpful. For this reason, many researchers are actively working on understanding the enzymes and proteins coded for in the fungus's DNA.

Fungus has a bad reputation. Most people associate it with mildew or athlete's foot. Like many misconceptions, this one is not completely accurate. Scientists developed an interest in white-rot fungus (Phanerochaete chrysosporium) when they discovered it was good at breaking down and digesting a large variety of compounds. In nature, white-rot fungus usually breaks down wood, but scientists have found it can also digest compounds similar to many human-made chemicals. It may even break down chemicals that are serious waste problems, such as plastic and toxic waste. The discovery that a fungus might be a safe solution to some waste problems is certainly an exciting one. But how does it do it? The answer is that it uses certain powerful enzymes. Organisms use enzymes to break down and digest food. And what are enzymes? Why, proteins, of course.

The pitcher plant produces enzymes that digest rats. The enzymes produced in pitcher plants are proteins. They are similar in function to your digestive enzymes. Read more about carnivorous plants http://www.botany.org/Carnivorous_Plants/index.php

In a dense patch of Borneo's rain forest, a light rain trickles down onto the colorful vegetation. A baby rat scurries to find shelter before the dampness turns into a downpour. It finds a large, rubbery plant that seems to be a perfect hiding spot. A strong, attractive smell further lures the unsuspecting rodent to peer into the pitcher plant. The cloudiness fogs the 3½ L of liquid death at the bottom of the plant. The rat slides into the predatory pool. The liquid is not simply evidence of rain. It contains powerful enzymes that start digesting the frantic rat alive. The baby rat tries to climb the sides of the pitcher plant, but a special waxy coating prevents its progress. How does the plant produce enzymes strong enough to digest a rat? Yes, you've got it now—through translation. Carnivorous plants create a variety of unique proteins. For more information, click here.

5.09 RNA makes Protein Translation is the process that converts the genetic code in DNA (and then mRNA) into a protein.

Many things you do in your daily life could be described as processes. For example, to bake cupcakes, you mix several ingredients in a bowl, stir them, and use the oven to cook them in a pan. Likewise, to make proteins, several steps and "ingredients" are required. This lesson goes through the instructions of making a protein, one step at a time.

Ribosomal RNA (rRNA) is part of the structure of a ribosome. Molecules of rRNA migrate from the nucleus to the cytoplasm, where they combine with proteins to make up a ribosome.

Molecules of rRNA Opens in modal popup window , which can be between 100 to 4,000 nucleotide sequences long, are formed in the nucleus and migrate to the cytoplasm. In the cytoplasm, rRNA combines with many small proteins to make up a ribosome. Each ribosome has two subunits, a larger one and a smaller one. The lesson on translation will discuss the role that ribosomes play in protein production. Turn to pages 104-105 of your reference book to read about these three types of RNA and their structures.

The enzyme RNA polymerase starts the process of transcription. The Start of Transcription Positioned vertically in the center of the space is a diagram of a molecule of D N A. The molecule has two exterior vertical bands resembling the sides of a ladder. Between the bands, like the rungs of a ladder, are pairs of nucleotides that fit together like jigsaw puzzle pieces. Beginning at the bottom of the molecule, the paired nucleotides are as follows. C and G. T and A. G and C. C and G. A and T. T and A. G and C. G and C. C and G. The nucleotide pair at the bottom is enclosed in an oval. The oval is labeled, enzyme. When the presentation activates, the enzyme moves up to the top of the D N A molecule. As the enzyme passes over each pair of nucleotides, the pair splits apart. The pair at the top of the molecule does not split apart. When the enzyme reaches that pair it begins to descend along the left side of the molecule. The enzyme stops about one-third of the way down on a T nucleotide. When that happens, a label appears. The label is, R N A polymerase. A caption under the presentation: After the D N A double helix unwinds, R N A polymerase moves into position to start the process of transcription.

Once the DNA strand is unwound, the enzyme RNA polymerase officially begins transcription. This enzyme has the ability to identify genes that are ready to be transcribed, and it carries out several jobs. Using keratin as an example, RNA polymerase binds to the segment of DNA that is the gene for keratin. Within the exposed gene is a special start sequence. RNA polymerase recognizes the sequence and moves to this point on the DNA

After leaving the nucleus, mRNA joins with a ribosome in the cytoplasm. mRNA-ribosome Complex Positioned to fill the space is a cell. The material of the cell is labeled cytoplasm. On the perimeter of the cell is the plasma membrane, which is also labeled. Positioned within the cell is the nucleus. The nuclear membrane is a thin line around the perimeter of the nucleus. The nuclear membrane is labeled, nuclear membrane. Positioned within the nucleus is a diagram of the double-helix strand of DNA. The middle segment of the helix pulls apart. Positioned immediately below this particular segment of DNA is the label, transcription. A strand of mRNA results from this process. At that time, the label transcription is replaced by the label mRNA. The mRNA moves out of the nucleus to the cytoplasm in the lower part of the cell. Positioned in the lower part of the cell is a ribosome. The mRNA joins with the ribosome. Two parts of a ribosome hold the mRNA like a clamp, keeping it in place. This is called the mRNA-ribosome complex.

Once the genetic code in DNA is used to assemble an mRNA Opens in modal popup window strand in the process of transcription, the newly transcribed mRNA molecule is moved through a nuclear pore into the cytoplasm. In the cytoplasm, mRNA joins with a ribosome. The ribosome acts kind of like a clamp, holding the mRNA in place. This is called the mRNA-ribosome complex.

The immune system and maintenance of metabolism include the actions of many proteins.

One day you are feeling fine and healthy, the next day your throat is a bit itchy, and the day after that you're stuck in bed with a fever. You are under attack by the flu virus. During the extra sleep you are getting, protein antibodies in your bloodstream work aggressively to tag and destroy the pesky virus. A few days later, your immune system is victorious, and you are back on your feet. Antibodies are not cells. They are defensive proteins produced by cells in the immune system. As you already know, an organism's metabolism is a complex balance of several factors, many of which are regulatory proteins. serotonin is a protein hormone that is involved in the regulation of sleep, mood, and appetite, among other things. Keep in mind that proteins do many jobs both in and out of the cellular environment.

mRNA travels to ribosomes, which the nucleolus produces. mRNA Travels to Ribosomes Positioned to fill most of the space is a cell. The interior of the cell is labeled, cytoplasm. A thin line outlines the cell. The line is labeled, plasma membrane. Positioned within the cell is the nucleus. A thin line outlines the nucleus. The line is labeled, nuclear membrane. Positioned within the nucleus is a diagram of a double-helix strand of D N A. The middle segment of the double-helix pulls apart. Positioned immediately below this particular segment is the label, transcription. A copy of a strand of m R N A appears in the center of the segment that has pulled apart. At that time, the label, transcription, is replaced by the label, m R N A. The m R N A moves out of the nucleus to the cytoplasm in the lower part of the cell. Positioned in the lower part of the cell is a ribosome. The ribosome is represented by a large circular structure and a smaller oval-shaped structure. The m R N A joins with the ribosome. The two parts of the ribosome hold the m R N A in place like a clamp. A caption below the presentation is, Ribosomes act kind of like a clamp, holding the m R N A in place where it can work with t R N A and r R N A to make proteins.

The cell making the keratin now has a portable copy of the blueprints (mRNA) to build the actual protein. That copy of the blueprints is then taken to the organelle where translation takes place—the ribosome Opens in modal popup window in the cytoplasm. Here the keratin will be produced, but you'll learn those details in an upcoming lesson. What makes up a ribosome? It may surprise you that the nucleolus, a suborganelle within the nucleus, is the site where ribosomes themselves are put together.

The genetic code translates mRNA codons into specific amino acids.

The codons on mRNA each correspond to specific amino acids, which is called the genetic code. The information is organized into a usable table. The genetic code shows the sequences of three nitrogenous bases for codons and their connection to the specific amino acids they represent. It is fascinating to think that the genetic code is the same for all organisms, including white-rot fungi, leafy sea dragons, and pitcher plants.

Translation uses the genetic code in mRNA and tRNA to build a protein. The shell of this hatchling tortoise is made primarily of the protein keratin. Meaning of Galapagos The oldest tortoise on record lived to be 154 years old, and this peaceful gian has become the icon for the Galapagos islands chain. The naming of the Galapagos islands is connected back to their famous giant tortoises. When Spaniards accidentally discovered the islands in the 1500's, the shell of the giant tortoise reminded them of a saddle. The Spanish word for saddle, galapago, became their name for the islands as well.

Twelve inches underground, a baby Galápagos tortoise breaks out of its shell. After some digging, it finally reaches the surface to soak up its first rays of sunlight. With some luck, the young giant tortoise will live a long life grazing on plants and fruits and resting in mud puddles. On its back is a tortoise shell made out of protein. Proteins are building blocks for many structures in living organisms. Specifically, the protein in the tortoise shell is keratin. As the baby tortoise developed in its underground environment, millions of its cells were busy translating messenger RNA (mRNA) that contained the code for keratin. Now that you are familiar with the components of translation Opens in modal popup window , it's time to learn about the process itself, which takes place in the aqueous environment of the cytoplasm with mRNA as the code and transfer RNA (tRNA) as the amino acid carrier.

Helicase and DNA polymerase are examples of enzymes used in replication.

Two examples of enzymes used in replication are helicase and DNA polymerase. Helicase acts to unwind DNA, moving down the molecule. It is often referred to as the protein that unzips DNA. Once the DNA is unwound, DNA polymerase is the protein that brings in the new nitrogenous bases and pairs them up on the original strands. DNA polymerase molecules are the central proteins involved in making or synthesizing new DNA. Reminder: An enzyme is a protein that speeds up a chemical reaction.

The exposed DNA gene becomes a template fortranscription.

With the gene for keratin unwound and exposed, RNA polymerase starts moving from the start sequence along the gene. Using the DNA as a template, RNA polymerase starts putting together the mRNA molecule. By following base-pairing rules, mRNA is constructed base by base to be a precise, complementary copy of the gene. Don't forget that uracil is used as a base in RNA instead of thymine. If part of the gene is CGGTAAT on the DNA strand, what is the complementary sequence on the mRNA? Answer CGGAUUA Explore the online activity on on transcription. Transcription DNA Transcription Here's a diagram of a DNA molecule in a eukaryotic cell. We show it untwisted for simplicity, even though it would normally be twisted into a double helix, with parts of it wound around histone proteins. Let's review how the RNA polymerase enzymes open up the paired nucleotides to prepare them for transcription. The polymerase enzyme is now positioned to allow an RNA nucleotide to pair its base with the complementary base in the DNA, to start making messenger RNA (mRNA). The RNA nucleotides pair to their template. The RNA nucleotides are parried to their template in this minigene to make mRNA. That's transcription. Continue to let the mRNA proceed to the cytoplasm for translation. The mRNA strand leaves the nucleus.

mRNA is a part of translation.

You already know how and where mRNA is made. You also know it is carrying the instructions for building a protein—in this example, keratin. The genetic language of the mRNA now needs to be translated into the language of proteins, one amino acid at a time. What you don't know is the nucleotides that mRNA is carrying are actually organized into a triplet code. Three nucleotides together code for a specific type of amino acid. These triplet codes are called codons. For example, if mRNA has the code CUUGGU, it is seen as two codons: CUU and GGU. Each codon Opens in modal popup window represents or translates into a certain amino acid. In this case, CUU represents the amino acid leucine, and GGU represents the amino acid glycine.

Before a cell can divide, its DNA must be replicated. Rapid growth through replication allows a strangler fig to apparently overwhelm its host tree. DNA is unique DNA is the only molecule capable of replicating itself.

You're probably familiar with the ferocious predators of the animal world—tigers, sharks, and piranhas—but how familiar are you with killer plants? Deep in an Australian jungle, a bird releases its waste onto the branch of a tree. In its waste is the seed of a strangler fig (Ficus destruens). Just like those of any other plant, the roots of the strangler fig search for soil. However, since the seed starts its life high in the branches of another tree, its roots climb down, or strangle, the trunk of its host until they reach the ground. Yet, this isn't as deadly of an embrace as it looks. The real trouble comes as the top of the strangler fig grows thick with leaves, blocking sunlight from the host tree. Strangler figs grow quickly, and growth includes the making of new cells. Before each mitotic division, the DNA has to be copied, or replicated. The process of DNA replication is similar in all organisms.

polypeptide

a chain of hundreds or thousands of amino acids

ribose

a monosaccharide; the sugar component of RNA

anticodon

a sequence of three bases in a tRNA molecule that is complementary to a codon in mRNA

Review transcription and translation using this interactive animation. Protein Synthesis Let's review the full process of protein synthesis from the first steps of transcription through the final stages of translation. Here's a diagram of a DNA molecule in a eukaryotic cell. We show it untwisted for simplicity, even though it would normally be twisted into a double helix, with parts of it wound around histone proteins. Let's watch the RNA polymerase enzyme open up the paired nucleotides to prepare them for transcription. The polymerase enzyme is now positioned to allow an RNA nucleotide to pair its base with the complementary base in the DNA, to start making messenger RNA (mRNA). The RNA nucleotides pair to their template. The RNA nucleotides are parried to their template in this minigene to make mRNA. That’s transcription. Continue to let the mRNA proceed to the cytoplasm for translation. The mRNA strand leaves the nucleus. The mRNA strand has left the nucleus and arrived at a ribosome located in the cytoplasm. Here transfer RNA (tRNA) molecules help translate the mRNA code into a polypeptide (shorter than the real ones in this simulation). Continue to begin translation. The correct anticodon matches with it’s amino acid following the base-pairing rules. Every protein begins with the start codon AUG. Watch how the polypeptide forms. Congratulations! Translation is complete and your polypeptide now has its primary structure. In the example, we created a polypeptide with three amino acids. In reality, some chains contain as many as 3,000 amino acids. Once the polypeptide is complete, it folds into more complicated structures.

ou learned about transcription in Lesson 8, and now you have learned about translation in Lesson 9. Remember, transcription is the creation of mRNA from DNA in the nucleus, and translation involves tRNA and ribosomes to create proteins. Since both processes are needed to build cell proteins, you will bring them together in this interactive animation. Turn to pages 98-101 of your reference book to read through both processes again.