Chapter 6: Transcription and Translation
mRNA
m stands for "messenger." Messenger RNA is the RNA that actually carries the information for protein synthesis (in the form of a nucleotide sequence) from DNA in the nucleus to the ribosomes in the cytoplasm.
rRNA
t stands for "transfer." Transfer RNA carries amino acids from the cytoplasm to the ribosomes during protein synthesis.
Diploid
(of a cell or nucleus) containing two complete sets of chromosomes, one from each parent.
Genes
A gene is a locus (or region) of DNA which is made up of nucleotides and is the molecular unit of heredity.
Codon
A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis.
Parent DNA
An original strand of DNA is called the parent DNA.
Template
DNA is made up of a double helix of two complementary strands. During replication, these strands are separated. Each strand of the original DNA molecule then serves as a template for the production of its counterpart, a process referred to as semiconservative replication.
What is DNA's job?
DNA's job is to carry the instructions for making proteins.
46: The Magic Number
Every cell in the human body (excluding sex cells) has 46 chromosomes.
How many chromosomes do somatic cells have?
Every non-sex, or somatic, cell has 46 chromosomes sitting in its nucleus.
Homologous Pair
Homologous chromosomes are made up of chromosome pairs of approximately the same length, centromere position, and staining pattern, for genes with the same corresponding loci. One homologous chromosome is inherited from the organism's mother; the other is inherited from the organism's father.
Step 3 - DNA Replication
Hydrogen bonds form between the base pairs, forming the new "rungs" of the DNA ladder. Bonds form between the sugar-phosphate components of the newly aligned nucleotides so that each newly formed ladder has a new side, too.
Twins
Identical twins have identical chromosomes. That is because identical twins are the result of a single early-stage embryo splitting in two. You can think of this as natural cloning. Each identical twin carries the same set of chromosomes from the original egg and sperm.
Marshall Nirenberg
In 1961, Marshall Nirenberg, a chemist, began to explore codons and the amino acids to which they correspond. He made an RNA molecule that contained only nucleotides of uracil. This RNA, therefore, had the codon UUU repeated over and over. He then placed this into a test tube with all 20 amino acids, and found that a single kind of protein was translated: polyphenylalanine.
Translation
In molecular biology and genetics, translation is the process in which ribosomes in a cell's cytoplasm create proteins, following transcription of DNA to RNA in the cell's nucleus. The entire process is a part of gene expression.
WHERE IN THE WORLD DOES THIS STUFF OCCUR?
Replication and transcription occur in the nucleus. But translation (protein synthesis) occurs in the cytoplasm of the cell. The three types of RNA must leave the nucleus and enter the cytoplasm for protein synthesis to take place.
Step 2 - DNA Replication
Next to each separated strand, an enzyme called DNA polymerase lines up nucleotides to form new second strands. The enzyme lines up nucleotides according to the base-pairing rules. Adenines are paired with thymines, and thymines are paired with adenines. Guanines are paired with cytosines, and cytosines are paired with guanines.
DNA Replicates Itself
One of the cool things about DNA is that it is able to make an exact copy of itself—in other words, it's able to replicate. This is necessary when cells want to divide. In order for the two new cells to be identical, the original cell first has to replicate its DNA, then divide the replicated DNA evenly between the two new cells. We'll talk about that process a little later.
Mom's Eyes, Dad's Nose
One set of your chromosomes came from your father in a sperm cell. One set of those chromosomes came from your mother in an ovum. When the sperm and ovum joined in fertilization, the resulting cell had two sets of 23 chromosomes each, for a total of 46 chromosomes. From that single cell, every other cell in the body was derived, so every other cell in the body has 46 chromosomes.
DNA contains genes, and genes tell your cells how to make protein. The step in between is the production of RNA.
RNA is the "middleman" between DNA and protein. This is known as the "central dogma of molecular biology." DNA to RNA to protein. In other words, DNA directs the synthesis of RNA, and RNA directs the synthesis of protein.
RNA polymerase
RNA polymerase, also known as DNA-dependent RNA polymerase, is an enzyme that produces primary transcript RNA. In cells, RNAP is necessary for constructing RNA chains using DNA genes as templates, a process called transcription.
In replication, the entire DNA molecule (the entire chromosome) is copied over. In transcription, only a portion of the DNA molecule is used (or transcribed) to make RNA.
Remember that much of the DNA in human cells does not carry instructions for protein synthesis, and that the portions that do carry instructions for protein synthesis are called genes. So we need to transcribe only the genes. Furthermore, we need to transcribe only some of the genes—the ones that correspond to proteins needed by the cell at that time.
Origins of Replication
Replication begins at specific sites called origins of replication. The process of copying the parent DNA occurs in both directions from this origin site.
All of your cells have exactly the same set of chromosomes in their nuclei.
The 46 chromosomes that are sitting in any one of your somatic cells are identical to the 46 that are sitting in every other of your somatic cells. It's not like your skin cells have one set of chromosomes and your kidney cells have another.
The nucleus contains the cell's genetic information—in other words, its DNA.
The DNA in eukaryotes is found in several pieces. Each piece is associated with various proteins and is called a chromosome. So when we talk about chromosomes, we're really just talking about the pieces of DNA in the nucleus of a cell.
RNA transcription
The DNA molecule unwinds, and an enzyme called RNA polymerase creates a complementary strand of RNA, using one of the strands of DNA as a template. Guanine (on DNA) is paired with cytosine (on RNA). Cytosine (on DNA) is paired with guanine (on RNA). Thymine on DNA is paired with adenine on RNA. And adenine (on DNA) is paired with uracil (on RNA).
Anticodon
The anticodon (contained within the anticodon loop) is a special region on the tRNA molecule that can base-pair with codons on mRNA. The anticodon must be complementary to a codon to base-pair with it. For example, if a codon has the sequence AUG, the only tRNA molecules that can base-pair with that codon are the tRNAs that have UAC for an anticodon.
Start Codon
The codon AUG (methionine) is known as the "start" codon, because it's the first codon on all mRNA, and methionine is the first amino acid in all proteins.
Step 1 - DNA Replication
The double helix unwinds, and the two strands separate.
Genetic Code
The genetic code is the set of rules by which information encoded within genetic material is translated into proteins by living cells. The Genetic Code is nothing more, really, than a list of the 64 possible codons and the amino acids to which they correspond.
Step 4 - DNA Replication
The new double-stranded molecules twist up into double helices.
Even though all chromosomes contain DNA, it is not true that all chromosomes are identical.
The nucleotide base sequences in two different pieces of DNA—two different chromosomes—can be (and are) different.
How Translation Works, Part 2: The Ribosome
The ribosome is the organelle that synthesizes protein. mRNA binds to ribosomes, and tRNAs carry amino acids to the ribosomes. There are two binding sites on a ribosome: the P-site and the A-site. The mRNA binds to the ribosome so that the first codon is in the P-site and the second codon is in the A-site. Once the mRNA is bound, tRNA, carrying the appropriate amino acids, comes and base-pairs with the codons on mRNA. Then the ribosome forms a peptide bond between the two amino acids. Once the mRNA is bound, tRNA, carrying the appropriate amino acids, comes and base-pairs with the codons on mRNA. Then the ribosome forms a peptide bond between the two amino acids. Once the peptide bond is formed, the first (and now empty) tRNA is released from the ribosome. It is free to return to the cytoplasm and bind to another amino acid. The ribosome, meanwhile, slides down one codon, so that the codon that was in the A-site shifts to the P-site, and the next codon in the sequence moves into the A-site: Now the sequence is just repeated. Another tRNA (carrying the appropriate amino acid) moves in and base-pairs with the codon in the A-site. The ribosome forms a peptide bond between the two amino acids (the one attached to the tRNA in the P-site and the one attached to the tRNA in the A-site), and the tRNA in the P-site is now released and returns to the cytoplasm. And the ribosome shifts over again: The ribosome continues shifting over a codon at a time, and the base-pairing, peptide bond-forming sequence is repeated until every codon has been base-paired with tRNA and peptide bonds have been formed between all amino acids. We then say that the codons on mRNA have been translated by tRNA and the ribosomes. When a stop codon (UAG, UGA, or UAA) appears in the A-site, the final tRNA is released from the ribosome, the completed protein is released, and translation is complete. By the way, the "P" in P-site stands for peptide. This is where the growing peptide is attached to the ribosome. And the "A" in A-site stands for amino acid. This is where the next amino acid is added to the growing protein.
Let's start with one DNA molecule.
The strands separate, and for each strand, the cell makes a new complementary strand, and we end up with two new, but identical, DNA molecules. The original DNA molecule has replicated.
TRANSLATION?
The translation of protein is no different. When you move from the language of nucleotides to the language of amino acids, you still need a "dictionary"; something that tells you which nucleotides correspond to which amino acids. More specifically, you need something to tell you which sequence of three nucleotides corresponds to which one amino acid. A sequence of three nucleotides is called a codon, and the order of codons on mRNA specifies the order of amino acids in a protein. The dictionary for protein translation is called the Genetic Code.
Daughter Strands
The two new DNA strands that form as a result of replication are called daughter strands.
Prokaryotes have DNA, too, even though they don't have a nucleus.
Their DNA is found as one large circular chromosome floating around in the cytoplasm. The DNA is still a double helix; the double helix is just joined at both ends to make a circle.
How many types of RNA are there?
There are three types of RNA that can be made, and each of them has a special role in protein synthesis.
Stop Codon
Three of the 64 possible codons do not specify an amino acid. They specify "stop." In other words, "stop translating, the protein is finished." The three stop codons are UAA, UGA, and UAG.
Transcription
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language.
An Exception: IDENTICAL TWINS
We just taught you that, within any individual, all cells have identical chromosomes. That's true. We also told you that two different individuals do not have the same sets of chromosomes. That's almost always true. The exception is identical twins. Each identical twin has exactly the same chromosomes as the other.
Chromosomes Come in Pairs
We said that human cells have a total of 46 chromosomes. But these 46 chromosomes come in pairs: Each human somatic cell has 23 pairs of chromosomes. Many other species also have their chromosomes situated in pairs. Forty-six just happens to be the number of chromosomes in human cells; hence, human cells have 23 pairs of chromosomes. Another way of saying this is to say that human cells have two sets of chromosomes, and each set consists of 23 different chromosomes.
tRNA
r stands for "ribosomal." Ribosomal RNA interacts with the cell's ribosomes to make them functional. The ribosomes are made of rRNA and protein. The ribosomes are the site of protein synthesis.
How Translation Works, Part 1: tRNA
tRNA is the molecule that carries amino acids from the cytoplasm to the ribosomes. It has a specific, three-dimensional shape (it looks somewhat like a pistol), and if it is flattened out, it has a "cloverleaf" shape.
tRNA: A Quick Review
• tRNA molecules carry amino acids from the cytoplasm to the ribosomes. • tRNA has an "anticodon" that's complementary to mRNA codons and that can base-pair with them. • The codon that tRNA can base-pair with determines the amino acid that can be carried by the tRNA.
