Biology, Study Guide 8, DNA and RNA

DNA has three functions: to store, copy, and transmit information. DNA is like a book, because books also have those functions. They hold information until the information is needed. The information in the books can be copied, and each copy of the book has the same information as the original book.

How is DNA like a book? Explain your answer.

The fact that the genetic code is the same in all organisms and is nearly universal suggests that all life-forms have a common evolutionary ancestor with a single genetic code.

What is the evolutionary significance of the genetic code?

Point mutations include substitutions, insertions, and deletions of single nucleotides in DNA. Insertions and deletions have a greater effect on proteins than do substitutions, because insertions and deletions can affect every amino acid that is specified by the nucleotides that follow the point of mutation. In contrast, a substitution affects a single amino acid. A change in more than one amino acid is more likely to alter the ability of the protein to function normally than is a change in a single amino acid.

Why do some kinds of point mutations generally result in greater changes in proteins than others?

Eukaryotic cells use DNA-binding proteins to regulate gene expression. The proteins bind to specific sites, called enhancer sequences, near the gene. Some of the proteins activate the transcription of the gene when they bind to the DNA. Others inhibit the transcription of the gene. Eukaryotic cells also use RNA interference to block gene expression. A Dicer enzyme cuts a small loop of miRNA into tiny pieces. Then, the pieces bind with proteins to make a silencing complex. When the complex finds a piece of mRNA that has a code that matches its own code, it sticks to the mRNA and shuts it down.

. How do eukaryotic cells regulate gene expression?

Every cell that has a nucleus in a multicellular organism has all the genes to build that organism. But not every cell needs every gene, so it is important that the unneeded genes are switched off. For example, nerve tissue needs to be flexible, not stiff and rigid. So, the genes that code for the proteins that create the rigidity of bones would be inappropriate in nerves. Genes that create bone structure need to be turned off in nerve cells.

. Why is gene regulation necessary in the development of multicellular organisms? Use a specific example to support your argument.

. Nondisjunction can occur during meiosis I if homologous chromosomes fail to separate during anaphase I. It can also occur during meiosis II if sister chromatids fail to separate during anaphase II.

At what two times during meiosis can nondisjunction occur?

In both prokaryotes and eukaryotes, DNA replication begins with the breaking of the hydrogen bonds between the DNA's base pairs, causing the two DNA strands to separate and unwind. Each strand serves as a template for the attachment of complementary bases, forming a new strand of DNA. As a result, two identical DNA molecules are formed, each with one original strand of DNA and one new strand of DNA. In most prokaryotes, however, DNA replication begins at a single point on the chromosome and often proceeds in two directions. In the larger chromosomes of eukaryotes, DNA replication begins at hundreds of points on the chromosomes and proceeds in two directions.

Compare and contrast DNA replication in prokaryotes and eukaryotes

Messenger RNA carries copies of instructions for assembling proteins from DNA to the ribosomes. Ribosomal RNA is a component of the ribosomes. Transfer RNA carries amino acids to the ribosomes for assembly into proteins.

Contrast the functions of the three main types of RNA

To begin the replication process, enzymes called helicases break the hydrogen bonds that hold the two complementary strands of the DNA double helix together, allowing the helix to unwind. The complementary strands are held apart by additional protein molecules. At the replication fork, the point at which the double helix separates, a molecule of DNA polymerase attaches and begins to add nucleotides to the exposed bases according to the base-pairing rules. This continues until the entire DNA sequence has been copied.

Describe how a molecule of DNA is replicated

Portions of DNA in a bacterial cell can move from one cell to another. If one bacterial cell has a mutation or change that allows it to resist antibiotics, it can transfer this genetic resistance to other bacteria. When the antibiotic kills the rest of the bacteria, only the resistant cells remain. These cells reproduce to create whole populations of bacteria that are resistant to the antibiotics.

Describe how some bacterial populations develop resistance to antibiotics over time.

Endosymbiotic theory describes how some cell organelles such as mitochondria and chloroplasts used to be independent primitive bacteria cells. These bacterial cells came to live together in a beneficial relationship that eventually evolved into a single organism, which became the modern eukaryotic cell.

Describe how the endosymbiotic theory explains how some cell organelles have their own DNA.

Often, the two chromosomes in a prokaryotic cell attach to different points inside the cell membrane and are separated when the cell splits to form two new cells. In eukaryotic cells, the chromosomes separate from each other during anaphase of mitosis.

Describe how two chromosomes separate after replication in a prokaryotic cell versus a eukaryotic cell?

Hershey and Chase grew bacteriophages in cultures containing radioactive isotopes of 32P and 35S. The 32P became incorporated into the bacteriophage's DNA, because DNA contains phosphorus. The 35S became incorporated into the bacteriophage's protein coat, because proteins contain sulfur. After the bacteriophages were allowed to infect bacteria, Hershey and Chase found that nearly all the radioactivity in the bacteria was from 32P. This indicated that the bacteriophage's DNA was injected into the bacteria. The results were important because they showed that the bacteriophage's genetic material was DNA, not protein.

Describe the Hershey-Chase experiment. Why were the results important?

The lac operon consists of three segments. These include a promoter, an operator, and three lactosemetabolizing genes. In addition, a repressor gene lies close to the lac operon. RNA polymerase is needed to transcribe the DNA code into mRNA. As long as the repressor is attached to the operon, the activity of the RNA polymerase is prevented. When lactose binds to and removes the repressor, the RNA polymerase can move to the lactose-metabolizing genes of the lac operon, and mRNA can be transcribed.

Describe the physical structure of the lac operon

After being transcribed from DNA, portions of the pre-mRNA molecule—called introns—are cut out from it. The remaining pieces, called exons, are then spliced together. Next, a cap and a tail are added to the opposite ends of the mRNA molecule, forming the final mRNA molecule.

Describe the process in which a molecule of pre-mRNA is converted into a final mRNA molecule.

A DNA molecule has the shape of a double helix, or that of a twisted ladder. Each strand of the helix is a chain of nucleotides. The two strands are held together by hydrogen bonds between the nitrogenous bases of the nucleotides on opposite strands. The nitrogenous bases form hydrogen bonds with one another in pairs. Adenine forms hydrogen bonds with thymine, and guanine forms hydrogen bonds with cytosine.

Describe the structure of a DNA molecule

Mutations may occur as a result of mistakes in base pairing during replication that are not repaired. Mutations may also be caused by mutagens such as chemicals or radiation.

Describe two causes of mutation

Gene rearrangements are mutations that move an entire gene to a new location. Changes in a gene's position often disrupt the gene's function because the gene is exposed to new regulatory controls in its new location. Gene alterations are changes in a gene itself. These include a change in one or many nucleotides in a gene. Point mutations, insertions, and deletions are examples of gene alterations. These types of mutations can disrupt a gene's function. (Students might also describe point mutations, insertions, deletions, or transpositions in response to this question.)

Describe two types of mutations and their effects

After a molecule of mRNA is transcribed in the nucleus it moves to the cytoplasm. A ribosome then positions itself at the start codon on the mRNA molecule. As each successive codon passes the ribosome, a molecule of tRNA brings an amino acid to the ribosome. Only a tRNA molecule with an anticodon that is complementary to the codon on the mRNA can attach an amino acid to the growing polypeptide chain. The ribosome attaches each new amino acid to the chain, and the bond holding the tRNA to the amino acid is broken. The ribosome moves to the next codon, and the process repeats until the entire mRNA molecule is translated.

Explain the process of translation.

Each nucleotide is made of three parts: a phosphate group, deoxyribose, and a base. Covalent bonds between the phosphate and the deoxyribose molecules gives DNA stability. There are two long bases (adenine and guanine), and two short bases (thymine and cytosine). There is always a long base and a short base in each pair: adenine always pairs with thymine, and guanine always pairs with cytosine. Because a long and a short base are always together, the backbones of the DNA molecule can be parallel and uniform and hydrogen bonds can form between the base pairs. This hydrogen bond is easily broken so DNA can "unzip" for replication, and the individual strands stay securely intact.

Figure 12-14 shows a short segment of DNA. Explain how the chemical components of a nucleotide give DNA a structure that allows it to hold itself together and yet come apart for replication

Information encoded in DNA molecules undergoes transcription as RNA polymerase makes an mRNA molecule with nucleotides having a sequence that is complementary to that of one of the original DNA strands. The mRNA molecule leaves the nucleus and associates with a ribosome, where the second step, translation, occurs. Translation involves the synthesis of the amino acid sequence of a protein molecule by the combined action of mRNA, tRNA, and rRNA. The sequence of mRNA nucleotides determines the sequence of amino acids in the assembled protein.

Genes control cellular activities through a two-step process known as gene expression. Name and discuss the significance of the two steps

Apoptosis can stop cells from reproducing if their DNA has been severely damaged, as when skin cells are damaged by exposure to sunlight. These cells experience sunburn and die instead of creating new cells. Also, during development, some cells die instead of continuing to multiply, such as the cells between fingers and toes, in order to develop functioning body parts.

Give some examples of how apoptosis is beneficial to an organism

Griffith killed disease-causing bacteria and mixed them with live, harmless bacteria. The harmless bacteria transformed into disease-causing bacteria. Because the ability to cause disease was an inherited by the offspring of the transformed bacteria, Griffith concluded that the transforming factor had to be a gene.

How did Griffith arrive at the conclusion that a gene from one kind of bacteria transformed another kind of bacteria?

The circular DNA molecules in prokaryotes usually have two replication forks that begin at a single start site. In eukaryotic cells, each start site also has two replication forks, but there are many start sites, thus enabling the genome to be replicated quickly.

How does the number of replication forks in the DNA of prokaryotic cells differ from the number of replication forks in the DNA of eukaryotic cells?

Accept any four of the following answers: RNA polymerase is involved in transcription, whereas DNA polymerase is involved in DNA replication. During transcription, free nucleotides base pair with the nucleotides on only one strand of a DNA molecule, not both strands as in DNA replication. In transcription, the free nucleotides are RNA nucleotides, not DNA nucleotides. Transcription continues until a stop signal is reached on the DNA strand. DNA replication continues until the entire chromosome is replicated. At the end of transcription, one single-stranded RNA molecule is formed, not two double-stranded DNA molecules. The newly formed RNA molecule leaves the nucleus, whereas the newly formed DNA molecules stay in the nucleus.

How does transcription differ from DNA replication? Describe at least four differences

The chromosome would first unravel into supercoiled strands of nucleosomes, which would unravel into looser coils. Then, the coils would unravel into strands of nucleosomes. The nucleosomes would unravel into clusters of histones and a single DNA molecule.

If you unraveled a eukaryotic chromosome, what would you observe as it became unraveled?

RNA polymerase is needed to transcribe the DNA code into mRNA. As long as the repressor is attached to the operon, the activity of the RNA polymerase is prevented. When lactose binds to and removes the repressor, the RNA polymerase can move to the lactose-metabolizing genes of the lac operon, and mRNA can be transcribed.

In the lac operon, how does RNA polymerase affect the expression of the lactose-metabolizing genes, and how is the activity of RNA polymerase controlled?

A bacteriophage has just two parts: the protein coat and the nucleic acid. So, Hershey and Chase needed to label just two parts to find out which part was responsible for transmitting the genetic code. Avery had to run many experiments in which he destroyed various kinds of biological chemicals, one at a time, to see which chemical was responsible for transmitting genetic information.

In what way did using a bacteriophage in an experiment on transformation make the Hershey-Chase investigation simpler than Avery's investigation?

Most amino acids can be specified by more than one codon. This can help neutralize the effect of some substitution mutations. If a substitution mutation results in a new codon that specifies the same amino acid, the mutation will have no effect. For example, if a substitution mutation occurs in a "CUG," codon, and a U is substituted for the C, the resulting codon, "UUG," still codes for the amino acid Leu. In fact, if another substitution mutation happens and "UUG," is changed to "UUA," the codon would still specify Leu.

Some substitution mutations have no effect on the final protein that is assembled by the ribosome. Use Figure 13-3 to provide specific examples that explain how having multiple codons specify the same amino acid makes this possible

DNA molecules are composed of two complementary strands of nucleotides arranged in a pattern resembling a spiral staircase. Each nucleotide consists of a sugar molecule, a phosphate group, and one of four possible bases. The double helix arrangement is maintained by the formation of hydrogen bonds between complementary bases on the two strands. The bases adenine and thymine pair with each other, and the bases cytosine and guanine pair. Thus, equal amounts of A and T, as well as equal amounts of C and G, are present.

The DNA molecule is described as a double helix. Describe the meaning of this expression and the general structure of a DNA molecule.

The three roles of DNA in heredity are storing information, copying information, and transmitting information. If a mistake is made in storing information, an existing cell might not be able to make all the materials it needs to function as a cell. If a mistake is made copying DNA, then a new cell would have a faulty code and might not have all the information needed to function as a cell. If a mistake is made in making the cells that transmit information to the next generation, then the offspring might not get all the information it needs to function as an organism.

What are the roles of DNA in heredity? Predict what would happen to each of the roles if a mistake were made in the genetic code

The lac repressor might be unable to bind with the operator. As a result, RNA polymerase would not be prevented from beginning the process of transcription, and the lac genes would be turned on permanently. Another effect of the mutation might be that the lac repressor would be unable to bind with lactose. As a result, the repressor would permanently bind with the operator, RNA polymerase would be prevented from binding to the promoter, and the lac genes would be turned off permanently.

What might be the effects of a mutation in the gene that codes for the lac repressor in E. coli?

When pesticides are present, mosquitoes that have a mutation that allows them to be resistant to pesticides have an advantage over mosquitoes without that mutation. In an area being sprayed with pesticide, mosquitoes lacking the mutation would die, and the resistant mosquitoes would live. So, having the mutation is beneficial to the mosquitoes.

Why is pesticide resistance considered a beneficial mutation in mosquitoes?