AP Biology : DNA Repair and Replication

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Dna, Rna, And Proteins

Which type of mutation does not change an organism's phenotype despite changing its genotype?

Possible Answers:

Frameshift

Silent

Nonsense

Missense

Correct answer:

Silent

Explanation:

Silent mutations will change a DNA sequence without affecting the phenotype of the organism. This can occur either in an intron, which will not be translated, or by replacing a single nucleotide with another nucleotide without changing the amino acid recruited by the codon. Silent mutations often result from the degenercy of codons.

Frameshift, missense, and nonsense mutations, however, change both an organism's genotype and phenotype by altering its DNA. A frameshift mutation results from the insertion or deletion of a nucleotide, causing a shift in the codon reading frame for every codon read after the mutation. Missense mutations replace one amino acid with another, and nonsense mutations result in a premature stop codon, terminating translation and resulting in a shortened protein.

Example Question #1 : Dna, Rna, And Proteins

What would be an immediate consequence for a cell with a mutant version of DNA polymerase III that has lost its proofreading function?

Possible Answers:

Cancer

Inability to replicate DNA

A higher rate of mutations during replication

Inability to complete the cell cycle

Correct answer:

A higher rate of mutations during replication

Explanation:

Proofreading is a function of DNA polymerase III that helps prevent errors during replication. An immediate consequence of a cell that cannot proofread would be a higher rate of mutations during replication. The other options could potentially happen later in the cell's life, but they would only occur as indirect results of the new mutations.

Example Question #841 : Ap Biology

Which of the following proteins are not situated within the core of the nucleosome?

Possible Answers:

H2A

H4

H2B

H3

H1

Correct answer:

H1

Explanation:

Histones are composed of several proteins, and are used to compact DNA within the nucleus. When DNA is wrapped around a group of eight histones, the resulting structure is a nucleosome.

The histone protein H1 is affixed on top of the nucleosome beaded structure, so as to keep the DNA that has wrapped around the nucleosome in place. It is not found in the core of the nucleosome.

H2A, H2B, H3, and H4 are very similar in structure and form the core of the histones.

Example Question #1 : Dna Repair And Replication

Which of the following classes of proteins are essential for DNA mismatch repair?

Possible Answers:

Nuclease

DNA polymerase

DNA ligase

All of these answers

Correct answer:

All of these answers

Explanation:

For correct mismatch repair all three of the choices are essential. A nuclease is required to remove the damaged DNA. DNA polymerase is required to synthesize new DNA. DNA ligase is essential for synthesizing a phosphodiester bond between the newly synthesized DNA and the original DNA.

Example Question #1 : Understanding Dna Repair

Which enzyme is not involved in DNA replication?

Possible Answers:

Lipase

Gyrase

Ligase

Helicase

DNA polymerase

Correct answer:

Lipase

Explanation:

Lipase is the general name for an enzyme that breaks down lipids. Ligase joins the Okazaki fragments on the lagging strand of the DNA during replication. DNA polymerase is the enzyme that catalyzes the polymerization of nucleotides in the 5' to 3' direction. Helicase separates the two strands of the double helix to facilitate formation of the replication bubble. Gyrase relieves strain on the DNA while it is being unwound by helicase. 

Example Question #2 : Dna, Rna, And Proteins

Which enzyme has a proofreading ability during DNA replication?

Possible Answers:

DNA gyrase

Primase

DNA helicase

DNA polymerase

Correct answer:

DNA polymerase

Explanation:

Proofreading is an important part of the DNA replication process to ensure that if mismatched base pairs are incorporated into the newly synthesized DNA strands, they get replaced with correct base paired nucleotides. Mismatched base pairs have the potential to cause disease. DNA polymerases have proofreading abilities. They are able to remove mismatched nucleotides from the end of a newly synthesized strand. Post-replication repair mechanisms also exist to prevent damage and error.

Example Question #1 : Understanding Dna Repair

You are trying to perform in vitro DNA replication on a small circular piece of DNA. You have DNA polymerase, Primase, Helicase, DNA ligase and all of their accessory proteins. You can get DNA replication to initiate but it never goes for very long without stopping. You visualize your small piece of DNA under an electron microscope and notice that after the initiation of replication, it looks all knotted up. What enzyme can you add to remedy this problem?

Possible Answers:

Reverse Transcipase

Topoisomerase

Telomerase

RNA polymerase

Knottase

Correct answer:

Topoisomerase

Explanation:

As the replication fork of DNA proceeds and continues to unwind the double helix, the DNA upstream of the fork gets over wound and knotted up which will eventually arrest replication as the fork will not be able to proceed any further. The enzyme topoisomerase corrects for this overwinding ahead of replication forks by swiveling and rejoining DNA strands

Example Question #1 : Dna Repair And Replication

Which of the following represents a step necessary to create the lagging strand, but not the leading strand, during DNA replication?

Possible Answers:

Helicase unwinds the parent strand of DNA

DNA is synthesized in the 5’ to 3’ direction

DNA polymerase synthesizes template DNA from the parent strand

DNA ligase joins Okazaki fragments together

Correct answer:

DNA ligase joins Okazaki fragments together

Explanation:

Okazaki fragments are only produced, and subsequently joined together, in the lagging strand to allow for replication in the opposite direction as replication fork movement. The leading strand, however, allows for continual replication.

All other choices reflect aspects of DNA replication for both the leading and lagging strands.

Example Question #1 : Understanding Dna Replication

Point mutations __________.

Possible Answers:

include nonsense mutations, which change an amino acid to a stop codon, affecting the entire polypeptide structure

include silent mutations, which have no effect on the organism's phenotype

All of the answer choices are correct

include missense mutations, which only affect the codon in which the mutation occurs

Correct answer:

All of the answer choices are correct

Explanation:

Point mutations replace a single nucleotide for a different one. This can change a certain codon to code for a different amino acid (missense), the same amino acid (silent), or lead to a stop codon (nonsense). Nonsense mutations are the most severe type of point mutation, as they will cause early termination of the protein.

Example Question #1 : Understanding Dna Replication

How many copies of DNA would you have after ten replication cycles if you start with four copies?

Possible Answers:

Correct answer:

Explanation:

This is really just a math equation. We need to double the amount of DNA each time it goes through a replication cycle.

Begin: 4

Cycle 1: 8

Cycle 2: 16

Cycle 3: 32

Cycle 4: 64

Cycle 5: 128

Cycle 6: 256

Cycle 7: 512

Cycle 8: 1024

Cycle 9: 2048

Cycle 10: 4096

After ten cycles, we would have 4096 copies from our original 4.

A shortcut calculation would be .

This is why PCR amplification is so effective.

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