AP Biology : Understanding DNA Repair

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #1 : Dna, Rna, And Proteins

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

Possible Answers:

Missense

Silent

Frameshift

Nonsense

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:

Inability to replicate DNA

Inability to complete the cell cycle

Cancer

A higher rate of mutations during replication

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 #2 : Dna, Rna, And Proteins

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

Possible Answers:

H1

H2A

H4

H2B

H3

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 #4 : Dna, Rna, And Proteins

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

Possible Answers:

DNA polymerase

Nuclease

All of these answers

DNA ligase

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 #3 : Dna, Rna, And Proteins

Which enzyme is not involved in DNA replication?

Possible Answers:

Gyrase

Lipase

Helicase

Ligase

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 #4 : 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 #7 : Dna, Rna, And Proteins

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

Knottase

RNA polymerase

Telomerase

Topoisomerase

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

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