GRE Subject Test: Biochemistry, Cell, and Molecular Biology : DNA Replication and Repair

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #11 : Dna

What is the main reason for there being both a leading and a lagging strand during DNA replication?

Possible Answers:

DNA polymerase can only synthesize one strand at a time

DNA polymerase can only read in the 3'-to-5' direction

There are not enough RNA primers to have both strands be synthesized simultaneously

Only one strand is available to be read at any given time

Correct answer:

DNA polymerase can only read in the 3'-to-5' direction

Explanation:

When the DNA helix is opened by DNA helicase, both strands are available to be read by DNA polymerase. However, since DNA polymerase can only read from 3'-to-5', one strand must be synthesized in segments (called Okazaki fragments), rather than one continuous strand. The leading strand is read in the 3'-to-5' direction away from the replication fork, while the lagging strand is read in the 3'-to-5' direction toward the replication fork. This results in a leading and a lagging strand due to the antiparallel structure of DNA.

Example Question #12 : Dna Replication And Repair

Which of the following enzymes is not involved in the process of replicating DNA in a replication bubble?

Possible Answers:

Methyltransferase

Polymerase

Topoisomerase

Primase

Ligase

Correct answer:

Methyltransferase

Explanation:

The correct answer is methyltransferase, which is involved in the methylation of DNA post transcription. The other 4 enzymes are directly involved in the DNA replication bubble. Topoisomerase unwinds the replication fork, polymerase elongates new DNA strands, primase creates primers on the discontinuous 5' to 3' side of the replication bubble, and ligase joins the Okazaki fragments on the discontinous 5' to 3' side.

Example Question #12 : Dna

Why do microsatellites, such as the one that causes Huntington's disease, occur in genomes?

Possible Answers:

The body is better able to interpret microsatellite sequences into proteins, so they are more common in the genome

None of these

Microsatellites are caused by insertion of DNA material by viruses

DNA polymerase "stutters" on repeated bases during DNA replication and adds or deletes bases

Microsatellites are a result of very slow evolutionary processes

Correct answer:

DNA polymerase "stutters" on repeated bases during DNA replication and adds or deletes bases

Explanation:

Microsatellites are generally composed of many repeated bases over and over again, which result from mistakes by DNA polymerase. Polymerase tends to create more errors in cases where many bases are repeated, often creating neutral mutations that evolve extremely quickly.

Example Question #13 : Dna

Which of the following is true with regards to the leading strand during DNA replication?

Possible Answers:

It can read the template strand in the 5' to 3' direction.

It only requires one DNA polymerase III in order to complete the strand.

It does not require an RNA primer.

It requires the joining of Okazaki fragments.

Correct answer:

It only requires one DNA polymerase III in order to complete the strand.

Explanation:

In DNA replication, the synthesis of new strands can be accomplished in both directions. In each direction, you will have both a leading strand and a lagging strand. While both new strands require an RNA primer in order to get started, the leading strand can be continuously synthesized because the template strand is exposed in a 3' to 5' direction. As a result, only one DNA polymerase III is required for this strand.

Keep in mind that while the new strand is synthesized in a 5' to 3' direction, the template strand is read in a 3' to 5' direction. This allows for the new strand to be complementary to the template.

Example Question #14 : Dna

Upon a double-stranded DNA break, a cell can repair the DNA by resecting damaged DNA by nucleases and resynthesis of DNA by DNA polymerase. Next, DNA ligase binds the resynthesized fragment to the original DNA strand. What best describes this process? 

Possible Answers:

Selective autophagy

V(D)J recombination

Homologous recombination

Homology-directed repair

Non-homologous end joining repair 

Correct answer:

Non-homologous end joining repair 

Explanation:

The correct answer is non-homologous end joining repair. This type of DNA repair is more common than homology-directed repair which repairs damaged DNA by using a homologous template. Homologous recombination is a specific type of homology-directed repair. V(D)J recombination is a form of genetic recombination that occurs in developing lymphocytes to give rise to diverse antibodies. Selective autophagy is the selective removal of damaged proteins from the cell following a stress event such as heat exposure. 

Example Question #13 : Dna

DNA ligase IV has been studied for its role in what process specifically?

Possible Answers:

Homology directed DNA repair

Non-homologous end joining DNA repair

None of the other answers 

Homologous recombination

DNA replication of lagging strand

Correct answer:

Non-homologous end joining DNA repair

Explanation:

The correct answer is non-homologous end joining DNA repair. This specific DNA ligase joins the double stranded phosphodiester bond break in DNA by consumption of ATP, however, this repair mechanism often times is error prone and results in indels (insertion/deletion mutations). Homology directed DNA repair and homologous recombination are very similar processes that rely on template sequences to "swap" DNA sequences with other parts of the genome, however, they do not rely on this ligase. DNA ligase IV does not have a role in DNA replication. 

Example Question #17 : Dna Replication And Repair

Primase is an enzyme that synthesize small primers for DNA polymerase to bind to so it can initiate DNA replication. What are these primers made of?

Possible Answers:

Lipids

RNA

Protein

Carbohydrates

DNA

Correct answer:

RNA

Explanation:

Primase synthesizes RNA primers for DNA polymerase to bind to and initiate DNA replication.

Example Question #16 : Dna Replication And Repair

What is translesion DNA synthesis?

Possible Answers:

A method of DNA synthesis by which a cell may replicate over a mutation encountered on the parental strand 

A method of RNA synthesis used to make highly specialized RNA molecules

A method of DNA synthesis used by prokaryotes only

A method of DNA synthesis used exclusively for the mitochondrial genome

Correct answer:

A method of DNA synthesis by which a cell may replicate over a mutation encountered on the parental strand 

Explanation:

Translesion DNA synthesis is a technique used by both prokaryotes and eukaryotes. The main purpose of translesion DNA synthesis is to bypass lesions encountered during DNA replication (commonly thymine dimers or AP sites). Translesion DNA synthesis is not exclusive to the mitochondria, nor does it create specialized RNA molecules. 

Example Question #17 : Dna Replication And Repair

Which of the following DNA repair mechanisms would be employed by the cell to repair DNA damage from UV light?

Possible Answers:

Nucleotide excision repair

Direct reversal

None of these answers are correct

Base excision repair

Correct answer:

Nucleotide excision repair

Explanation:

UV light causes the formation of thymidine dimers. A thymidine dimer is two thymine molecules that dimerize and cannot be recognized by the DNA transcription machinery, which would cause a mutation in the gene if the dimerization occurs on a gene.

Base excision repair and direct reversal only work on individual bases on the DNA molecule. Nucleotide excision repair cuts out a section of damaged DNA and repolymerizes the molecule, as would be required in this case.

Example Question #15 : Dna Replication And Repair

If a double strand break (DSB) is not repaired during G1 phase of the cell cycle, what type of replication error would result during S phase at the site of the DSB?

Possible Answers:

Catenane formations

Gap in the newly replicated DNA strand

Collapsed replication fork

Reversed replication fork

Correct answer:

Collapsed replication fork

Explanation:

Unrepaired doublestrand breaks will result in collapse of the DNA fork because the replication fork cannot continue beyond the area that has the doublestrand break. The other answers require the presence of at least one continuous strand of DNA.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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