GRE Subject Test: Biology : Understanding DNA Replication

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Dna Replication

Several enzymes are required for DNA replication. What is the class of enzymes that is required for unwinding the DNA at the replication fork?

Possible Answers:

Topoisomerase

DNA polymerase

Telomerase

DNA helicase

Correct answer:

DNA helicase

Explanation:

DNA helicases use ATP to break the hydrogen bonds that separate complementary strands of DNA. During DNA replication, DNA helicases move along the DNA backbone with the replication fork and are responsble for unwinding the DNA at the fork.

Example Question #2 : Understanding Dna Replication

Which of the following proteins is not necessary during DNA replication?

Possible Answers:

DNA polymerase

RNA polymerase

Helicase

Single-strand binding proteins

Correct answer:

RNA polymerase

Explanation:

RNA polymerase is an enzyme that transcribes RNA from DNA; it is not essential for DNA replication. This enzyme is easy to confuse with primase, whose primary function is to synthesize the RNA primers necessary for replication. DNA polymerase add nucleotides during replication, synthesizing the daughter strand from the parental template. Helicase is responsible for separating double-stranded DNA. Single-strand binding proteins are needed to keep DNA from reannealing after it has been denatured by helicase.

Example Question #3 : Understanding Dna Replication

Which statement correctly describes the process of DNA replication?

Possible Answers:

The old DNA strands stay together and are replicated to form an entirely new DNA molecule

The original molecule is separated and each daughter molecule will have one old DNA strand and a new DNA strand

Parent DNA strands are cut into pieces and the segments are replicated and combined to form two new DNA molecules

New DNA is replicated from parental template strands and an enzyme cuts the new strands away from the old strands

Correct answer:

The original molecule is separated and each daughter molecule will have one old DNA strand and a new DNA strand

Explanation:

DNA replicates in a semiconservative process. Parental strands are used as templates to synthesize daughter strands, which remain adhered to the parental template creating hybrid molecules of old and new DNA.

The original DNA molecules is "unzipped" by helicase to create the replication fork. DNA polymerase then begins to recruit nucleotides to bind to the exposed template, building the new DNA strand along the parental strand.

Example Question #4 : Understanding Dna Replication

Which of the following proteins is known for its ability to break hydrogen bonds?

Possible Answers:

Topoisomerase

Primase

DNA helicase

DNA ligase

Correct answer:

DNA helicase

Explanation:

Before replication, the DNA helix must be unwound so that the strands can be replicated by DNA polymerase. This unwinding is accomplished by DNA helicase, which interferes with the hydrogen bonds between nucleotide pairs. This intervention creates a small separation between the two strands, known as the replication fork. DNA polymerase binds to the replication fork and recruits nucleotides to build the new DNA strand.

Topoisomerase is responsible for cleaving phosphodiester bonds in order to release torsional tension in the DNA backbone. DNA ligase synthesizes phosphodiester bonds, both on the daughter strand of DNA and in the regions cleaved by topoisomerase. Primase is responsible for synthesizing RNA primers that serve to help recruit and bind DNA polymerase in the replication fork.

Example Question #5 : Understanding Dna Replication

__________ is a protein that synthesizes RNA primers on __________ during DNA replication.

Possible Answers:

RNA polymerase . . . both the leading and lagging strands

RNA polymerase . . . the lagging strand

Primase . . . both the leading and lagging strands

Primase . . . the lagging strand

Correct answer:

Primase . . . both the leading and lagging strands

Explanation:

In order for DNA polymerase to begin synthesizing base pairs, an RNA primer is needed to assist the binding of DNA polymerase to the DNA template strand. This primer is synthesized by the enzyme primase. Because DNA polymerase always needs an RNA primer before it can bind, primase must synthesize RNA primers on both the leading and lagging strands.

RNA polymerase transcribes molecules of RNA from DNA sequences during transcription, and is not involved in DNA replication.

Example Question #6 : Understanding Dna Replication

Which of the following is not true of DNA replication?

Possible Answers:

The lagging strand is synthesized in short fragments, directed away from the replication fork

The DNA must be denatured at the replication fork

Replication occurs during prophase of mitosis

Replication occurs in the 5' to 3' direction

Correct answer:

Replication occurs during prophase of mitosis

Explanation:

DNA replication occurs during the S phase of the cell cycle, significantly before prophase of mitosis. During prophase chromosomes are condensed into easily segregated forms, but replication has already occurred. The S phase is the intermediate period of interphase in the cell cycle. The G2 phase follows the S phase, and is subsequently followed by the M phase (mitosis).

The short fragments synthesized on the lagging strand are known as Okazaki fragments. DNA replication does occur in the 5' to 3' direction; this is also the reason that the lagging strand must be synthesized away from the replication fork. DNA is denatured (separated) at the replication fork by an enzyme known as helicase, which breaks the hydrogen bonds between base pairs to allow DNA polymerase and other replication proteins to bind to single-strand DNA.

Example Question #7 : Understanding Dna Replication

Which prokaryotic polymerase is primarily responsible for filling in DNA nucleotides into the gap created by the removal of RNA primers?

Possible Answers:

RNA polymerase

Reverse transcriptase

DNA polymerase I

DNA polymerase III

DNA polymerase II

Correct answer:

DNA polymerase I

Explanation:

DNA polymerase I replaces the RNA primer gap with DNA nucleotides. This polymerase is unique in that it has 5'  3' exonuclease activity. This RNA primer is created by primase, it is removed and replaced with DNA by DNA polymerase I, and the remaining nick is sealed by DNA ligase. Bacterial DNA polymerase III, in contrast, is the main polymerase for bacterial elongation. The function of DNA polymerase II is not completely understood. The remaining answer choices are not involved in prokaryotic DNA replication.

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