GRE Subject Test: Biology : Genetics, DNA, and Molecular Biology

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

varsity tutors app store varsity tutors android store

Example Questions

1 2 3 4 5 6 8 Next →

Example Question #1 : Understanding Dna Replication

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

Possible Answers:

Topoisomerase

DNA ligase

DNA helicase

Primase

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 #4 : Understanding Dna Replication

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

Possible Answers:

RNA polymerase . . . the lagging strand

Primase . . . the lagging strand

RNA polymerase . . . both the leading and lagging strands

Primase . . . both the leading and lagging strands

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 #5 : Understanding Dna Replication

Which of the following is not true of DNA replication?

Possible Answers:

Replication occurs in the 5' to 3' direction

Replication occurs during prophase of mitosis

The DNA must be denatured at the replication fork

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

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 #1 : Dna Replication And Repair

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

DNA polymerase III

DNA polymerase II

DNA polymerase I

Reverse transcriptase

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.

Example Question #1 : Dna Replication And Repair

Which of the following is not true of DNA repair?

Possible Answers:

Irreparable DNA damage may activate pathways that lead to apoptosis

p53 is a protein responsible for activating many DNA repair pathways

DNA repair cannot occur during replication

Several DNA polymerases contain an exonuclease function 

Correct answer:

DNA repair cannot occur during replication

Explanation:

DNA repair can, and does, occur during replication. An easy example of this is the proofreading function of several DNA polymerases. This function is carried out due to the enzymes containing an exonuclease function that allows them to excise incorrect base pairs. p53 is an incredibly important protein that is expressed heavily when DNA damage is detected. It is responsible for activating both DNA repair pathways and apoptotic pathways, preventing the cell from passing replication and cell cycle checkpoints. If the DNA damage is irreparable, the cell may undergo apoptosis. 

Example Question #2 : Dna Replication And Repair

Upon double-stranded DNA break, the cell can search the genome for a homologous sequence to serve as a template for repairing the damaged sequence. What is this process known as?

Possible Answers:

Crossing over 

Directed reversal

Base excision repair

Homology directed repair

Non-homologous end joining

Correct answer:

Homology directed repair

Explanation:

The correct answer is homology directed repair. Using flanking homologous regions upstream and downstream of the double stranded break, the cell is able to determine the precise sequence that is in the damaged region and repair that sequence. 

Example Question #1 : Dna Replication And Repair

Which of the following enzymes is responsible for catalyzing the formation of phosphodiester bonds in single- and double-stranded DNA breaks?

Possible Answers:

DNA polymerase I

DNA ligase

Endonuclease

DNA polymerase III

Exonuclease 

Correct answer:

DNA ligase

Explanation:

DNA ligase catalyzes the formation of phosphodiester bonds between the elements of the DNA backbone. DNA polymerases function in replication of DNA, whereas exonucleases and endonucleases break apart DNA strands by disrupting phosphodiester bonds. 

1 2 3 4 5 6 8 Next →
Learning Tools by Varsity Tutors