All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources
Example Questions
Example Question #1 : Dna Replication And Repair
Which of the following is NOT a DNA repair mechanism?
Nucleotide excision repair
Binding-protein excision repair
Mismatch repair
Base excision repair
Binding-protein excision repair
There are three main single-stranded DNA repair mechanisms.
The first is nucleotide excision repair. In this mechanism, specific endonuclease enzymes remove nucleotides containing damaged bases. DNA polymerase then replaces the region with undamaged bases, and ligase seals the addition with phosphodiester bonds.
The second mechanism is base excision repair. In this mechanism, glycosylase enzymes detect and excise damaged bases. DNA polymerase then replaces the region with undamaged bases, and ligase seals the addition with phosphodiester bonds.
Finally, there is mismatch repair. In this mechanism a new strand of DNA is tested for pairing with the template strand, prior to methylation. Any mismatched nucleotides are removed, replaced, and joined into the complete strand.
Example Question #1 : Dna
Which of the following enzymes adds DNA to the ends of chromosomes to avoid loss of genetic material with duplication?
Telomerase
Primase
Helicase
Polymerase
Telomerase
Telomeres are regions of non-coding DNA at the ends of the DNA strands. The telomeres function as regions of acquired damage and mutation, protecting the actual genome. Telomerase is the enzyme responsible for adding additional nucleotides to the 3' end of the chromosome to maintain the telomere.
Helicase unwinds the DNA helix and separates the strands to form the replication fork. Primase synthesizes short RNA primers on the DNA template to help recruit DNA polymerase, which then adds nucleotides to build the new DNA strand.
Example Question #1 : Dna Replication And Repair
What is the role of helicase?
Facilitates formation of phosphodiester bonds
Unwinds DNA template at the replication fork
Nicks the DNA backbone to relieve supercoils
Prevents DNA strands from reannealing
Unwinds DNA template at the replication fork
Helicase is one of the first proteins necessary for initiating DNA replication. It is responsible for unwinding the DNA double-helix and separating the hydrogen bonds that hold the two strands together. This allows DNA polymerase to enter the replication fork and recruit nucleotides to build daughter DNA molecules.
Single-strand binding proteins attach to the DNA in the replication fork to prevent it from reannealing. Topoisomerase breaks phosphodiester bonds in the DNA backbone to relieve tension, while DNA ligase reestablishes these bonds after replication is complete and fuses Okazaki fragments on the lagging strand.
Example Question #1 : Dna Replication And Repair
Which of the following structures indicates where DNA replication begins?
Origin of replication
Replication fork
Helicase
DNA polymerase III
Origin of replication
The origin of replication is the particular sequence in the genome where DNA replication begins. In prokaryotes, there is a single origin of replication, whereas there are multiple origins of replication in eukaryotes. At the origin of replication in eukaryotes, certain proteins bind to form the origin recognition complex. This complex is then used to recruit replication proteins and initiate the process of DNA replication.
Example Question #1 : Dna Replication And Repair
__________ is the primary prokaryotic replicatory polymerase that can proofread DNA and fix incorrect base pairs due to its __________.
DNA polymerase III . . . 3'-5' endonuclease function
DNA polymerase I . . . 3'-5' endonuclease function
DNA polymerase III . . . 3'-5' exonuclease function
DNA polymerase I . . . 3'-5' exonuclease function
DNA polymerase III . . . 3'-5' exonuclease function
DNA polymerase III is the main replicatory polymerase in prokaryotic cells, responsible for synthesizing daughter DNA strands during replication. DNA polymerase I performs more specialized functions, such as synthesizing DNA during DNA repair pathways.
The difference between an endonuclease and an exonuclease is whether or not the cleavage takes place in the middle or at the end of a strand, respectively. DNA polymerase III is cleaving bases at the end of the strand, meaning it has exonuclease function.
Example Question #1 : Help With Dna Replication Proteins
During DNA replication, single-stranded DNA is kept from reannealing due to the function of __________.
DNA topoisomerase
single-strand binding proteins
helicase
histones
single-strand binding proteins
Single-strand binding proteins, as the name suggests, bind single-stranded DNA. This is important because it helps prevent the strands from reannealing prematurely. These proteins are essential for maintaining the replication fork.
Helicase is responsible for separating the strands at the replication fork, but is not directly responsible for preventing single-stranded DNA from reannealing. It creates the replication fork, but is incapable of maintaining it. DNA topoisomerase cuts the DNA backbone ahead of the replication fork to avoid topological problems. Histone proteins are removed during DNA replication, and are not involved in this process.
Example Question #1 : Help With Dna Replication Proteins
A select mutation is causing a cell lineage to be unable to replicate DNA successfully. When observed under a microscope, researchers observe that the DNA is able to be separated, but the template strands keep coming back together before the new strands can be replicated.
Based on this observation, which protein involved in DNA replication is most likely mutated?
Single-stranded binding protein
DNA primase
DNA polymerase
DNA helicase
Single-stranded binding protein
Since the strands can be successfully "unzipped" from one another, this suggests that DNA helicase is working just fine. There is also nothing in the prompt that states the synthesis of new strands is not working, so DNA polymerase is fine as well. The problem involves keeping the strands separated for a long enough time. This is the job of single-stranded binding proteins. Because of this, we can argue that this protein is mutated in the cell.
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All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources
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