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

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.

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.

The correct answer is abortive initiation. This is a normal transcription event is found in both prokaryotes and eukaryotes and occurs prior to promoter clearance, or the event when RNA polymerase escapes the promoter and begins elongation of synthesized transcripts. Abortive initiation is thought to occur when the RNA polymerase complex is not stable enough on the DNA. DNA scrunching describes the mechanism by which RNA polymerase transcribes, rather than RNA polymerase moving along DNA, it actually pulls DNA into the complex and unwinds it.

The correct answer is Holliday junction. These intermediates in genetic recombination have symmetrical sequence and are mobile to preserve specific base pairing at recombination and damage loci. Repair enzymes recognize and subsequently localize to this DNA structure to facilitate locus specific enzymatic activity.

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.

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.

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

DNA synthesis requires a free 3' hydroxyl (-OH) group to add the next nucleotide base. Drugs that block DNA replication often have a modified 3' hydroxyl group, which prevents the addition of the next nucleotide and results in chain termination.

In order for a nucleotide to be added to a growing DNA molecule, two reactions must occur involving magnesium. First the 3'-OH group on the end of the growing DNA molecule is bound by magnesium and removed from the deoxyribose sugar so that the phosphate from the new nucleotide can bind in its place. Second, nucleotides exist in the nucleus as dNTPs (deoxyribose nucleoside triphosphates). This means that there are three phophates attached to the deoxyribose sugar on the free nucleotide. Only one phosphate (the primary phosphate) binds to the growing DNA molecule. Magnesium binds the other two phosphates and removes them from the dNTP so that the reaction can continue.

In order for DNA polymerase III to lengthen the new DNA strands, RNA primers must be put in place as a template. Once the strands are done being made, the RNA primers are removed and replaced with DNA nucleotides by DNA polymerase I.

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.

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.

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.

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.

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.

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.

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.

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.

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.