Cell Biology Chapter 6 True False

DNA ligase is required to seal the sugar-phosphate backbone between all the DNA fragments on the sagging strand.

True

The 3′ overhang "invades" the homologous DNA duplex, which can be used as a primer for the repair DNA polymerase

True.

The first step in repair requires a nuclease to remove a stretch of base pairs from the 5′ end of each strand at the site of the break.

True.

Ionizing radiation and oxidative damage can cause DNA double‐strand breaks.

True

Primase is needed to initiate DNA replication on both the leading strand and the lagging strand.

True

Telomerase is a DNA polymerase that carries its own RNA molecule to use as a primer at the end of the lagging strand.

True

There pair polymerase does not require the aid of the sliding clamp, because it is only synthesizing DNA over very short stretches.

True

DNA replication is a bidirectional process that is initiated at multiple locations along chromosomes in eukaryotic cells.

True.

Meselson and Stahl ruled out the dispersive model for DNA replication

True.

The sliding clamp is loaded once on each DNA strand, where it remains associated until replication is complete.

False. Although the sliding clamp is only loaded once on the leading strand, the lagging strand needs to unload the clamp once the polymerase reaches the RNA primer from the previous segment and then reload it where a new primer has been synthesized.

DNA replication origins are typically rich in G‐C base pairs

False. DNA replication origins are typically rich in A‐T base pairs, which are held together by only two hydrogen bonds (instead of three for C‐G base pairs), making it easier to separate the strands at these sites.

Depurination of DNA is a rare event that is caused by ultraviolet irradiation.

False. Depurination occurs constantly in our cells through spontaneous hydrolysis of the bond linking the DNA base to the deoxyribose sugar.

Homologous recombination cannot occur in prokaryotic cells, because they are haploid, and therefore have no extra copy of the chromosome to use as a template for repair.

False. Homologous recombination also occurs in prokaryotic cells, and typically occurs very shortly after DNA replication, when the newly replicated duplexes are in close proximity.

Nonhomologous end joining is a mechanism that ensures that DNA double‐strand breaks are repaired with a high degree of fidelity to the original DNA sequence.

False. Homologous recombination can repair double‐strand breaks without any change in DNA sequence, but nonhomologous end joining always involves a loss of genetic information because the ends are degraded by nucleases before they can be ligated back together.

After damaged DNA has been repaired, nicks in the phosphate backbone are maintained as away to identify the strand that was repaired.

False. It is believed that the nicks are generated during DNA replication as a means of easy identification of the newly synthesized strand but are sealed by DNA ligase shortly after replication is completed

There is a single enzyme that degrades the RNA primers and lays down the corresponding DNA sequence behind it.

False. Primase does not have a proofreading function, nor does it need one because the RNA primers are not a permanent part of the DNA. The primers are removed, and a DNA polymerase that does have a proofreading function fills in the remaining gaps.

The DNA template used to repair the broken strand is the homologous chromosome inherited from the other parent

False. Although it is called homologous recombination, this is not a process that depends on the proximity of parental homologs. When used as a mechanism for DNA repair, homologous recombination uses the sister chromatids in an undamaged, newly replicated (homologous) DNA helix as a template.

When DNA is being replicated inside a cell, local heating occurs, allowing the two strands to separate.

False. The two strands do need to separate for replication to occur, but this is accomplished by the binding of initiator proteins at the origin of replication.

There pair polymerase is the enzyme that proofreads the newly synthesized strands to ensure the accuracy of DNA replication.

False. There pair polymerase is used to fill in the spaces left vacant after the RNA primers are degraded