Ch 6 Study Guide Cell Bio

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. Ionizing radiation and oxidative damage can cause DNA double-strand breaks.

true

T/F: DNA ligase is required to seal the sugar-phosphate backbone between all the DNA fragments on the lagging strand.

true

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

true

T/F: Messelson and Stahl ruled out the dispersive model for DNA replication.

true

T/F: Primase is needed to initiate DNA replication on both the leading strand and the lagging strand.

true

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

true

T/F: 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

T/F: The repair polymerase does not require the aid of the sliding clamp, because it is only synthesizing DNA over very short stretches.

true

T/F: telomerase is a DNA polymerase that carries its own RNA molecule to use as a primer at the end of the lagging strand.

true

T/F: 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.

T/F: 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.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. 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.

T/F: 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.

T/F: After damaged DNA has been repaired, nicks in the phosphate backbone are maintained as a way 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.

T/F: Primase requires a proofreading function that ensures there are no errors in the RNA primers used for DNA replication

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.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. The repair polymerase is the enzyme that proofreads the newly synthesized strands to ensure the accuracy of DNA replication.

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

T/F: 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.

The events listed below are all necessary for homologous recombination to occur properly: A. Holliday junction cut and ligated B. strand invasion C. DNA synthesis D. DNA ligation E. double-strand break F. nucleases create uneven strands What is the correct order of events during homologous recombination?

E, F, B, C, D, A

T/F: 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

Because all DNA polymerases synthesize DNA in the 5′-to-3′ direction, and the parental strands are antiparallel, DNA replication is accomplished with the use of two mechanisms: continuous and discontinuous replication. Indicate whether the following items relate to (1) continuous replication, (2) discontinuous replication, or (3) both modes of replication. ______ primase ______ single-strand binding protein ______ sliding clamp ______ RNA primers ______ leading strand ______ lagging strand ______ Okazaki fragments ______ DNA helicase ______ DNA ligase

___3___ primase ___2___ single-strand binding protein ___3___ sliding clamp ___3___ RNA primers ___1___ leading strand ___2___ lagging strand ___2___ Okazaki fragments ___3___ DNA helicase ___2___ DNA ligase

T/F: 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.

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

false. This is a two-step process that requires two different enzymes. First, a nuclease removes the RNA primers. Then, the repair polymerase fills in the complementary DNA sequence.