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Chapter 35: Chromatin: DNA Structure and Replication

When dividing cells are progressing through the cell cycle, many checks are imposed to ensure that the process is occuring with fidelity. A critical checkpoint in the cell cycle occurs in response to DNA damage, such as that induced by UV light. Which of the following cell-cycle proteins is involved in DNA damage–mediated cell-cycle arrest?

A. CDK2

B. cyclin A

C. cyclin D

D. E2F

E. p53

Answer E: One major function of the p53 protein is to serve as a component of the checkpoint that controls whether cells enter as well as progress through S-phase. The action of p53 is induced in response to DNA damage. Under normal circumstances p53 levels remain very low due to its interaction with a member of the ubiquitin ligase family called MDM2. In response to DNA damage, for instance, as a result of UV-irradiation or γ-irradiation, cells activate several kinases including checkpoint kinase 2 (CHK2) and ataxia telangiectasia mutated (ATM). One target of these kinases is p53. ATM also phosphorylates MDM2. When p53 is phosphorylated it is released from MDM2 and can carry out its transcriptional activation functions. One target of p53 is the cyclin inhibitor p21Cip1 gene. Activation of p21Cip1 leads to increased inhibition of the cyclin D1-CDK4 and cyclin E-CDK2 complexes, thereby halting progression through the cell cycle either prior to S-phase entry or during S-phase.

You are carrying out experiments to study DNA replication in a cell line derived from a breast cancer tumor. You discover that the replication process does not completely copy all of each chromosome. Examination of the process using electron microscopy indicates that replication ceases near to where the DNA strands are attached to the chromatin scaffold. Given these observations, which of the following activities is most likely to be defective in these cells?

A. DNA ligase

B. helicase

C. polymerase-α

D. single-stranded binding proteins

E. topoisomerase

Answer E: The progression of the replication fork requires that the DNA ahead of the fork be continuously unwound. Due to the fact that eukaryotic chromosomal DNA is attached to a protein scaffold the progressive movement of the replication fork introduces severe torsional stress into the duplex ahead of the fork. This torsional stress is relieved by DNA topoisomerases. Topoisomerases relieve torsional stresses in duplexes of DNA by introducing either double- (topoisomerases II) or single-stranded (topoisomerases I) breaks into the backbone of ...

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