A. What is the purpose of cell-cycle checkpoints, such as the DNA-damage checkpo

Question

A. What is the purpose of cell-cycle checkpoints, such as the DNA-damage checkpoint or the spindle-attachment checkpoint?

B. You must isolate mutant cells defective in the DNA-damage checkpoint, but you find that under normal laboratory conditions, the mutant cells grow and divide at the same rate as wild-type cells. Why doesn’t the defect in the DNA-damage checkpoint have a large effect on cell growth and division rates under normal conditions?

C. Please describe an experimental condition in which wild-type cells and mutant cells defective in the DNA-damage checkpoint would show a difference in cell growth and division rates.

D. How would this condition affect the cell growth and division rates in the wild-type and mutant strains from Part B?

Explanation / Answer

Ans A. A checkpoint is a stage in the eukaryotic cell cycle. In this stage the cell examines both the internal and the external cues to decide whether to go ahead with the division or not. The cell cycle is halted when the checkpoint detect problems with the DNA. So, the cell attempts to either complete DNA replication or repair the damaged DNA.

The M checkpoint is also known as the spindle checkpoint. Here, the cell examines whether all the sister chromatids are correctly attached to the spindle microtubule and the cell will halt if a chromosome is misplaced and this gives time for the spindle to capture the stray chromosome.

Ans B. Cells that have a defective G2-M checkpoint do not have a large effect on cell growth and division because they enter mitosis before repairing their DNA and this leads to death after cell division.

Ans C: Analysis of cell cycle mutants in both Saccharomyces cerevisiae and Schizosaccharomyces pombe has led to the identification of a number of proteins that are involved in both the initiation and elongation of DNA replication. It affects replication by depletion of dNTPs

Ans D: Atr1 and Chk1 are the main players of the DDR pathway and if it fails or if it lacks functioning, then it results in the production of high sensitivity genotoxic agents in the cells, most commonly to agents that cause DNA replication stress such as methylmethane sulfonate (MMS) and hydroxyurea (HU). This affects replication by depletion of dNTPs

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