A synchronously dividing cell population is treated with S35-Methionine when the

Question

A synchronously dividing cell population is treated with S35-Methionine when the cells entered into S phase. At G2/S phase the cells are lysed and the protein extract is fractionated through column chromatography. The samples fractions are collected and injected into a cell line that is unable to cross from G1 into S phase. The fractions that allowed the cells to cross into S phase didn’t show any signal on photographic plate.

Choose the statement that best explains the reason for lack of S35-Methionine signal in the fractions.

S-Cyclin sequence has no Methionine and hence there is no incorporation of S35 Methionine in the fractions that contain S-Cyclin.

Explanation / Answer

Choice 4 is correct.

Explanation: The S phase of cell cycle represents the chechpoint where synthesis of DNA takes place. When a cell successfully passes this phase, it enters the G2 phase where protein translational takes place. Thus, no signal from radiolabelled protein or amino acid must be expected during S phase. Here, isolation and injection of labelled methionine took place during S phase and these cells couldn't pass through S phase checkpoint. Thus, no translational took place and thus no radioactive signal was obtained.

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