Question

Select the correct even from below that is most likely to occur during the three subphases of the interphase of the mitotic cell cycle.

• A. During G₁ phase, the cell is metabolically active and replicates its DNA.
• B. During S phase, duplication of centrioles takes place.
• C. The number of chromosomes becomes double in the synthesis phase.
• D. Replication of DNA starts during the G₂ phase and proteins are synthesized in order to prepare the cell for mitosis.

Answer 1

Answer: (B)

Interphase can be subdivided into 3 further phase: G₁, S and G₂.
G₁ phase corresponds to the interval between mitosis and initiation of DNA replication. During G₁ phase the cell is metabolically active and continuously grows but does not replicate its DNA. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G₁, even after S phase the number of chromosomes remains the same, i.e., 2n. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G₂ phase, proteins are synthesised in preparation for mitosis while cell growth continues