Question

In which stage of cell cycle, does DNA replication occur?
a) G1 phase
b) S phase
c) G2 phase
d) M phase

Answer 1

Hint: The cell cycle of an animal cell has two main stages: the Interphase and the M phase. DNA synthesis occurs during interphase.

Complete step-by-step answer:
The cell cycle of animal cells has two main stages: the Interphase and M phase. Interphase is the preparatory phase that is further divided into Gap1 (G1) phase, Synthesis (S) phase, and Gap2 (G2) phase. M phase is the mitotic phase where true cell division occurs.

Interphase is the longest phase. It consists of the following stages:
> G1 phase is known as the Growth phase. The chromosome unwinds to form euchromatin to transcribe DNA to RNA for synthesizing proteins. Cell doubles in size. There is a checkpoint at the end of this phase to make sure that cells can enter the next stage.

> S phase is the replication phase. The majority of the resources of the cell are used for DNA replication. In human cells, all 46 chromosomes are replicated. The original and replicated chromosomes are joined by special proteins at a region called the centromere. At this point, the cell contains 92 chromatids ( 42 original chromatids and 42 replicated chromatids)

> G2 phase is the last phase of interphase. This phase makes sure that the cell is well prepared for mitosis. Hence protein synthesis continues here. The G2 phase has checkpoints too. At the checkpoint, the cell checks for the concentration of the Mitosis promoting factor (MPF). If the concentration level is high enough, the cell exits the G2 phase, and the cell enters the M phase.

> M phase is a mitotic phase which comprises two events- karyokinesis and cytokinesis. Karyokinesis involves the division of the nucleus and is divided into four stages: Prophase, metaphase, anaphase, and telophase. Cytokinesis involves the division of the cytoplasm resulting in independent daughter cells.

Thus, the correct answer is option B, i.e. S phase

Note: G1 phase gathers machinery for DNA replication, but the process actually occurs in S phase.