Describe the main events in each stage of a cell cycle
The correct answer and explanation is:
The cell cycle consists of two main phases: interphase and the mitotic (M) phase. These phases are further divided into stages that involve specific processes.
Interphase:
- G1 (Gap 1) Phase: This is the first stage of interphase. The cell grows in size and carries out its normal functions. It also prepares the necessary organelles and molecules required for DNA replication. The G1 checkpoint ensures that the cell is ready for DNA synthesis.
- S (Synthesis) Phase: During this phase, the cell’s DNA is replicated, ensuring that each daughter cell will receive an identical set of chromosomes. The DNA strands unwind and duplicate to form sister chromatids.
- G2 (Gap 2) Phase: The cell continues to grow and produces proteins necessary for cell division. It also checks the newly replicated DNA for errors. The G2 checkpoint ensures that the DNA replication is complete and the cell is ready to enter mitosis.
M Phase (Mitotic Phase):
- Prophase: The chromatin condenses into visible chromosomes. The nuclear membrane begins to break down, and the mitotic spindle, which will help separate the chromosomes, starts to form. The centrosomes (microtubule organizing centers) move toward opposite poles of the cell.
- Metaphase: The chromosomes align at the metaphase plate, which is an imaginary line equidistant from the two centrosomes. The spindle fibers attach to the centromeres of the chromosomes, ensuring that each sister chromatid will be pulled toward opposite poles.
- Anaphase: The sister chromatids are separated at the centromere and are pulled toward opposite poles of the cell by the spindle fibers. This ensures that each daughter cell will receive an identical set of chromosomes.
- Telophase: The separated chromatids, now individual chromosomes, begin to de-condense back into chromatin. The nuclear membrane re-forms around each set of chromosomes, creating two nuclei.
Cytokinesis: This process, often considered part of the M phase, is the final step where the cytoplasm divides, resulting in two daughter cells. In animal cells, the cell membrane pinches inwards to divide the cytoplasm, while in plant cells, a cell plate forms to separate the two daughter cells.