Cell Cycle and Mitosis: Biological Processes Explained

1. Interphase

The cell cycle begins with interphase, which lasts longer than mitosis. During this stage, the cell duplicates its components and DNA.

G1 Phase (5 hours)

Occurring after mitosis and before DNA synthesis, the cell increases in size and accumulates substances necessary for division. This stage includes the Restriction Point (R); if the cell does not pass this point, it enters the G0 phase (quiescent state, e.g., neurons). By the end of this phase, protein synthesis increases, histones are transported to the nucleus, and the number of centrioles and microtubules grows.

S Phase (6-8 hours)

During this phase, DNA replication occurs. Euchromatin is replicated first, followed by heterochromatin. Metabolic activity decreases, while histone synthesis and mRNA production persist. Centrioles also duplicate.

G2 Phase (3-5 hours)

This is the latency period between DNA replication and the start of nuclear division. Regulatory processes prevent the accumulation of excess gene products. This phase concludes when chromosomes begin to condense, signaling the start of mitosis.

2. Mitosis

Mitosis is the mechanism that ensures the regular division of hereditary components (chromosomes). It requires both karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Stages of Mitosis

• Prophase: The nucleus increases in size, and chromosomes become distinct. Each chromosome consists of two chromatids joined by a centromere. The nuclear membrane dissolves, and the spindle apparatus develops.
• Metaphase: Chromosomes align along the equatorial plane of the cell, attached to the spindle fibers by their centromeres.
• Anaphase: Spindle filaments shorten, pulling sister chromatids apart toward opposite poles of the cell.
• Telophase: Daughter nuclei are reconstructed. Chromatids fade back into chromatin, the nuclear membrane reforms, and the spindle disappears.

Cytokinesis

• Plant Cells: A cell plate forms to create a new cell wall between daughter cells.
• Animal Cells: A cleavage furrow forms at the cell periphery, strangulating the equatorial zone until the mother cell divides into two daughter cells.