Answer the following questions in about 150 words each : (a) Define cell cycle. Describe different types and phases of cell cycle.

Definition of Cell Cycle

The cell cycle is a series of events that takes place in a cell leading to its division and duplication. It involves cell growth, DNA replication, and cell division, ensuring that two daughter cells are produced, each with identical genetic material and cellular components from the parent cell.

Types of Cell Division

While the cell cycle broadly governs cell reproduction, there are two primary types of cell division that occur in eukaryotic cells:

• Mitosis: This is an equational division that results in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. It is crucial for growth, tissue repair, and asexual reproduction.
• Meiosis: This is a reductional division that occurs in germ cells, producing four haploid daughter cells (gametes) with half the number of chromosomes of the parent cell. It is essential for sexual reproduction and genetic diversity.

Phases of the Cell Cycle (Eukaryotic Cells)

The eukaryotic cell cycle is typically divided into two main stages: Interphase and the M-phase (Mitotic Phase).

1. Interphase (Preparatory Phase)

This is the longest phase, during which the cell grows and prepares for division. It consists of three sub-phases:

• G₁ Phase (Gap 1):
  • The cell grows significantly, synthesizes proteins and RNA, and recovers from the previous mitosis.
  • Organelles duplicate, and the cell prepares for DNA replication.
  • At the end of G1, there is a crucial checkpoint (G1 checkpoint or Restriction Point) that determines if the cell proceeds to S phase or enters a quiescent G₀ phase.
• S Phase (Synthesis):
  • DNA replication occurs, resulting in the duplication of chromosomes. Each chromosome now consists of two sister chromatids.
  • The amount of DNA per cell doubles (e.g., from 2C to 4C), but the chromosome number remains the same (e.g., if 2n, remains 2n).
• G₂ Phase (Gap 2):
  • The cell continues to grow and synthesizes proteins necessary for mitosis, such as spindle fibers.
  • It checks for any DNA damage or replication errors before entering the M-phase.

2. M-Phase (Mitotic Phase)

This is the actual cell division phase, which is relatively short and involves both nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).

• Mitosis (Karyokinesis): Division of the nucleus, typically involving four stages:
  • Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and the mitotic spindle begins to form.
  • Metaphase: Chromosomes align at the metaphase plate (equator of the cell), attached to spindle fibers.
  • Anaphase: Sister chromatids separate and move to opposite poles of the cell, becoming individual chromosomes.
  • Telophase: Chromosomes decondense at the poles, nuclear envelopes reform, and the mitotic spindle disappears.
• Cytokinesis: Division of the cytoplasm, which usually overlaps with telophase, resulting in the formation of two distinct daughter cells. In animal cells, a cleavage furrow forms, while in plant cells, a cell plate forms.

G₀ Phase (Quiescent Stage)

Some cells exit the cell cycle after the G₁ phase and enter an inactive stage called G₀. These cells remain metabolically active but do not proliferate unless stimulated to re-enter the G₁ phase. Examples include nerve cells and mature muscle cells, which are terminally differentiated and typically do not divide.