Cell division

1. Introduction to Cell Cycle

Cell division is a fundamental biological process by which a parent cell divides into two or more daughter cells. The sequence of events by which a cell duplicates its genome, synthesises other constituents, and eventually divides into two daughter cells is termed the cell cycle.

2. Phases of Cell Cycle

The cell cycle is broadly divided into two basic phases:

1. Interphase (Resting Phase)

   • Represents the phase between two successive M phases.
   • The cell prepares for division by undergoing cell growth and DNA replication in an orderly manner.
   • Occupies more than $95\%$ of the duration of the cell cycle.
   • Subdivided into three stages:
     • $G_1$ Phase (Gap 1): Cell grows continuously and is metabolically active.
     • $S$ Phase (Synthesis): DNA replication takes place. The amount of DNA per cell doubles (e.g., from $2C$ to $4C$), but chromosome number remains the same ($2n$).
     • $G_2$ Phase (Gap 2): Proteins are synthesised in preparation for mitosis while cell growth continues.
     • $G<em>0$ Phase (Quiescent Stage): Cells that do not divide further exit the $G</em>1$ phase and enter an inactive stage called the $G_0$ stage.

2. M Phase (Mitosis Phase)

   • Represents the phase where actual cell division occurs.
   • Involves nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis).

[Cell Cycle Flow Diagram]
       +-----------------------------------+
       |            Interphase             |
       |  (G1 Phase -> S Phase -> G2 Phase)|
       +-----------------+-----------------+
                         |
                         v
       +-----------------+-----------------+
       |             M Phase               |
       |  (Prophase -> Metaphase ->       |
       |   Anaphase  -> Telophase)         |
       +-----------------+-----------------+
                         |
                         v
                    Cytokinesis

3. Mitosis (Equational Division)

Mitosis results in the production of diploid daughter cells with identical genetic material ($2n \rightarrow 2n$).

Stages of Karyokinesis in Mitosis

1. Prophase:

   • Chromatin condenses to form distinct chromosomes.
   • Nucleolus, nuclear envelope, and Golgi apparatus disappear.
   • Centrosomes move to opposite poles and begin assembling spindle fibers.

2. Metaphase:

   • Chromosomes align at the equator of the cell (metaphase plate).
   • Spindle fibers attach to the kinetochores of chromosomes.

3. Anaphase:

   • Sister chromatids split at the centromere and move towards opposite poles.

4. Telophase:

   • Chromosomes reach opposite poles and lose their individuality.
   • Nuclear envelope reforms around each chromosome cluster; nucleolus and organelles reappear.

Cytokinesis

• Animal Cells: Formed by a cleavage furrow in the plasma membrane deepening inwards.
• Plant Cells: Formed by cell plate formation growing outwards from the center.

4. Meiosis (Reductional Division)

Meiosis reduces the chromosome number by half ($2n \rightarrow n$), producing four haploid daughter cells.

[Meiosis Overview Diagram]
       Parent Cell (2n)
              |
              v (Meiosis I)
       Cell 1 (n)   Cell 2 (n)
           |             |
           v (Meiosis II) v
     (n)  (n)       (n)  (n)  ---> 4 Haploid Daughter Cells

Stages of Meiosis I

1. Prophase I: Longest and most complex phase, divided into 5 substages:
   • Leptotene: Chromatin condenses into visible chromosomes.
   • Zygotene: Homologous chromosomes pair up (synapsis) forming a bivalent/tetrad.
   • Pachytene: Crossing over occurs between non-sister chromatids leading to genetic recombination.
   • Diplotene: Synaptonemal complex dissolves; chiasmata (X-shaped structures) become visible.
   • Diakinesis: Terminalisation of chiasmata; nuclear membrane breaks down.
2. Metaphase I: Bivalents align on the equatorial plate.
3. Anaphase I: Homologous chromosomes separate to opposite poles (sister chromatids remain attached).
4. Telophase I: Nuclear membrane reappears; followed by interkinesis.

Stages of Meiosis II

Equational division similar to mitosis, separating sister chromatids to produce four distinct haploid ($n$) cells.

5. Key Differences: Mitosis vs Meiosis

• Mitosis: Takes place in somatic cells; produces 2 identical diploid cells ($2n$); no crossing over.
• Meiosis: Takes place in germ cells; produces 4 genetically varied haploid cells ($n$); involves crossing over.