1.12 Cell Division

Cell division is the fundamental process by which a parent cell divides into two or more daughter cells, ensuring the continuity of life. It is essential for growth, repair, and reproduction. The two main types are mitosis (for somatic cells) and meiosis (for germ line cells).

Cell Cycle

The cell cycle is the ordered series of events a cell undergoes, leading to its division and the production of two daughter cells. It is broadly divided into two main phases: Interphase and the M (Mitotic) phase.

• Interphase: The longest stage of the cell cycle, where the cell grows and copies its DNA in preparation for division.

  • G1 phase (First Gap): The cell grows by producing proteins and cytoplasmic organelles.
  • S phase (Synthesis): The cell duplicates its chromosomes (DNA replication).
  • G2 phase (Second Gap): The cell continues to grow and completes its preparations for division.

• G0 phase: A quiescent stage where cells exit the cell cycle and perform their normal metabolic functions without active division.

• M phase (Mitotic phase): The shortest part of the cell cycle, which includes nuclear and cytoplasmic division.

  • Mitosis (Karyokinesis): Division of the nucleus.
  • Cytokinesis: Division of the cytoplasm.

• Cell Cycle Checkpoints: Control mechanisms (at G1, G2, and during mitosis) that ensure the cell cycle proceeds correctly and without errors.

Mitosis

Mitosis is the division of a cell's nucleus in which the chromosomes are duplicated and distributed equally, resulting in two genetically identical diploid ($2n$) daughter cells. It is a continuous process divided into five phases for descriptive purposes.

Phases of Mitosis

1. Interphase: The cell prepares for mitosis. The nuclear envelope is intact, chromosomes are duplicated but not condensed (visible as chromatin), and two centrosomes have formed.

2. Prophase:

   • Chromatin fibers coil and condense into visible chromosomes.
   • Each chromosome consists of two identical sister chromatids joined at the centromere.
   • The nucleoli disappear.
   • The mitotic spindle begins to form from the centrosomes, which move apart.

3. Prometaphase:

   • The nuclear envelope breaks down.
   • Spindle microtubules invade the nuclear area.
   • Each chromatid develops a kinetochore (a protein structure at the centromere) to which microtubules attach.

4. Metaphase:

   • Centrosomes are at opposite poles of the cell.
   • Chromosomes align on the metaphase plate, an imaginary plane equidistant between the two poles.
   • The kinetochores of sister chromatids are attached to microtubules from opposite poles.

5. Anaphase:

   • The shortest stage of mitosis.
   • Sister chromatids suddenly separate, becoming individual chromosomes.
   • The newly formed chromosomes are pulled towards opposite poles as kinetochore microtubules shorten.
   • The cell elongates.

6. Telophase:

   • Two daughter nuclei begin to form.
   • Nuclear envelopes reform around the two sets of chromosomes.
   • Chromosomes become less condensed.
   • Mitosis (karyokinesis) is complete.

Cytokinesis

Cytokinesis is the division of the cytoplasm, which typically begins during late telophase.

• Animal cells: A cleavage furrow (a pinching of the cell membrane) forms and deepens until the cell is split into two daughter cells. This is driven by a contractile ring of actin microfilaments.
• Plant cells: A cell plate (phragmoplast) forms due to the fusion of Golgi-derived vesicles containing pectin and cellulose at the equatorial plane.

Significance of Mitosis

• Essential for the growth and multiplication of cells.
• Ensures daughter cells have the same number of chromosomes and genetic information as the parent cell.
• Crucial for wound healing, regeneration, and replacing old or damaged cells.
• Uncontrolled mitosis can lead to the formation of tumors or cancer.

Stem Cells→

Meiosis

Meiosis is a specialized type of cell division where a single diploid ($2n$) cell divides twice to produce four haploid ($n$) daughter cells, each with half the original amount of genetic information. It is the process that produces gametes (sperm and eggs) in animals and spores in plants.

Meiosis I: The Reductional Division

This division separates homologous chromosomes, reducing the chromosome number from diploid to haploid.

1. Prophase I: A long and complex phase — unique to meiosis.
   • Chromosomes condense and become visible.
   • Synapsis: Homologous chromosomes pair up lengthwise to form a bivalent or tetrad (a structure of four chromatids).
   • Crossing Over: Genetic material is exchanged between non-sister chromatids of homologous chromosomes at points called chiasmata. This creates new combinations of genes (genetic recombination).

2. Metaphase I:
   • Homologous pairs (tetrads) line up at the metaphase plate.
   • Microtubules from each pole attach to one chromosome of each homologous pair.
3. Anaphase I:
   • Homologous chromosomes are pulled apart and move to opposite poles.
   • Crucially, sister chromatids remain attached at their centromeres (unlike mitosis anaphase).
4. Telophase I and Cytokinesis:
   • Chromosomes arrive at opposite poles.
   • Each pole now has a haploid set of chromosomes, but each chromosome still consists of two sister chromatids.
   • The cytoplasm divides, forming two haploid daughter cells.

Meiosis II: The Equational Division

This division separates sister chromatids and is mechanistically similar to mitosis.

1. Interphase II: A brief phase where DNA replication does NOT occur.
2. Prophase II: Spindle apparatus forms.
3. Metaphase II: Chromosomes (each still with two chromatids) align at the metaphase plate.
4. Anaphase II:
   • Centromeres divide, and sister chromatids separate.
   • The separated chromatids (now individual chromosomes) move to opposite poles.
5. Telophase II and Cytokinesis:
   • Nuclei reform around the chromosomes at each pole.
   • Cytokinesis occurs, resulting in a total of four genetically unique haploid daughter cells.

Significance of Meiosis

• Reduces the chromosome number by half, ensuring the chromosome count remains constant from one generation to the next after fertilization.
• Introduces genetic variability through crossing over and the independent assortment of homologous chromosomes.
• Provides the raw material for natural selection and evolution.

Comparison: Mitosis vs. Meiosis