4: Cell division (mitosis and meiosis)

4: Cell Division (Mitosis and Meiosis)

Cell division is the fundamental process by which organisms grow, develop, and reproduce. In genetics, understanding the two main types of eukaryotic cell division – mitosis and meiosis – is crucial because they govern how genetic material is distributed to daughter cells, forming the basis of inheritance.

Mitosis: Ensuring Genetic Uniformity in Somatic Cells
Mitosis produces two genetically identical daughter cells from one parent cell. Its primary role is in growth, tissue repair, and asexual reproduction. The process involves several distinct phases:

1. Prophase: Chromosomes condense and become visible. The nuclear envelope breaks down, and the mitotic spindle (composed of microtubules) begins to form.
2. Metaphase: Chromosomes align precisely along the equatorial plane (metaphase plate) of the cell, attached to spindle fibers at their centromeres.
3. Anaphase: Sister chromatids (identical copies of each chromosome) separate and are pulled towards opposite poles of the cell by the shortening spindle fibers.
4. Telophase: Chromosomes decondense at the poles. Nuclear envelopes re-form around each set of chromosomes, creating two distinct nuclei. Cytokinesis (cytoplasmic division) then physically splits the cell into two identical diploid ($2n$) daughter cells, each possessing the same number and type of chromosomes as the parent cell. This maintains genetic stability in somatic (body) cells.

Meiosis: Generating Genetic Diversity for Sexual Reproduction
Meiosis produces gametes (sperm and egg cells) and is essential for sexual reproduction. It involves two consecutive divisions (Meiosis I and Meiosis II) but only one round of DNA replication. This results in four non-identical haploid ($n$) daughter cells, each with half the number of chromosomes (one set) of the original diploid cell.

• Meiosis I (Reduction Division): Homologous chromosomes (one inherited from each parent) pair up during prophase I in a process called synapsis, forming tetrads. Crucially, crossing over occurs here – homologous chromosomes exchange segments, shuffling alleles and creating new genetic combinations. During metaphase I, homologous pairs line up at the equator. In anaphase I, homologous chromosomes (each still consisting of two sister chromatids) separate and move to opposite poles. This reduces the chromosome number by half. Telophase I and cytokinesis result in two haploid cells, but each chromosome still has two chromatids.
• Meiosis II (Equational Division): Resembles a mitotic division but occurs in haploid cells. In prophase II, a new spindle forms. Metaphase II sees chromosomes align at the equator. Anaphase II involves the separation of sister chromatids, which are pulled to opposite poles. Telophase II and cytokinesis result in four haploid gametes, each with a unique combination of chromosomes and alleles due to independent assortment (random alignment of homologs in Meiosis I) and crossing over.

Key Differences & Genetic Significance

• Products: Mitosis: 2 identical diploid ($2n$) cells. Meiosis: 4 unique haploid ($n$) gametes.
• Genetic Variation: Mitosis maintains genetic identity. Meiosis generates enormous genetic diversity through crossing over (prophase I) and independent assortment (metaphase I).
• Role: Mitosis is for growth and maintenance. Meiosis is essential for sexual reproduction and the formation of gametes, enabling the combination of genetic material from two parents.