6.3 - Meiosis

Meiosis is a process of nuclear division that reduces the number of chromosomes in daughter cells to half of that in the parent cell. Meiosis produces haploid gametes through gametogenesis. During sexual reproduction in animals, the gametes (sperm and ovum) fuse together to restore the number of chromosomes, forming a diploid zygote.

The Need for Meiosis

Meiosis provides genetic variation which occurs every next generation. Meiosis is important to maintain the diploid number of chromosomes from one generation to the next. Meiosis consists of two separate nuclear divisions:

• Meiosis I , which consists of prophase I, metaphase I, anaphase I and telophase I
• Meiosis II , which consists of prophase II, metaphase II, anaphase II and telophase II (works like mitosis)

Figure 14 - 1. During prophase I , chromatin condenses to form chromosomes (shorter and thicker). Homologous chromosomes pair up together to form bivalents/tetrad through a process called synapsis. Crossing over can occur at any locations or several locations on the chromosomes where the exchange of genetic materials between non-sister chromatids. The points where the chromatids cross over is called chiasmata (singular, chiasma). By the end of prophase I, nucleoli and the nuclear membrane start to disappear. Centrioles migrate to the opposite poles of the cell. The spindle fibres start to form.

Figure 15 - 2. During metaphase I , the homologous chromosomes are arranged side by side at the metaphase plate. Each chromosome of the homologous pair is attached to the spindle fibres from the opposite poles. The centromeres do not divide.

Figure 16 - 3. Anaphase I begins when the homologous chromosomes are pulled apart to the opposite poles of the cell. Each chromosome still has two sister chromatids that move as a single unit.

Figure 17 - 4. During telophase I , the chromosomes reach the opposite poles. Each pole now has a haploid daughter nuclei (4 chromosomes to 2 chromosomes). The spindle fibres disappear. Nucleoli and nuclear membrane are reformed in each two daughter cells.

Figure 18 - 5. In prophase II , the nuclear membrane and nucleoli start to disappear. The spindle fibres reforms in each daughter cell. Each chromosome has two sister chromatids joined together at the centromere.

Figure 19 - 6. In metaphase II , the chromosome are lined up randomly at the metaphase plate. Each sister chromatid is attached to the spindle fibres at the centromere. Metaphase II ends when the sister chromatids separate.

Figure 20 - 7. In anaphase II , the centromeres of the sister chromatids separate. Now, the sister chromatids are individual chromosomes. The individual chromosome moves towards the opposite poles of the cell.

Figure 21 - 8. Finally in telophase II , the nucleoli and nuclear membrane are reform. The spindle fibres break down. Cytokinesis follows and four haploid daughter cells are formed. Each haploid cell has half the number of chromosomes as the parent cell. They are genetically different as well. These haploid cells become gametes.

Comparison between Mitosis and Meiosis

• Similarities
  • DNA replication happens once only
• Differences
  • Mitosis
    • Type of cell: All somatic cells
    • Purpose: For growth and repair
    • Synapsis: Absent
    • Crossing over: Absent
    • Metaphase: Chromosomes are arranged randomly at the metaphase plate
    • Anaphase: Sister chromatids separate
    • Number of divisions: One
    • Number of daughter cells produced: Two
    • Chromosomal number of the daughter cells: Diploid (2n)
    • Genetic content: Genetically identical
    • Genetic variation: Absent
  • Meiosis
    • Type of cell: Cells in the reproductive organs
    • Purpose: Produces gametes for sexual reproduction
    • Synapsis: Present
    • Crossing over: Present
    • Metaphase: Homologous chromosomes line up adjacent to each other at the metaphase plate
    • Anaphase: Homologous chromosomes separate during anaphase I
    • Number of divisions: Two
    • Number of daughter cells produced: Four
    • Chromosomal number of the daughter cells: Haploid (n)
    • Genetic content: Genetically different
    • Genetic variation: Present