10.1.1 The Process of Meiosis

Like mitosis, meiosis is preceded by DNA replication

• Like most other eukaryotic cells, gamete-producing cells perform a cell cycle
  • Gamete-producing cells are found in the testes and ovaries
    • The cells that give rise to sperm cells are found in the testes
• If a new sperm-producing cell is being generated in the testes, mitosis occurs as with any other diploid body cell
  • The G1, S and G2 stages of the cell cycle proceed 
  • The sperm-producing cell undergoes DNA replication and the amount of DNA within that cell doubles
  • The cell still contains 2n number of chromosomes; although each chromosome has doubled in size
• DNA replication also happens before meiosis 
  • Unlike mitosis, replication in meiosis is followed by 2 rounds of chromosome separation as opposed to 1 round in mitosis
• Hormones and other stimuli trigger cells in the testes to enter meiosis; at this stage the sperm-producing cell ceases to be diploid and fulfils its function to produce haploid gametes (spermatids, which then develop into spermatozoa, also known as sperm cells)
  • The triggering of ova generation in female mammals is less well understood because of the different times of a female mammal's life when eggs are produced
    • Male mammals tend to produce sperm throughout their adult life

Mitosis or Meiosis?

• Gamete-producing cells are unique in that they can divide by both well-known cell division routes
• Considering a sperm-producing cell as an example, it has two possible routes of cell division
  • Mitosis - to regenerate itself and during growth of the tissue in the testes
    • Like any other somatic cell in this regard
  • Meiosis - to fulfil its specialisation ie. to produce sperm (called spermatogenesis) at the required time of the male's life
• Both routes begin with DNA replication within the diploid cell's nucleus

The two possible routes of cell division for a spermatocyte. Both start with DNA replication. The right hand side describes spermatogenesis

DNA Replication

• During interphase, the cell increases in mass and size and carries out its normal cellular functions
  • eg. synthesising proteins and the reactions of respiration
• Interphase consists of three phases:
  • G1 phase
  • S phase
  • G2 phase
• During interphase the DNA in the nucleus replicates, after which each chromosome consists of two identical sister chromatids
  • This phase of interphase during which DNA replication occurs is called the S phase – S stands for synthesis (of DNA)

Following DNA replication, the fate of the sperm-producing cell is determined by hormonal and other stimuli

• If the male is sexually mature, some cells in the testes will enter meiosis and begin producing sperm
  • The individual sperm-producing cell ceases to exist when it enters meiosis
• Other sperm-producing cells will divide by mitosis
  • To ensure a healthy population of sperm-producing cells for future sperm production
  • A sperm-producing cell retains its identity by this route

Overview of Meiosis

• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
• It produces gametes in plants and animals that are used in sexual reproduction
• It has many similarities to mitosis but it has two successive divisions: meiosis I and meiosis II
• As with mitosis, within each division there are four stages; prophase, metaphase, anaphase and telophase
• Meiosis occurs:
  • In the testes of male animals and the ovaries of female animals
  • In the anthers and ovaries of flowering plants
• Meiosis leads to the production of the following haploid gametes:
  • Spermatozoa, or sperm cells, in male animals, ova (singular ovum) in female animals
  • Male plant gametes are carried in pollen grains and female plants gametes are held in the ovules within the plant ovary.

Overview of meiosis, showing chromosome numbers and quantities of DNA present in each cell in humans (Homo sapiens)

Formation of Bivalents

• After DNA replication, the first step of the meiotic pathway (prophase I), is that the chromosomes condense
  • This means that they shorten, become denser and so become visible
  • Condensation separates the jumble of chromatin in the nucleus and allows the chromosomes to segregate properly later, in meiosis
• Each chromosome is visible as a pair of chromatids
  • Sister chromatids are so-called because they originate from the same parent
    • This is not a reference to gender/sex chromosomes
  • Two homologous chromosomes (exact copies of each other) align alongside to each other
    • This is called a bivalent because it is composed of two chromosomes
    • It is also called a tetrad because it is composed of four chromatids
    • This process is called synapsis
  • An example of a bivalent would be for human chromosome number 11 (see image below)
  • The original chromosome pair 11 has one chromosome no. 11 inherited from the paternal line ie. from the organism's father and one no. 11 chromosome from the maternal line
    • Each chromosome 11 copies itself in interphase
      • So there are 2 identical copies of paternal chromosome 11
      • And 2 identical copies of maternal chromosome 11
    • Such a bivalent is also a tetrad because each of the two copies of chromosome 11 is made up of 2 chromatids, making 4 chromatids in total