Meiosis

What is meiosis?

• Meiosis is a special type of division which occurs in both plants and animals during different stages of sexual reproduction.
• Meiosis always occurs in specific diploid (2n) cells and results in the reduction of the number of chromosomes to half. So it is also called reductional division.
• The cell undergoing meiosis is called a meiocyte, and the haploid cells formed after completion are called meiotic products.
• Examples:
  • Spore mother cell (2n) produces spores (n).
  • Microspore mother cell (2n) produces microspores (n).
  • Megaspore mother cell (2n) produces megaspores (n).
• Meiosis occurs after the completion of DNA replication during the S-phase of interphase, and involves 2 sequential cycles of nuclear and cell division (meiosis I and meiosis II), but only a single cycle of DNA replication.

Types of meiosis

• Zygotic meiosis – the meiocyte is the zygote. E.g. many algae (like Chlamydomonas) and fungi.
• Sporic meiosis – occurs in spore mother cells.
• Gametic meiosis – occurs during gametogenesis.

Meiosis I

Meiosis I is further divided into:

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

Prophase I

• The longest and most complex phase of meiosis.
• For ease of study, it is divided into 5 sub-stages based on chromosomal behaviour:
  • Leptotene
  • Zygotene
  • Pachytene
  • Diplotene
  • Diakinesis

Leptotene

• Condensation of chromatin starts.
• All the condensing chromatin aligns together on the nuclear lamina, resulting in the formation of a bouquet-like structure. So it is also called the bouquet stage.

Zygotene

• Pairing of homologous chromosomes starts, and the process is called synapsis.
• Formation of the synaptonemal complex to stabilise synapsis.
• The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or tetrad.

Pachytene

• The 4 chromatids of each bivalent become distinct and clearly appear as tetrads.
• Appearance of recombination nodules at the sites of crossing over.
• Recombination nodules consist of enzymes like recombinase, endonuclease and ligase.
• Crossing over – exchange of genetic material between non-sister chromatids of a homologous pair of chromosomes.
• Crossing over is completed in pachytene, but the chromosomes remain attached at the sites of crossing over.

Diplotene

• Dissolution of the synaptonemal complex.
• The sites of crossing over become visible in the form of X-shaped structures called chiasmata.
• In the oocytes of some vertebrates, the diplotene stage lasts for several months or years.

Diakinesis

• Marked by the terminalisation of chiasmata.
• Chromosomes are completely condensed.
• Meiotic spindles are assembled.
• The nucleolus disappears, and the nuclear envelope also breaks down.

Metaphase I

• Spindle fibres from opposite poles of the cell attach to the kinetochores of homologous chromosomes.
• The bivalent chromosomes align at the equator of the cell, resulting in the formation of a double metaphasic plate.

Anaphase I

• Homologous chromosomes (of each bivalent) separate, while sister chromatids remain associated at the centromere.
• Splitting of the centromere does not occur during anaphase I.
• Anaphase I results in the reduction of the number of chromosomes to half.

Telophase I

• The nuclear envelope and nucleolus reappear.
• Chromosomes do not decondense completely.
• Cytokinesis follows and results in the formation of a dyad of cells.

Interkinesis/ Intra-Meiotic Interphase

• A short-lived stage between meiosis I and meiosis II, in which DNA replication is absent.

Meiosis II

• The events of meiosis II are similar to mitosis.
• Mitosis can occur in both haploid and diploid cells, but meiosis II always occurs in cells formed by meiosis I.

Prophase II

• Chromatin condenses to form chromosomes.
• The nuclear envelope disappears.

Metaphase II

• Chromosomes align at the equator of the cell, resulting in the formation of a single metaphasic plate.
• Microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.

Anaphase II

• Simultaneous splitting of the centromere of each chromosome.
• Sister chromatids move towards opposite poles by the shortening of microtubules.

Telophase II

• The nuclear envelope reforms.
• Chromatin decondenses.
• Cytokinesis follows, and as a result, 4 haploid cells are formed.