Meiosis I : Reductional Cell Division

Sexual reproduction in organisms takes place through the fusion of male and female gametes, the sperm and the egg respectively. Gametes are haploid in nature, i.e., they contain only half the number of chromosomes. This genetic content makes them different from other body cells. Meiosis leads to the formation of haploid cells.

Let us have a detailed look at meiosis 1 and the different stages and phases of meiosis 1.

Meiosis 1

Mitotic cell division is equational in nature while meiosis is a reduction division. The salient features of meiotic division that make it different from mitosis are as follows:-

1. It occurs in two stages of the nuclear and cellular division as Meiosis I and Meiosis II. DNA replication occurs, however, only once.
2. It involves the pairing of homologous chromosomes and recombination between them.
3. Four haploid daughter cells are produced at the end, unlike two diploid daughter cells in mitosis.

Meiosis 1 separates the pair of homologous chromosomes and reduces the diploid cell to haploid. It is divided into several stages that include, prophase, metaphase, anaphase and telophase.

Also Read: Significance of Meiosis

Meiosis 1 Stages

The different stages of meiosis 1 can be explained by the following phases :

• Prophase 1
• Metaphase 1
• Anaphase 1
• Telophase

Phases of Meiosis 1

Meiosis 1 Prophase 1

• Prophase I is longer than the mitotic prophase and is further subdivided into 5 substages,
  • leptotene
  • zygotene
  • pachytene
  • diplotene
  • diakinesis
• The chromosomes begin to condense and attain a compact structure during leptotene.
• In zygotene, the pairing of homologous chromosomes starts a process known as chromosomal synapsis, accompanied by the formation of a complex structure called synaptonemal complex. A pair of synapsed homologous chromosome forms a complex known as bivalent or tetrad.
• At pachytene stage, crossing over of non-sister chromatids of homologous chromosomes occurs at the recombination nodules. The chromosomes remain linked at the sites of crossing over.
• Diplotene marks the dissolution of the synaptonemal complex and separation of the homologous chromosomes of the bivalents except at the sites of cross-over. The X-shaped structures formed during separation are known as chiasmata.
• Diakinesis is marked by the termination of chiasmata and assembly of the meiotic spindle to separate the homologous chromosomes. The nucleolus disappears and the nuclear envelope breaks down.

Meiosis 1 Metaphase 1

The bivalents align at the equatorial plate and microtubules from the opposite poles attach to the pairs of homologous chromosomes.

Meiosis 1 Anaphase 1

The two chromosomes of each bivalent separate and move to the opposite ends of the cells. The sister chromatids are attached to each other.

Meiosis 1 Telophase 1

The nuclear membrane reappears and is followed by cytokinesis. This gives rise to a dyad of cells.

Also Read: Meiosis II

For more information on meiosis 1, stages of meiosis, phases of meiosis 1 such as meiosis 1 prophase 1, meiosis 1 metaphase 1, etc. and other related topics, keep visiting BYJU’S website or download BYJU’S app for further reference.