Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)
- Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids
- A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage
The first meiotic division is a reduction division (diploid →haploid) in which homologous chromosomes are separated
Prophase – I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occur.
Metaphase – I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell.
Anaphase – I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell.
Telophase – I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells.
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
Prophase – II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
Metaphase – II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
Anaphase – II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles.
Telophase – II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells.
The final outcome of meiosis is the production of four haploid daughter cells.
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
significance of meiosis :
- Formation of gametes − Meiosis form gametes that are essential for sexual reproduction.
- Genetic information − Meiosis switches on the genetic information for the development of gametes.
- Maintenance of chromosome number − Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms.
- Assortment of chromosomes − In meiosis paternal and maternal chromosomes assort independently. It causes reshuffling of chromosomes and the traits controlled by them.
- Crossing over − It introduces new combination of traits or variations.
- Mutation − Mutations take place due to irregularities of meiotic division.