a) G1 Phase:
- period between the beginning of interphase and the beginning of the DNA synthesis.
- It is the longest phase and constitutes up to 50% of the total inter phase duration.
- Marked events includes Marked protein and RNA synthesis, Increase the cell volume by imbibing water and nutrients
b) S Phase:
- period between G1 and G2 phases
- DNA synthesis takes place (chromosomes replicate)
- RNA and protein synthesis are very low.
c) G2 Phase:
- period between S and M phases
- There is considerable amount of RNA and protein synthesis takes place in this phase.
- M phase:
- Structural and functional protein is synthesized.
- RNA and ribosomes are synthesized.
- DNA of each chromosome replicates.
- Cell volume increases.
- Division of mitochondria and chloroplast.
- Doubling of centrioles
- High cell metabolic rate
- Uncoiling of chromatids
- Disintegration of nucleolus
- Disappearance of nuclear membrane
- Formation of spindle
- Arrangement of chromosome on equilateral region.
- Splitting of centromere or division of chromosome.
- Movement of chromosomes to opposite poles.
e) Telophase :
- Formation of daughter nuclei
- Reappearance of nuclear membrane and nucleolus.
- Meiosis I:
A) Prophase I: Homologous chromosomes pairing (Synapsis) and crossing over (Recombination) occurs.
a) Leptotene: These cells and nuclei are usually bigger than the other cells, like a “bouquet”
b) Zygotene: Pairing of homologous chromosomes begins. It is also called synapsis and the resulting structure synaptic complex
c) Pachytene: The pairing stabilizes. The number of synaptic complexes corresponds to the number of chromosomes in a haploid set of the respective species. The pairs are also called bivalents.
d) Diplotene: The bivalents separate and formed cross-like structures, single or multiple loops: the chiasmata (sing. chiasma).
e) Diakinesis: The chromosomes condense and become more compact.
B) Metaphase I: Bivalents are randomly oriented along the metaphase plate.
C) Anaphase I:
- Kinetochore microtubules shorten and pull chromosome towards opposite poles forming two haploid chromosomal sets.
- Non-kinetochore microtubules lengthen pushing the centrioles apart and the cell elongates.
D) Telophase I:
- Microtubules disappears, new nuclear membrane surrounds each haploid set
- Chromosomes uncoil and cytokinesis occurs.
E) Interkinesis: Rest period of cell and hence no DNA replication occurs during this stage.
4. Meiosis II
- Results in formation of 4 haploid cells.
a) Prophase II:
- Disappearance of nucleoli and the nuclear envelope.
- Shortening and thickening of chromatids.
- Centrioles moves to the polar regions and arrange spindle fibers for the second meiotic division.
b) Metaphase II:
- Metaphase equatorial plate is rotated by 90 degrees when compared to meiosis I.
c) Anaphase II:
- Microtubules pull the sister chromatids towards opposite poles.
d) Telophase II:
- Uncoiling and lengthening of the chromosomes and the disappearance of spindle fiber.
- Nuclear envelops reform and cleavage and produces four daughter cells.