CELL DIVISION: Mitosis & Meiosis
Can be divided into 2 stages:
- G1 Growth phase 1.
- S Synthetic phase.
- G2 Growth phase 2.
- During Synthetic phase, DNA duplicates. The genetic material doubles mean from 2N it becomes 4N.
- M Mitosis/Meiosis.
- C Cytokinesis.
Mnemonic:“Go Sally Go! Make Children!”
It is an equational division. Occurs in somatic cells.
Prophase, Metaphase, Anaphase, Telophase
Mnemonic: “People Meet And Talk”
- Coiling of chromatin occurs, forming thin long threads.
- By the end, chromosomes start forming,
- Nucleolus & nuclear membrane starts disappearing by the end.
- Spindle fiber formation starts.
- Centriole in animal cells starts moving towards the poles.
- Nuclear membrane and nucleolus has disappeared,
- Spindle fibers have formed,
- 2 types of spindle fibers occur chromosomal fibers that are attached to chromosomes at the centromere & continuous fibers that join the 2 poles.
- Chromosomes having two chromatids are seen,
- Chromosomes align themselves on the equatorial plate due to contraction of spindle fibers.
- Amphiastral mitosis occurs in animal cells & anastral mitosis occurs in plant cells.
- Shortest phase.
- Centromere splits
- Chromosomes start moving towards poles due to contraction of spindle fibers.
- Various shapes of chromosomes are seen.
- Chromosomes have reached the poles,
- Uncoiling of chromosomes occur,
- Nucleolus and nuclear membrane reappear,
- Spindle fibers disappear.
- 2 nuclei are formed by the end.
- Starts in mid-Anaphase and ends by the end of Telophase dividing the cell into 2 daughter cells.
- Occurs by invagination of the cell membrane in animal cells & by cell plate method in plant cells.
It occurs in 2 stages:
Meiosis I – reductional division:
Prophase I, Metaphase I, Anaphase I, Telophase I.
Prophase I: divided into 5 substages.
Mnemonic: Little Zara in Pink Dress is Dancing.
- Chromatin coils forming thin long threads.
- Further coiling of chromatin occurs.
- Pairing of homologous chromosomes occurs due to the mutual attraction between them.
- Synapsis is pairing of homologous chromosomes.
- Bivalents are seen.
- Synaptinemal complex occurs between homologous chromosomes, that helps in precise pairing.
- Each chromosome splits longitudinally to form two chromatids attached at the centromere.
- Bivalent changes into tetrad.
- Crossing over, i.e., exchange of segments between non-sister chromatids occurs.
- Crossing over occurs by the help of recombinase enzyme.
- Homologous chromosomes try to separate.
- Chromosomes remain attached at regions where crossing over has occurred.
- Chaisma is the regions where crossing over has occurred.
- Chromosomes pull themselves apart from the centromere, as a result chaisma starts moving towards ends.
- This is Terminalization, which starts in diplotene stage.
- Terminalization completes forming ring-shaped chromosomes.
- Nucleolus & nuclear membrane starts disappearing, spindle fiber formation starts.
- Nucleolus & nuclear membrane has disappeared, spindle fiber formation is completed.
- Chromosomes align on equatorial plate.
- Homologous chromosomes separate due to contraction of spindle fibers,
- Terminal chaisma opens up & the chromosomes start moving towards poles.
- Chromosomes reach poles and uncoiling starts.
- Nucleolus & nuclear membrane reappear, spindle fibers disappear.
- Two nuclei one at each pole are formed.
Meiosis II – equational division:
Prophase II, Metaphase II, Anaphase II, Telophase II.
Meiosis II is same as Mitosis.
- 4 daughters cells each having haploid number of chromosomes are produced.
Also watch:Cell Cycle, Chromosomes.