Cell Division – Mitosis & Meiosis

CELL DIVISION: Mitosis & Meiosis

Cell Cycle

Can be divided into 2 stages:

INTERPHASE.

  • 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.

DIVISIONAL PHASE.

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  • M Mitosis/Meiosis.
  • C Cytokinesis.

Mnemonic:Go Sally Go! Make Children!”

cell_cycle

Mitosis

It is an equational division. Occurs in somatic cells.

Prophase, Metaphase, Anaphase, Telophase

Mnemonic:People Meet And Talk”

 

Prophase:

  • 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.

Metaphase:

  • 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.

Anaphase:

  • Shortest phase.
  • Centromere splits
  • Chromosomes start moving towards poles due to contraction of spindle fibers.
  • Various shapes of chromosomes are seen.

Telophase:

  • Chromosomes have reached the poles,
  • Uncoiling of chromosomes occur,
  • Nucleolus and nuclear membrane reappear,
  • Spindle fibers disappear.
  • 2 nuclei are formed by the end.

Cytokinesis:

  • 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.

Meiosis

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.

Leptotene:
  • Chromatin coils forming thin long threads.
Zygotene:
  • 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.
Pachytene:
  • 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.
Diplotene:
  • 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.
Diakinesis:
  • Terminalization completes forming ring-shaped chromosomes.
  • Nucleolus & nuclear membrane starts disappearing, spindle fiber formation starts.

Metaphase I:

  • Nucleolus & nuclear membrane has disappeared, spindle fiber formation is completed.
  • Chromosomes align on equatorial plate.

Anaphase I:

  • Homologous chromosomes separate due to contraction of spindle fibers,
  • Terminal chaisma opens up & the chromosomes start moving towards poles.

Telophase I:

  • 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.

MEIOSIS

Also watch:Cell CycleChromosomes.

Also read:

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