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Introduction

  • Process by which chromosome number of a diploid cell (2n) is reduced to half i.e. it changes to haploid (n).
  • Results in daughter cells differing from parent cell, genetically.
  • Resulting haploid cell either directly becomes gamete or differentiate/divide to produce cell that later becomes a gamete.
  • Plays a key role in eukaryotic reproduction.
  • Homologous Chromosomes or Homologues: Chromosomes in a diploid cell comes in distinct pairs, each carrying different set of genes. Members of a pair are called homologous chromosomes or homologues. Therefore, homologues carry same set of genes but different alleles. During meiosis, homologues associate intimately.
  • Chromosomes from different pairs are called heterologues.

Meiosis occurs by two cell divisions:

Steps of Meiosis

Remember: Nucleus divides twice during meiosis but DNA is replicated only once.

Final Products of Meiosis I & Meiosis II = 4 Haploid daughter cells, each having ‘n’ number of chromosomes.

Unique Features of Meiosis I :

  • Homologues come together pairing along their entire lengths. Such pairing absent in Mitosis.
  • Homologues separate but individual chromosome consisting of two sister chromatids remain intact. Chromatids will separates during Meiosis II.
  • Preceded by an interphase, like Mitosis, during which each chromosome is replicated. Result: each chromosome consists of two sister chromatids which are held together by cohesin proteins.
  • At end of Meiosis I, two nuclei are formed – each with half of the original number of chromosomes i.e. one member of each homologous pair.
  • Centromeres do not separate in Meiosis I. Implying that each chromosome still consists of two sister chromatids which separate during Meiosis II, as mentioned above.

Unique Features of Meiosis II :

  • Involves separation of two sister chromatids into daughter nuclei, like Mitosis.
  • Unlike Mitosis, due to crossing over in Meiosis I, sister chromatids are not necessarily identical to each other.
  • Chance assortment occurs during Meiosis II, leading to further genetic diversity.

MEIOSIS I

Meiosis I

1) Prophase I

  • Leptotene (= Leptonema) (= Thin threads)
    • Duplicated chromosomes condense out of diffused chromatin network.
    • Under electron microscope (but not light microscope), each chromosome appears to consist of two sister chromatids.
    • Chromosomes appear as long, slender threads with bead-like structures (chromomeres) along their lengths.
    • Nucleus enlarges in size.
    • Nucleolus becomes prominent and increases in size.
Leptotene – Prophase I
Overview – Prophase I
  • Zygotene (= Zygonema) (= Paired threads)
    • Homologues intimately come together and the process of pairing of homologous chromosomes is called synapsis, which roughly lasts from Prophase I till end of Metaphase I.
    • Synapsis is usually accompanied by formation of synaptonemal complex (a protein structure) between pairing chromosomes.
    • Synaptonemal complex consists of –
      • Three parallel rods of two lateral elements associated with each chromosome and a central element located midway.
      • Large number of ladderlike rungs connecting lateral elements with central element.
    • Proterminal Synapsis – When synapsis starts from ends of homologues & proceeds towards centromere.
    • Procentric Synapsis – When synapsis starts near centromere & advances towards ends of homologues.
    • Random Synapsis – When synapsis starts from any point on homologues.
    • Synapsis is highly specific, occurring between all homologous chromosome parts, even if present on non-homologous chromosomes.
    • Usually, in triploid chromosomes, pairing occurs between only two chromosomes in any one region i.e. synapsis is confined to two homologuous chromosome areas at a time, with exceptions for pairing of all three together.
    • Nucleolus remains prominent during zygotene.
Synaptonemal Complex
  • Pachytene (= Pachynema) (= Thick threads)
    • Synapsis progress & duplicated chromosomes continue to condense.
    • Resulting thickened chromosomes from condensation are characteristic of pachytene i.e. thick threads.
    • Paired chromosomes can be seen under light microscope.
    • Each pair consists of two duplicated homologous chromosomes, further each chromosome of a pair consists of two sister chromatids.
    • Difference between Bivalent & Tetrad terms: When we count homologues, the pair is referred to as bivalent of chromosomes. When we count strands, it is referred to as tetrad of chromatids.
    • Crossing over occurs during primarily during pachytene.
    • Crossing over – Process during which chromosomes exchange material through breaking & rejoining of their DNA molecules, increasing genetic variation.
    • Individual sister chromatids may be broken & the broken pieces may be swapped between chromatids within tetrad. Thus, crossing over results in recombinant chromatids or recombination of genetic material between paired chromosomes.
Pair of Homologous chromosomes & Bivalents
Recombinant Chromatids
  • Diplotene (= Diplonema) (= Two threads)
    • Gradually paired chromosomes separate.
    • However, remaining in close contact at sites of crossing over. These contact points are called chiasmata (singular: chiasma; = cross).
    • May last a very long time e.g. in human females, it may last for more than 40 years.
    • Nucleolus & nuclear membrane almost disappear.
  • Diakinesis (= Movement through)
    • Chromosomes continue to condense.
    • Nuclear membrane fragments.
    • Spindle apparatus forms.
    • Spindle microtubules gets attached to kinetochores of chromosomes.
    • Chromosomes then move to central plane of cell perpendicular to axis of spindle apparatus.
    • Nucleolus either disappears or detaches from associated chromosome.

2) Metaphase I

  • Paired chromosomes orient toward opposite poles of the spindle, ensuring one member of each pair go to each pole in next phase of cell division.
  • Chiasmata moves away from centromere towards end of chromosomes, a phenomenon called terminalisation – reflecting repulsion between the two homologues of a pair.
Metaphase I (Meiosis I)

3) Anaphase I

  • Paired chromosomes separate from each other, phenomenon called chromosome disjunction.
  • Separation is mediated by spindle apparatus acting on each bivalent.
Anaphase I (Meiosis I)

4) Telophase I

  • Separating chromosomes gather at opposite poles – marking end of Meiosis I.
  • Spindle apparatus gets disassembled.
  • Chromosomes are decondensed.
  • Daughter cells formed. Also, nucleus is formed in each daughter cell.
  • In some species, chromosome decondensation is incomplete & daughter nuclei do not form. Instead daughter cells immediately proceed to Meiosis II.

Note: If homologous chromosomes fail to separate during meiosis I i.e. Non disjunction occurs resulting in Aneuploidy.

MEIOSIS II

1) Prophase II

  • Chromosomes condense.
  • Chromosomes gets attached to new spindle apparatus.
  • Nuclear envelope fragments (into vesicles).
  • Centrosomes move towards opposite poles. They duplicated during interkinesis.

2) Metaphase II

  • Sister chromatids move to equatorial plane in the cell.
  • Sister chromatids are now maximally condensed.
  • Spindles attach to kinetochores of each sister chromatid.

3) Anaphase II

  • Sister chromatids are pulled apart by kinetochore microtubules. They move towards opposite poles. Phenomenon known as chromatid dysjunction.
  • Cell is elongated by non-kinetochore microtubules.
Anaphase II (Meiosis II)

4) Telophase II

  • Note: Separated chromatids are now called chromosomes.
  • Chromosomes gather at poles & daughter nuclei are formed around.
  • Each daughter nuclei consists of haploid set of chromosomes.

(To be continued – Diagrams pending)

Harjeet Kaur

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