Cell Division – Meiosis

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:

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

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.

• 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.

• 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.

• 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.

3) Anaphase I

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

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.

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)