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)
