• 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

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.


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

Website Renovation Under Process

Our Top Ranked Posts on Google and Yahoo Search Results

Click over the link below for our top ranked posts on Google and Yahoo Search Results:

Top Ranked Posts

Special Thanks to Our Visitors & Supporters

Google Language Translator

Since we have global visitors, we have enabled Google Language Translator for better learning on our website.

We aim to provide free and quality learning. Please share our website as much as you can.

Social Media

Follow us on Social Media & Stay Updated with our Latest Content on Website

You can also freely DM your suggestions for the website there, including topics you demand. We will definitely try our best to bring them up.

Author wise Posts

Click on the name of the author to study their notes:

Harjeet Kaur

Smriti Kanchan

Priyanka Sethia

Nancy Gupta

Anjali Thakur

Amrinder Singh Panesar

Translate »
error: Content is protected !!