Meiosis | Complete Phases of Meiosis I & Meiosis II (CELL CYCLE PART 3)

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What is Meiosis?

Meiosis is a special type of cell division that reduces the chromosome number from diploid (2n) to haploid (n).

It occurs in reproductive cells to produce:

  • Sperm cells in males
  • Egg cells in females

Importance of Meiosis

  • Maintains chromosome number in species
  • Produces genetic variation
  • Essential for sexual reproduction


Meiosis consists of two successive divisions:

Division Purpose
Meiosis I Reduction division
Meiosis II Equational division

MEIOSIS I (Reduction Division)

In Meiosis I:

  • Homologous chromosomes separate
  • Chromosome number becomes half

Phases of Meiosis I

1. Prophase I

This is the longest and most important phase.

It is divided into five sub-phases:

A. Leptotene

Main Events

  • Chromosomes become visible as thin threads
  • Each chromosome already has two chromatids
  • Chromosomes start condensing

Key Point:

“Lepto” means thin.


B. Zygotene

Main Events

  • Homologous chromosomes pair together
  • Pairing is called synapsis
  • Synaptonemal complex forms

Key Point

Bivalents are formed.

C. Pachytene

Main Events

  • Chromosomes become shorter and thicker
  • Crossing over occurs
  • Exchange of genetic material happens

Key Point

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Genetic variation is produced here.

D. Diplotene

Main Events

  • Homologous chromosomes begin separating
  • Chiasmata become visible

Key Point

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Chiasma = point of crossing over.

E. Diakinesis

Main Events

  • Chromosomes become highly condensed
  • Nuclear membrane disappears
  • Spindle fibers form

Key Point

Cell prepares for metaphase.

2. Metaphase I

  • Bivalents arrange at equator
  • Spindle fibers attach to centromeres

Homologous pairs line up together.

3. Anaphase I

  • Homologous chromosomes separate
  • Chromatids remain attached
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Chromosome number reduces to half.

4. Telophase I

  • Chromosomes reach poles
  • Nuclear membrane may reform
  • Cytokinesis occurs

Two haploid daughter cells are formed.

Interkinesis

A short resting stage between Meiosis I and Meiosis II.

  • No DNA replication occurs

MEIOSIS II (Equational Division)

Meiosis II is similar to mitosis.

In this division:

  • Sister chromatids separate

Phases of Meiosis II

1. Prophase II

  • Chromosomes condense again
  • Nuclear membrane disappears
  • Spindle fibers form

2. Metaphase II

  • Chromosomes arrange at equator
  • Spindle fibers attach

3. Anaphase II

  • Centromeres divide
  • Sister chromatids separate

Chromatids move to opposite poles.

4. Telophase II

  • Nuclear membranes reform
  • Cytokinesis completes

Four haploid daughter cells are formed.

Final Result of Meiosis

Feature Result
Number of divisions 2
Number of daughter cells 4
Chromosome number Half
Genetic variation Present
Type of cells produced Haploid gametes

Difference Between Meiosis I and Meiosis II

Feature Meiosis I Meiosis II
Type Reduction division Equational division
Chromosomes separate Homologous chromosomes Sister chromatids
DNA replication Before division only No replication
Result Two haploid cells Four haploid cells

Simple Flow Chart of Meiosis

Meiosis I ↓ Prophase I (Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis) ↓ Metaphase I ↓ Anaphase I ↓ Telophase I ↓ Interkinesis ↓ Meiosis II ↓ Prophase II ↓ Metaphase II ↓ Anaphase II ↓ Telophase II ↓ 4 Haploid Cells

Important Terms

Term Meaning
Synapsis Pairing of homologous chromosomes
Crossing Over Exchange of genetic material
Chiasma Point where crossing over occurs
Bivalent Paired homologous chromosomes
Haploid Single set of chromosomes

One-Line Summary

Meiosis is a two-step cell division process that produces four genetically different haploid cells for sexual reproduction.


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