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 changes the chromosome number from diploid (2n) to haploid (n). It is important for sexual reproduction because it produces cells that contain half the usual number of chromosomes.




Meiosis occurs in cells involved in reproduction and helps in the formation of gametes such as:

  • Sperm cells in males
  • Egg cells in females

Unlike normal body-cell division, meiosis involves two cell divisions and produces cells that are genetically different from one another. This variation helps explain why offspring are not exactly the same as their parents or siblings.

Why is Meiosis Important?

Meiosis is important because it helps organisms reproduce sexually and keeps the chromosome number of a species stable from one generation to the next.

  • Reduces chromosome number: It produces haploid cells containing one set of chromosomes.
  • Helps maintain chromosome number: During fertilization, two haploid gametes join and restore the diploid chromosome number.
  • Produces genetic variation: Crossing over and the independent assortment of chromosomes create different combinations of genetic material.
  • Supports sexual reproduction: Meiosis produces the haploid cells needed for sexual reproduction.


How is Meiosis Organized?

Meiosis has two successive divisions. DNA is copied only once before Meiosis I. It is not copied again between Meiosis I and Meiosis II.

Division What Happens?
Meiosis I Homologous chromosomes separate and the chromosome number is reduced.
Meiosis II Sister chromatids separate and four haploid cells are produced.

Important idea:

DNA is copied once, but the cell divides twice.

MEIOSIS I (Reduction Division)

Meiosis I is called the reduction division because homologous chromosomes separate and the chromosome number changes from diploid to haploid.

Before Meiosis I starts, DNA has already been copied during the S phase of interphase. Because of this, each chromosome has two sister chromatids when Meiosis I begins.

  • Homologous chromosomes pair with each other and later separate.
  • Crossing over can occur between homologous chromosomes.
  • The chromosome number is reduced by half.
  • Sister chromatids stay together during Meiosis I.

Phases of Meiosis I

1. Prophase I

Prophase I is the longest and one of the most important stages of meiosis. During this stage, homologous chromosomes pair with each other and exchange of genetic material may occur.

Prophase I has five sub-phases:

  • Leptotene
  • Zygotene
  • Pachytene
  • Diplotene
  • Diakinesis

A. Leptotene

Main Events

  • Chromosomes start becoming condensed and can be seen as thin threads.
  • Each chromosome already has two sister chromatids because DNA has been copied.
  • The chromosomes become easier to see as they continue to condense.

Key Point:

“Lepto” means thin. This can help you remember that chromosomes appear as thin threads during this stage.


B. Zygotene

Main Events

  • Homologous chromosomes pair with each other.
  • This pairing is called synapsis.
  • A structure called the synaptonemal complex develops between the paired chromosomes.
  • The paired chromosomes form structures called bivalents.

Why is this stage important?

Pairing brings homologous chromosomes close together. This prepares them for crossing over in the next stage.

Key Point

Synapsis = pairing of homologous chromosomes.

C. Pachytene

Main Events

  • Chromosomes become shorter and thicker as they continue to condense.
  • Crossing over occurs between non-sister chromatids of homologous chromosomes.
  • Small sections of DNA are exchanged between homologous chromosomes.

Why is this stage important?

Crossing over creates new combinations of genetic material. This is one of the processes that produces genetic variation.

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Key Point: Crossing over increases genetic variation.

D. Diplotene

Main Events

  • Homologous chromosomes start moving away from each other.
  • They remain connected at certain points called chiasmata.
  • Chiasmata are visible points where homologous chromosomes remain connected after crossing over.

Why is this stage important?

Chiasmata help keep homologous chromosomes connected until they separate during Anaphase I.

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Key Point: Chiasmata are visible points where homologous chromosomes remain connected.

E. Diakinesis

Main Events

  • Chromosomes become highly condensed.
  • Chiasmata become easier to see toward the ends of the chromosomes.
  • The nuclear membrane breaks down.
  • Spindle fibers form as the cell prepares for Metaphase I.

Why is this stage important?

Diakinesis is the last stage of Prophase I. It prepares the chromosomes for their arrangement at the equator of the cell.

2. Metaphase I

  • Homologous chromosome pairs, called bivalents, arrange themselves at the equator of the cell.
  • Spindle fibers attach to the chromosomes.
  • The random orientation of chromosome pairs contributes to genetic variation.

Key Point: Homologous chromosome pairs line up together at the equator.

3. Anaphase I

  • Homologous chromosomes separate and move toward opposite poles.
  • Sister chromatids remain attached to each other.
  • This separation reduces the chromosome number in each new cell.
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Key Point: Homologous chromosomes separate, but sister chromatids stay together.

4. Telophase I

  • Chromosomes reach opposite poles.
  • A nuclear membrane may form around each chromosome set, depending on the organism.
  • Cytokinesis may divide the cytoplasm.

Result: Two haploid cells are produced.

Each chromosome still has two sister chromatids at this stage. This is an important difference between the end of Meiosis I and the end of Meiosis II.

Interkinesis

Interkinesis is a short interval between Meiosis I and Meiosis II in some cells. It is not another normal interphase because DNA is not copied again.

  • No second round of DNA replication occurs.
  • The cells prepare for Meiosis II.

MEIOSIS II (Equational Division)

Meiosis II is called the equational division because sister chromatids separate without another round of DNA replication. The basic steps are similar to mitosis.

The important point is that the cells entering Meiosis II are already haploid.

  • Sister chromatids separate.
  • Centromeres separate during Anaphase II.
  • The final result is four haploid cells.

Phases of Meiosis II

1. Prophase II

  • Chromosomes condense again if they had become less condensed after Meiosis I.
  • Nuclear membranes break down where they are present.
  • Spindle fibers form.

2. Metaphase II

  • Chromosomes arrange individually at the equator of each cell.
  • Spindle fibers attach to the chromosomes.

3. Anaphase II

  • Centromeres divide.
  • Sister chromatids separate.
  • The separated chromatids move toward opposite poles.

Key Point: Sister chromatids separate during Anaphase II.

4. Telophase II

  • Chromosomes reach opposite poles.
  • Nuclear membranes form again.
  • Cytokinesis completes cell division.

Result: Four haploid daughter cells are formed.

Final Result of Meiosis

Feature Result
Number of divisions 2
DNA replication 1 round before Meiosis I
Number of daughter cells 4
Chromosome number Reduced to half
Genetic variation Produced through crossing over and independent assortment
Type of cells produced Haploid cells involved in sexual reproduction

Difference Between Meiosis I and Meiosis II

Feature Meiosis I Meiosis II
Type Reduction division Equational division
What separates? Homologous chromosomes Sister chromatids
DNA replication Occurs before Meiosis I Does not occur before Meiosis II
Chromosome number Reduced from diploid to haploid Remains haploid
Result Two haploid cells Four haploid cells after completion

Meiosis vs Mitosis: What is the Difference?

Meiosis and mitosis are both types of cell division, but they have different purposes and produce different results.

Feature Meiosis Mitosis
Main role Formation of cells involved in sexual reproduction Growth, repair and replacement of cells
Number of divisions 2 1
Number of daughter cells 4 2
Chromosome number Reduced by half Usually maintained
Genetic similarity Daughter cells are generally genetically different Daughter cells are generally genetically similar to the parent cell
Crossing over Occurs during Prophase I Does not normally occur

Simple Flow Chart of Meiosis

DNA Replication ↓ Meiosis I ↓ Prophase I (Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis) ↓ Metaphase I ↓ Anaphase I ↓ Telophase I ↓ 2 Haploid Cells ↓ Meiosis II ↓ Prophase II ↓ Metaphase II ↓ Anaphase II ↓ Telophase II ↓ 4 Haploid Cells

Important Terms

Term Meaning
Synapsis Pairing of homologous chromosomes during Prophase I.
Crossing Over Exchange of corresponding DNA sections between non-sister chromatids of homologous chromosomes.
Chiasma A visible point where homologous chromosomes remain connected after crossing over.
Bivalent A pair of homologous chromosomes joined together during Prophase I.
Haploid A cell containing one set of chromosomes.
Diploid A cell containing two sets of chromosomes.
Homologous Chromosomes A pair of chromosomes that have the same types of genes at corresponding locations, with one chromosome inherited from each parent.
Sister Chromatids Two copies of the same replicated chromosome that remain connected until they separate during cell division.

Common Student Mistakes

  • Confusing homologous chromosomes with sister chromatids: Homologous chromosomes are a matching pair, while sister chromatids are copies of the same replicated chromosome.
  • Thinking DNA replicates twice: DNA is copied once before Meiosis I. It is not copied again between Meiosis I and Meiosis II.
  • Thinking sister chromatids separate in Meiosis I: Homologous chromosomes separate during Anaphase I. Sister chromatids separate during Anaphase II.
  • Forgetting why chromosome number is reduced: The chromosome number is reduced when homologous chromosomes separate during Meiosis I.
  • Confusing crossing over with chromosome separation: Crossing over involves exchanging DNA between homologous chromosomes, while chromosome separation happens later during Anaphase I.

Quick Revision: Meiosis in One Minute

Before Meiosis: DNA is copied once.

Meiosis I: Homologous chromosomes pair, crossing over can occur, and homologous chromosomes separate.

After Meiosis I: Two haploid cells are formed. Each chromosome still has two sister chromatids.

Meiosis II: Sister chromatids separate.

Final Result: Four haploid cells are produced.

Key Points to Remember

  • Meiosis has two successive cell divisions.
  • DNA replication occurs only once before Meiosis I.
  • Meiosis I is the reduction division.
  • Meiosis II is the equational division.
  • Crossing over occurs during Prophase I.
  • Homologous chromosomes separate during Anaphase I.
  • Sister chromatids separate during Anaphase II.
  • The final result is four haploid cells.
  • Meiosis helps produce genetic variation.

One-Line Summary

Meiosis is a special type of cell division in which DNA is copied once and the cell divides twice, producing four haploid cells and helping create genetic variation during sexual reproduction.

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