Meiosis: Cell Division and Gamete Formation

• Meiosis is the formation of gametes (egg and sperm).
• It is a type of cell division that occurs in sexually reproducing organisms.
• It involves two consecutive divisions, known as meiosis I and meiosis II.
• It results in the production of four daughter cells, each with half the number of chromosomes as the parent cell.
• Meiosis is preceded by interphase, which includes chromosome replication.
• The original cell is diploid (2n).
• The four daughter cells produced are haploid (1n).
• Daughter cells contain half the number of chromosomes as the original cell.
• It occurs in our germ cells that produce gametes.
• It occurs in the testes in males (spermatogenesis).
• It occurs in the ovaries in females (oogenesis).
• Meiosis starts with 46 double stranded chromosomes (2n).
• After the first division, there are 23 double stranded chromosomes (n).
• After the second division, there are 23 single stranded chromosomes (n).
• Meiosis is fundamental to sexual reproduction.
• Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote.
• Meiosis reduces the chromosome number by half.
• Fertilization then restores the 2n number.

Meiosis I: Reduction Division

• Meiosis I includes:
  • Early prophase I (chromosome number doubled)
  • Late Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I (haploid)

Prophase I

• Early prophase involves:

  • Chromosomes condensing
  • Homologous pairing
  • Crossing over

• Late prophase involves:

  • Spindle forming
  • Nuclear envelope fragments

Stages of Prophase of Meiosis I

• Leptotene stage: replicated chromosomes condense.
• Zygotene stage: synapsis begins and a bivalent forms.
• Pachytene stage: A bivalent has formed and crossing over has occurred.
• Diplotene stage: synaptonemal complex dissociates.
• Diakinesis stage: nuclear membrane fragments, ending prophase I.
• Homologous chromosomes are pairs of chromosomes in a diploid organism that are similar in structure, size, shape, and gene content, but may contain different alleles.
• Synapsis is the process during which homologous chromosomes pair up and align closely with each other.

Synaptonemal Complex (SC)

• This is a protein structure that forms between homologous chromosomes during prophase I of meiosis.
• Plays a critical role in facilitating synapsis.
• Facilitates the pairing of homologous chromosomes.
• Essential for crossing over and genetic recombination.
• A tetrad is a structure formed during prophase I of meiosis, consisting of two homologous chromosomes duplicated into two sister chromatids.
• A chiasma is the physical point where two homologous chromosomes exchange genetic material through the process of crossing over during meiosis.
• During crossing over, homologous chromosomes in a tetrad cross over each other.
• Pieces of chromosomes or genes are exchanged.
• Genetic recombination occurs in the offspring.

Metaphase I

• The homologous pair of chromosomes align along the equator of the cell.

Anaphase I

• Homologous chromosomes separate and move to opposite poles.
• Sister chromatids remain attached at their centromeres.

Telophase I

• Nuclear envelopes reassemble.
• The spindle disappears.
• Cytokinesis divides the cell into two.

Meiosis II

• It is the second division in the process of meiosis, following Meiosis I.
• Separates sister chromatids to create four haploid daughter cells, each with a unique genetic composition.

Prophase II

• The nuclear membrane breaks down.
• Spindle fibers reappear.

Metaphase II

• Chromosomes align along the equator of the cell.

Anaphase II

• Sister chromatids separate and move to opposite poles.

Telophase II

• The nuclear envelope assembles.
• Chromosomes decondense.
• The spindle disappears.
• Cytokinesis divides the cell into two.

Results of Meiosis

• Gametes (egg and sperm) form.
• Four haploid cells are produced.
• They have one copy of each chromosome as well as one allele of each gene
• Different combinations of alleles for different genes occur along the chromosome.

Gametogenesis

• This is the process of oogenesis and spermatogenesis.

Spermatogenesis

• Occurs in the testes.
• Two divisions produce 4 spermatids.
• Spermatids mature to form spermatozoa.
• Men produce approximately 200-300 million spermatazoa every day.

Oogenesis

• Occurs in the ovaries.
• Two divisions produce 3 polar bodies that die and 1 egg.
• Polar bodies die because of unequal division of cytoplasm.
• Immature egg is called an oocyte.
• Starting at puberty, one oocyte matures into an ovum (egg) every 28 days.

Comparison of Mitosis and Meiosis

• Mitosis involves one division while meiosis involves two.
• The number of daughter cells in mitosis is 2, while in meiosis it is 4.
• Mitosis produces genetically identical daughter cells, while meiosis does not.
• Mitosis results in the same number of chromosomes as the parent, while meiosis halves the number of chromosomes.
• Mitosis occurs in somatic cells, while meiosis occurs in germ cells.
• Mitosis occurs throughout life, while meiosis occurs at sexual maturity.
• Mitosis plays a role in growth and repair, while meiosis plays a role in sexual reproduction.