Meiosis

• 2 rounds of division

• Final gametes are haploid (23+23= 46 chromosomes)

  </p>

A male gamete is called a sperm cell.

A female gamete is called an egg cell.

Human Life Cycle

Zygote → Embryo → Fetus → Human

Stages of meiosis:

1. Interphase
2. Meiosis I
3. Meiosis II

Prophase

Chromosomes will find each other and exchange DNA.

Meiosis I

• Chromosomes replicate
• Same size, shape and gene location
• Alleles may not be the same.
• Meiosis II splits the cells into haploids

\
Prophase I

• Chromatin condenses into chromosomes.
• Centrioles move to opposite poles
• Spindle fibres form.
• The nuclear membrane breaks down.
• A tetrad is a pair of duplicated homologous chromosomes (together, they have 4 chromatids).
• Synopsis: The process of forming a tetrad.

\
Metaphase I

• Tetrads line up along the equator randomly
• This is called random assortment and it allows genetic variation.
• Spindle fibres attach to the pair of sister chromatids.

\
Anaphase I

• Chromosome pairs are separated and pulled to the opposite ends.
• The sister chromatids do NOT separate (this only happens in mitosis).

\
Telophase I

• Nuclear membrane reforms.
• Spindle fibres retract.

\
Prophase II

• The nuclear membrane begins to break down.
• Spindle fibres begin to form.
• Centrioles begin to move to poles.
• NO REPLICATION OF DNA

\
Metaphase II

• Chromosome pairs align along the equator of the cell.
• Spindle fibres attach to centromeres of chromatids.
• Chromatids are no longer identified because of crossing over.

\
Anaphase II

• Spindle fibres pull chromatids apart.

\
Telophase II

• Nuclear membrane assembles.
• Chromosomes decondense
• Spindle fibres disappear.

\
Humans have 23 pairs of chromosomes.

• 22 are autosome pairs
• 1 pair that is sex-determining

\
Female: XX

Male: XY

\
For mitosis, check this link.

https://knowt.io/note/874e8eda-7744-4668-86c6-1eecebf7689c/Mitosis

\