Cell biology/Meiosis

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Asexual vs. Sexual ReproductionEdit

In the first stage of sexual reproduction, "meiosis", the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilization", haploid gametes come together to form a diploid zygote and the original number of chromosomes is restored.
Copied from WP: w:Sexual reproduction

Aseuxal Reproduction: New individuals are created when a parent makes an exact copy of itself. Examples of this is in Mitosis, Binary Fission, Budding, etc.

Sexual Reproduction: An individual is created when two haploid gametes (sex cells) join together during fertilization. Example: Meiosis.

Reproductive CellsEdit

Explanation of homologous chromosomes.

Somatic Cells: Body cells → Chromosome # in humans (2n) = 46

  • Somatic cells are diploid in number meaning they have a full set of chromosomes. Abbreviated as 2N.

Gametes/Germ Cells: Sex cells → Chromosome # in humans (n) = 23

  • Gametes are haploid in number meaning they only have half a set of chromosomes.
  • Sperm (n) → Chromosome # = 23
  • Ova = egg (n) → Chromosome # = 23
  • Abbreviated as N

Homologous chromosomes: This is a pair of chromosomes in which each parent donates 1 chromosome to the pair.


Meiosis is when a full set of chromosomes in a cell is cut in half through the separation of homologous chromosomes. This occurs in two parts: Meisois I and Meisois II.

  • You could call Meiosis the "reduction division".

Spermatogenisis: The creation of four sperm cells through the process of Meiosis.
Oogenesis: Creation of an ova (egg) and three polar bodies through Meiosis.

Tetrads and Crossing Over
  • When each pair of chromosome pairs with its homologous chromosome they form a structure called a tetrad.
  • Crossing Over - When tetrads are formed, homologous chromosomes may exchange portions of their chromatids (DNA).

Stages of MeiosisEdit

Meisois IEdit

Stage name Description
Prophase I
  1. Chromosomes become visible.
  2. Synapsis occurs: Coupling of homologous chromosomes
  3. Tetrads form.
  4. Crossing Over occurs
Metaphase I
  1. Homologous chromosomes attach to spindle fibers and line up in the middle of the cell.
Anaphase I
  1. The fibers pull the homologous chromosomes toward opposite ends.
Telophase I & Cytokenisis
  1. Number of cells produced: 2
  2. Chromosome # in cells at this point: 46

Meisois IIEdit

Stage name Description
Prophase II
  1. Stage begins with 2 daughter cells, each with 46 chromosomes. Similar to prophase in Mitosis.
Metaphase II
  1. The chromosomes line up in the middle similar to the metaphase stage of mitosis.
Anaphase II
  1. The sister chromatids separate and move towards opposite ends of the cell.
Telophase II & Cytokinesis
  1. Number of cells produced: 4
  2. Chromosome # in cells at this point: 23 (haploid cells)
Overall Diagram


When male and female sex cells, or gametes, (n=23) combine to create a diploid zygote (2n=46).


Description Mitosis Meiosis
Parent cell goes through how many divisions? 1 2
# of Daughter cells produced? 2 4
Type of Cells produced? Body cells Sex cells
Diploid (2N) or Haploid (N) cells? Diploid (2N) Haploid (N)
# of chromosomes in Humans 46 23
Daughter cells indentical or different from parent cell? Identical Different
Synonyms Synonyms for body cells: Somatic Cells Synonyms for sex cell: Gamete, Sperm, Egg
Why is mitosis necessary?
  • To replace dead, worn out or damaged cells.
  • So that an organism can grow and develop.
Why is meiosis necessary?
  • To create gametes (sex cells) necessary for fertilization.
  • Increase genetic diversity in a population.