Meiosis
* All human cells possess 46 chromosomes (2n) with 2 copies of each chromosome, except for
gametes that contain 23 chromosomes (n).
* Gametes are formed by a process of cell division called Meiosis.
* Sperm is the gamete of male: It contains 22 chromosomes (from autosomes) + X
Or
22 chromosomes (from autosomes) + Y.
* Ovum is the female gamete: It contains 22 chromosomes (from autosomes) + X.
* Somatic cells (body cells) are diploid (2n), and divide by mitosis.
* Sex cells result by meiosis, and are haploid cells (1n).
* Meiosis starts with interphase (duplication of chromatin) and it consists of 2 successive divisions:
Meiosis I (reductional division) & Meiosis II (equational division).
I- The Reductional Division (Meiosis I):
1- Prophase 1:
- Nuclear membrane disappears.
- Centrosome changes into asters that migrate to opposite poles.
- Spindle fibers appear between the 2 asters.
- Chromatin condenses into chromosomes.
- Homologous chromosomes pair up to form tetrads.
2- Metaphase 1:
- Homologous chromosomes pair & line up at the equator, forming the “equatorial plate”.
3- Anaphase 1:
- “Polar Ascension”: Homologous chromosomes separate & each chromosome migrates toward
opposite poles.
4- Telophase 1:
- Two daughter cells are formed. Each cell contains (1n) chromosomes i.e. half the number of
chromosomes of the mother cell.
- Each chromosome is still made up of 2 sister chromatids.
II- The Equational Division (Meiosis II):
5- Prophase 2: Same events as in Meiosis I, but each cell contains (1n) chromosomes.
6- Metaphase 2: Same events as in Meiosis I, but each cell contains (1n) chromosomes.
7- Anaphase 2: Sister chromatids separate & migrate toward opposite poles.
8- Telophase 2: - 4 daughter cells are formed: Each cell contains “n” chromosomes, and each
chromosome is made of 1 chromatid.
III- Genetic Recombination:
* At the end of meiosis, gametes contain chromosomes of either a paternal origin, or a maternal
one. Thus, gametes are genetically different.
* The segregation (separation) of chromosomes during anaphase 1 is random and dependent on
chance. This causes chromosomal & genetic recombination.
* The number of different possible gametes = 2n
Where n = number of homologous chromosomes pair.
* Ex: In human beings: 2n = 46 => n = 23
=> The number of possible gametes = 223
IV- Fertilization:
* Is characteristic of sexual reproduction, by which the sperm (the male gamete) & the ovum (the
female gamete) unite. It results in the formation of the zygote which is the first cell of a new living
organism.
* It is also a random process, yielding more genetic recombination.
* The newborn will have a different unique genetic program.
* It is the male who determines the gender (sex) of the newborn. This depends on whether his
gamete contains an X or Y chromosome.
V- Example:
* If 2n = 4 chromosomes.
* 4 chromosomes = 2 pairs of homologous chromosomes.
* At the beginning, the cell is made of 2n = 4 chromosomes. Each chromosome is made of 2 sister
chromatids.
* By the end of Meiosis I, each cell contains n = 2 chromosomes. Each chromosome is made of 2
sister chromatids.
* By the end of Meiosis II, each cell contains n = 2 chromosomes. Each chromosome is made of
1 chromatid.
Prophase 1
Anaphase 1
Metaphase 1
Telophase 1
Prophase 2
Telophase 2
Metaphase 2
Anaphase 2