Meiosis

THE CELLS IN OUR BODIES

Two types: (1) somatic cells, and (2) reproductive cells

(1) Somatic cells

- Reproduce by mitosis (cells are genetically identical!!).

- ex. skin cells, muscle cells, nerve cells, intestinal cells, etc.

- These cells are diploid....

- Diploid cells have a complete set of chromosomes (50% paternal, 50% maternal).

- ex. human somatic cells have 46 chromosomes.

(2) Reproductive cells

- Also known as sperm and eggs.

- Sperm/eggs are also called sex cells or gametes.

- Produced through a process called Meiosis.

- Meiosis occurs in the gonads (reproductive organs).

- These cells are haploid....

- Haploid cells have half of the number of chromosomes as the original reproductive cell.

- ex. human reproductive cells have 23 chromosomes.

MEIOSIS

- Resembles mitosis EXCEPT meiosis involves TWO cell divisions instead of just one!

- A diploid cell undergoes two cell divisions to produce four haploid cells.

o Meiosis I - the first division.

- Homologous chromosomes (copied during interphase) are joined together in tetrads.

- Homologous chromosomes are of similar shape/size and carry genes that code for the same trait in the same position on the chromosome.

- One of the homologues comes from the mother, while the other homologue comes from the father.

- Exceptions: the X and Y chromosomes!

- Tetrads are separated and the sister chromatids move to opposite poles of the dividing cell.

-A diploid cell becomes two haploid cells, hence this stage is called reduction division.

o Meiosis II - the second division

- Sister chromatids are now divided and move to opposite poles.

- Four haploid cells have been produced.

- Genetically identical gametes are rare!

- Meiosis results in cells with HALF of the number of chromosomes as compared to the rest of the body. Why is this important for fertilization???