Meiosis = making gametes (sex cells) Types of gametes = sperm, eggs, and spores

Similar to mitosis same phases (PMAT) Difference mitosis has one cell division resulting in two (diploid) cells meiosis has two cell divisions resulting in * four (haploid) cells Mitosis Meiosis

A normal cell is diploid, it has 2 sets of chromosomes (2n) One set comes from the mother and the other set comes from the father Gametes are haploid, they only have 1 set of chromosomes (1n)

Most chromosomes are homologous pairs = the two chromosomes of each pair carry genes for the same trait at the same location, but not necessarily the same form of that gene Eye color (Brown) Eye color (Blue)

Sex chromosomes are not necessarily homologous Sex chromosomes are not necessarily homologous. Males have an X and a Y chromosome (For females the sex chromosomes are homologous they have 2 X’s) The Y chromosome is missing some genes that are on the X chromosome All of the other nonsex chromosomes are called autosomes

Phases of Meiosis Prophase I Metaphase I Anaphase I Telophase I First cell division Prophase II Second cell division

What occurs in each of the phases is essentially the same in both mitosis and meiosis Major difference: In Prophase I of meiosis crossing over occurs which mixes the genes Creates a variety of individuals (diversity)

Prophase I homologous chromosomes come close to each other and twist around each other

crossing over during this time pieces of chromosomes may switch from one chromosome to the other.

Cytokinesis The cells divide The cells are haploid (1n) Male gametes produce four haploid cells

Egg Formation Females produce one ovum (egg) from each original cell (as opposed to the male which produces 4 sperm)

first division in meiosis one complete set of chromosomes is cast off in a small cell called a polar body. Original cell first division 1st polar body second division of meiosis another set of chromosomes is cast off in a second polar body. For the most part the polar body just contains the chromosomes. Most of the cytoplasm (which contains the organelles) remains in the larger cell. In the In most species of animals the polar bodies disintegrate. (The remaining cell becomes the ovum egg) 1n second division 2nd polar body 1st polar body 2n

When are gametes produced? In males, gametes are not produced until puberty In females meiosis begins in the embryo, millions of potential ova form in the ovaries of the female embryo but they stop meiosis at prophase I and remain in this state until puberty.

About once a month 5 to 20 cells will continue the meiotic process up to metaphase II and then stop again. One of the cells will be released from the ovary. If the ova (egg) is fertilized then it will continue meiosis II (from metaphase II on)

Females produce about 400 to 500 ova in a lifetime Males produce about 200 million sperm per day. It takes about 60 days to produce a mature sperm. Males can store billions of sperm and release about 200 million to 500 million sperm at a time Sperm that are not released are reabsorbed by the body