REMEMBER Gametes are reproductive cells, or sex cells, and contain a haploid (n) number of chromosomes. Remember this is half the chromosomes found in each normal cell. Meiosis is the process in which gametes are formed. This occurs ONLY in reproductive cells!

Meiosis Meiosis produces 4 daughter cells. Each cell has HALF of the original diploid number which means they are (haploid) The new cells ARE NOT identical (unlike in mitosis where they are)

Meiosis Why is the process of meiosis important? Meiosis keeps the number of chromosomes constant over generations When the egg (n) and sperm (n) unite during fertilization, the offspring produced is a diploid cell (n + n= 2n)

Homologous Pair: Matching genes from the mother and the father. Homologous Pairs Homologous Pair: Matching genes from the mother and the father. Each of these bands is called a gene locus. A gene locus is a specific location of a gene on a chromosome. An example would be the gene for eye color. The alleles of a gene will reside in the same location in a homologous pair (maternal & paternal).

Homologous Pair Remember that each chromosome in the pair is the exact same in terms of size, gene location and position on the centromere WITH THE EXCEPTION of sex chromosomes. The sex chromosomes for a female are XX, and the sex chromosomes for a male are XY Humans have 46 total chromosomes, but 23 pairs. 22 pairs of normal genes (autosomes) 1 pair of sex chromosomes

How does all this work? How are cells able to change from diploid to haploid? ---Meiosis. this animation on the process of meiosis. Notice that there are two distinct divisions for this process--meiosis I and meiosis II: http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter28/animation__how_mei osis_works.html Right click to follow the hyperlink

Goal of Meiosis Create 4 cells. There is one main goal for meiosis... All different from each other All have haploid (n) number of chromosomes

There are actually two separate divisions that take place for Meiosis. Two Processes There are actually two separate divisions that take place for Meiosis. Meiosis I Meiosis II

Meiosis I Interphase I Cells undergo a round of DNA replication, forming duplicate chromosomes.

Meiosis I Prophase I Each chromosome pairs with its corresponding homologous chromosome and forms a tetrad. As homologous chromosomes pair up and form tetrads, they exchange portions of their chromatids in a process called crossing over. Tetrad: a structure that forms 4 chromatids. Crossing over: results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles and results in increase diversity

Meiosis I Metaphase I The chromosomes align at the center of the cell. Spindle fibers attach to the chromosomes. Notice the areas on each chromosome where crossing over has occurred.

Meiosis I Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell.

Meiosis I Telophase I & Cytokinesis Nuclear membranes form and the cell separates into two new cells. NOTICE: EACH CELL HAS DIPLOID NUMBER OF CHROMOSOMES (2N)

Meiosis II Where we left off after Meiosis I... We now have two cell with 46 chromosomes of NON-IDENTICAL DNA What is going to happen in Meiosis II... We will divide again leaving half of each DNA in each cell... 4 cells with 23 chromosomes of non-identical DNA

Meiosis II Prophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell

Meiosis II Metaphase II The chromosomes line up in a similar way to the metaphase stage of mitosis.

Meiosis II Anaphase II The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II Telophase II Meiosis II results in four haploid (N) daughter cells, each with just 2 chromosomes.

Meiosis II Summary

Right click to follow the hyperlink Mitosis vs. Meiosis We have now discussed both mitosis and meiosis in full. If you are still having trouble, you may find the video below to be helpful. Right click to follow the hyperlink https://www.dropbox.com/s/4ecw87c8l18kzaj/lifes-greatest-miracle-how-cells- divide-mitosis-vs-meiosis.webarchive?m