The production of Gametes Meiosis The production of Gametes

Meiosis The production of gametes During this process specialized cells in the gonads, produce sex cells that contain only one set of chromosomes. Each cell produced contains one member from each pair of homolgous chromosomes

Meiosis In humans cells that contain two sets of chromosomes (2N=46) produce cells that contain 23 chromosomes (N=23) Keep in mind that the 23 chromosomes are not just any 23, but one member from each pair.

Meiosis In our example we will for simplicity sake use cells that contain 2N=4 chromosomes. That is the cell undergoing meiosis contains two pairs of homologous chromosomes.

Meiosis Step by Step

Interphase

Interphase As in mitosis, meiosis is proceeded by Interphase

Prophase 1

Prophase I During this phase: the homologous chromosomes pair up to form Tetrads. Each tetrad contains 2 chromosomes and 4 chromatids This pairing up is referred to as synapsis

Metaphase I

Metaphase I the tetrads are lined up at the center of the cell The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Anaphase I

Anaphase I Chromosomes, each with two chromatids, move to separate poles.

Telophase I

Telophase I As you can see the two new daughter cells will contain 2 chromosomes each. However the chromosomes still consist of two chromatids. During telophase, cytokinesis occurs. This is the division of the cytoplasm.

Meiosis II The next set of cell divisions will separate the chromatids. This process is very similar to the process of mitosis

Metaphase II

PROPHASE II No tetrads form Chromosomes already doubled Chromatin condenses to from doubled chromosomes

Metaphase II In this stage the chromosomes line up at the equator

Anaphase II

Anaphase II The chromatids separate and move towards opposite poles

The End Product of Meiosis

The End Product of Meiosis In the end from one cell in the gonads, 4 new cells will be formed. Each of the new cells will contain only one member from each homologous pair.

Differences in Sperm and Egg production Remember that sperm and egg production differ: In males 4 viable sperm are produced In females 3 of the cells produce are known as polar bodies and do not survive. Only one egg is formed

Meiosis and Variation Unlike mitosis, meiosis does not produce identical cells The cells produced will not only have half the number but also the chromosomes will differ in specific information.

Meiosis and Variation What chromosomes end up in what cell all depend upon how the chromosomes line up in Metaphase I

Meiosis and Variation If the two blue chromosomes line up on the same side and the two red chromosomes line up on the same side, then the following will be produced

Meiosis and Variation If the a red and a blue line up on the same sides then the following will result.

Meiosis and Variation So in effect, in our example of a cell that contains only 4 chromosomes, Four different gametes can be produced. Keep in mind that they contain the same kinds of chromosomes BUT the specific information on these chromosomes differ