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