Meiosis The production of Gametes
Jan 14, 2016
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