Introduction to Meiosis

Diploid Diploid is what a cell is called when it has both copies (2N) of each chromosome (N). The diploid number is abbreviated 2N. In humans, our body cells are diploid cells. Therefore, we have two copies of each chromosome in each of our body cells.

Question Since we have 46 total chromosomes in each body cell… How many pairs of chromosomes are in each body cell? 23

What is N? N is the number of single sets of chromosomes or the haploid number. Haploid is what a cell is called when there is only one copy of each chromosome present. Ex: In a chimpanzee, a body cell has 48 chromosomes (2N). Therefore, N equals 24 in chimps.

Chromosomes Present in other Species

Mitosis In the human body, somatic (body) cells have 46 chromosomes. These cells undergo mitosis to create 2 identical cells, each with 46 chromosomes. Purpose: organism growth and cell replacement

Number of Chromosomes throughout Mitosis

Stages of Mitosis

Anaphase Prophase Metaphase Telophase Picture adopted from: Real Pictures

Meiosis The process by which the number of chromosomes in a cell is cut in half to create sex cells. This occurs in the reproductive organs. Purpose: Producing haploid sex cells (gametes) for reproduction.

Process of Meiosis During S phase (prior to meiosis), the cell goes through a round of DNA replication. This DNA replication doubles the amount of chromosomes already present. NormalS phase # of chromosomes 4692

Number of Chromosomes throughout Meiosis

Crossing Over: This is good! During prophase I, tetrads can exchange portions of their chromatids in a process called crossing over. Crossing-over produces genetically different homologous chromosomes, which increases genetic diversity. Non-disjunction: This is bad. This can occur during meiosis and leads to extra or missing chromosomes in the gametes formed. Non-disjunction results in chromosomal issues such as Down Syndrome (an extra copy of chromosome 21). Two processes unique to meiosis….

Prophase I Homologous chromosomes twist and wind around each other forming a tetrad (means 4) and crossing over occurs.

Metaphase I Tetrads line up at the equator of the spindle. Homologous chromosomes are positioned in pairs, side by side.

Anaphase I Paired chromatids move to opposite ends. Centromeres do not split.

Telophase 1 Chromatids are still joined at centromeres. These two cells will enter interphase and then eventually meiosis II.

These two new cells have chromosomes (and genes) that are different. They are not identical.

Meiosis Picture adopted from: Metaphase I Prophase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II 2n nn nn n n

Prophase II

Metaphase II The chromosomes line up in a similar way to metaphase in mitosis.

Anaphase II

Telophase II

Cytokinesis The end result of meiosis is 4 haploid (N) gametes that are NOT identical to each other.

Meiosis Picture adopted from: Metaphase I Prophase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II 2n nn nn n n

Meiosis Animation