Grade 11 University Biology Miss MacKay

Meiosis A special type of cell division that occurs in the gonads Purpose: to create haploid reproductive cells known as gametes. Meiosis is name for the process that separates the duplicated nuclear material. Cytokinesis is needed to split up the organelles and cytoplasm. The process of meiosis occurs somewhat differently in males and females.

Introduction Just like mitosis, Interphase occurs prior to meiosis, and the chromosomes are duplicated. Meiosis then has the same stages as mitosis, however they occur twice in order to reduce the chromosome number from diploid to haploid (reduction division). Unlike mitosis, meiosis produces 4 non-identical haploid cells whose purpose is for reproduction only.

First Half of Meiosis Reduction Division Meiosis I First Half of Meiosis Reduction Division

Prophase I Many similarities to Prophase in Mitosis. Nuclear membrane dissolves. Chromosomes are attached to their copy by a centromere. Centrioles appear and move to the poles

Prophase I But also, all the chromosomes of the same chromosome number, known as homologous chromosomes (i.e. all chromosome #1) and their copies from DNA duplication, join together to form a tetrad or bundle of four chromosomes (2 from each parent). This is different to mitosis!

Prophase I During the tetrad formation, non-sister chromatids can exchange genes to create unique allele combinations that are not seen in the parent. This is called crossing over, and is one of the reasons for the variation seen in offspring.

Crossing Over

Metaphase I The tetrads align themselves at the equator. All tetrads align independent of one another. This is called independent assortment and is the other reason for the variation seen in offspring.

Metaphase I Spindle fibres released from the centrioles at the poles attach to the centromeres of the sister chromatids.

Anaphase I The spindle fibres contract and pull one pair of sister chromatids to each pole Notice that the centromere did not split apart during this phase. It still holds the chromosome copies (sister chromatids) together.

Telophase and Cytokinesis A nuclear membrane reforms around each new nucleus and the cytoplasm is divided by cytokinesis creating 2 non- identical cells. These two cells proceed into the next round of meiosis. There is no second round of interphase.

Meiosis II Second Half of Meiosis

Meiosis II The phases of meiosis II are all exactly identical to mitosis. The only difference is that they are occurring in two cells at the same time. The end product is four, non-identical, haploid cells.

Prophase II The nuclear membrane dissolves. The chromosomes become more visible and are still attached to their copy by the centromere. Centrioles move towards the “poles” of the cell.

Metaphase II The sister chromatids line up at the “equator” of the cell. Spindle fibres emerge from the centrioles and attach to the centromere of each chromosome pair.

Anaphase II The spindle fibres contract, breaking the centromere, and pull apart the sister chromatids. One copy of the chromosome pair goes to one pole, while the other copy goes to the other pole.

Telophase II The nuclear membrane starts to reform around the chromosomes at each pole. The cell membrane starts to pinch inward at the equator signaling the end of meiosis and the beginning of cytokinesis. The end product is four, non- identical, haploid cells (sperm in male, eggs in female).

Spermatogenesis and Oogenesis

Spermatogenesis

Oogenesis

Meiosis Meiosis Animation