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.
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.
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 centromeredid 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).
Meiosis • Meiosis Animation