Meiosis

Meiosis A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Diploid – 2 sets of chromosomes Haploid – 1 set of chromosomes Homologous – chromosomes that each have a corresponding chromosome from the opposite sex parent

Meiosis Meiosis usually involves 2 distinct stages Meiosis I Meiosis II

Prophase I Part of this phase is similar to mitosis. The doubled chromosomes coil up and become visible.  The nuclear membrane and nucleolus disappear.  The spindle fibers form Here is how Prophase I is different from prophase in mitosis.  In prophase I, doubled homologous chromosomes group together to form a tetrad.  Thus a tetrad is defined as the grouping of doubled homologous chromosomes.  (Tetra means four.  You can see there are four sister chromatids in a tetrad.)  In mitosis, the doubled chromosomes all remain independent of one another.

  In this picture, the chromosomes that are the same size are homologous.  The red are inherited from mom while the blue came from dad.  You can see the doubled homologs forming a tetrad.  Crossing over is occurring between the sister chromatids on the inside of the tetrad.

Prophase 1 Crossing over members of the tetrad swap DNA.  Think about this.  Because of crossing over, the four gamete cells produced at the end of meiosis will all be different from one another. 

Metaphase 1 the tetrads will line up in the middle of the cell. 

Anaphase 1 the tetrad splits in half and each doubled chromosome moves away to opposite sides of the cell.

Telophase 1/Cytokinesis 1 each set of homologous chromosomes have been separated into two daughter cells and the cytoplasm has been divided.

Interphase II Some cells will skip interphase II. There is no S because the chromosomes are only copied during the first interphase.

Prophase II This occurs just like prophase in mitosis. The nuclear membrane break down. Spindle fibers form  

Metaphase II The doubled chromosomes line up in the middle of the cell.

Anaphase II The doubled chromosomes split and each sister chromatid moves to opposite sides of the cell.

Telophase II/Cytokinesis II The chromosome uncoil.  The nucleolus and nuclear membrane reform.  The spindle fibers break down. 

Result of Meiosis Meiosis produces four different haploid cells (half the number of chromosomes as the parent) that will become sperm in males and eggs in females. Notice that each cell has two chromosomes (the parent had four), and all the cells are different. 

Mitosis vs Meiosis Mitosis Meiosis Results in 2 Diploid Cells (2N) 4 Haploid Cells (N) Cells are Genetically Identical Genetically Different Occurs in Somatic (Body) Cells Sex Cells

https://www.youtube.com/watch?v=kVMb4Js99tA&feature=related