Bio Meiosis

The word "meiosis" comes from the Greek verb meioun, meaning "to make small,"

A process of reductional division in which the number of chromosomes per cell is cut in half.

Prophase I takes the most time. Chromosomes begin to condense. Cell is 2N (diploid) and 2 chromatids/chromosomes (like before mitosis). Synapsis occurs: homologous chromosomes pair, their DNA aligns. Crossing over may occur: chromosomes exchange homologous loci (between homologous pairs). The point of crossing over is called the chiasma.

Homologous chromosomes align at cell equator to form tetrads.

Chromosomes are pulled to either side of the dividing cell; one of each pair is pulled toward each pole.

The nuclear envelope may form again.

Two daughter cells are formed.

Nuclear envelope disperses

Chromosomes align between poles

Chromatids segregate.

After cytokinesis, four daughter cells are formed.  

Meiotic errors Nondisjunction- homologues don't separate in meiosis 1 1.Results in aneuploidy 2.Usually embryo lethal 3.Trisomy 21, exception leading to Downs syndrome 4.Sex chromosomes 1.Turner syndrome: monosomy X 2.Klinefelter syndrome: XXY Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Comparing Meiosis and Mitosis Chromosome behavior •Mitosis: Homologous chromosomes independent •Meiosis: Homologous chromosomes pair forming bivalents until anaphase I Chromosome number •Mitosis- daughter cells diploid •Meiosis- daughter cells haploid Genetic identity of progeny: