Chapter 11.4 Meiosis

Mendel’s principles of genetics require 2 things: each organism must inherit a single copy of every gene from each of its parents. When an organism produces its own gametes, the 2 sets of genes must separate so that each gamete has 1 set of genes.

Chromosome Number A fruit fly has 8 chromosomes – it inherits 4 from the male parents and 4 from the female parent. These 2 sets are called homologous chromosomes. A cell that has both sets of homologous chromosomes is called diploid (2 sets). For fruit flies 2N (the diploid number) = 8 Gametes (egg and sperm) usually only have 1 set of chromosomes, these are called haploid cells. In fruit flies N (haploid number) = 4.

Phases of Meiosis Meiosis is the production of haploid cells. It reduces the number of chromosomes per cell by separating homologous chromosomes. There are 2 divisions in meiosis – meiosis 1 and meiosis 2. The end result is 4 haploid cell from 1 diploid cell.

Meiosis 1 Before meiosis 1 every chromosome makes a copy of itself. In prophase the corresponding chromosomes form a tetrad. There are 4 chromatids in a tetrad. At this point the chromatids can exchange pieces – this is called crossing over. This gives a new combination of genes that didn’t exist before. During meiosis 1 the homologous chromosomes are separated.

Meiosis 2 The 2 cells from meiosis 1, they go again through similar stages at mitosis. There is no DNA replication at this time. Each of the cells is made of 2 chromatids. At the end of meiosis 1 4 daughter cells are produced, each with a haploid number. We will go over this well in class!

Comparing Mitosis and Meiosis Mitosis made 2 genetically identical diploid cells. Meiosis makes 4 genetically different haploid cells.