The production of male and female gametes Meiosis The production of male and female gametes

Why Meiosis? 1) Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops 2) Meiosis is the process of producing sperm (in the testes) and eggs (in the ovaries) These are called gametes. 3) Meiosis ensures that there is the correct number of chromosomes present when sperm cells and egg cells unite.

Significance of meiosis Leads to halving of chromosome number, so to ensure that the diploid number of chromosomes can be restored after fertilization

Why have sexual reproduction? 1) Shuffling of information between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

2) Cells having pairs of homologous chromosomes - diploid (2n) e.g. body (somatic) cells 46 chromosomes total in humans 3) Cells having one chromosome from each homologous pair - haploid (n) e.g. gametes 23 chromosomes total in humans

Diploid and Haploid Body Cell Gamete

How many divisions? Two nuclear divisions Meiosis I and meiosis II Four haploid cells are produced Occurrence of Meiosis: Plants: anthers and ovules Mammals: testes and ovaries

Prophase I Chromosomes become visible Nuclear membrane disappears

Prophase I Halves of chromosomes pair up and crossing over can occur. 92 chromosomes total = doubled from 46 in interphase.

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Metaphase I Homologous chromosomes line up at the middle of the cell randomly This is called independent assortment 92 chromosomes

Independent assortment

Metaphase 1 Additions 1) Chromatids – when a chromosome replicates before meiosis two chromatids are produced. (sister chromatids) 2) Each pair of sister chromatids lines up with its homologous pair and joins together at their centromeres forming a tetrad or bivalent. This process is called synapsis

Anaphase I Homologous chromosomes separate and are pulled to opposite ends. 92 chromosomes

Telophase I Nuclear membrane reforms and cytoplasm divides. Each cell has half the chromosome number as the parent cell 92 chromosomes total = 46 in each cell.

Second meiotic division Separation of chromatids of each chromosome 4 daughter cells with half of the chromosome number of the parent cells are formed

Prophase 2 Same as meiosis one, but with 2 cells, 46 in each cell.

Metaphase 2 Same as meiosis one, but with 2 cells, 46 chromosomes in each cell.

Anaphase 2 Same as meiosis one, but with 2 cells, 46 chromosomes in each cell.

Telophase 2 Same as meiosis one, but with 4 cells. 23 chromosomes in each cell.

Significance of meiosis Produce genetic variation at crossing-over between homologous chromosomes during prophase I independent assortment of chromosomes during metaphase I

Sources of genetic variation Crossing-over between homologous chromosomes during meiosis Independent assortment of chromosomes during meiosis Random fusion of gametes during fertilization Mutation

Why do the members of this family look different?