Meiosis

What is meiosis? A special type of cell division. Only occurs in reproductive organs Reduces the number of chromosomes in the nucleus -Reduction division

What does meiosis make? End result: - Gametes in animals - Spores in plants

Body Cells A.K.A. somatic cells Contain the total number of chromosomes in an organism Diploid number (2n) Humans have a diploid number of 46 chromosomes

Sex Cells A.K.A. gametes Contain half of the total number of chromosomes Hap/oid number (n) Humans have a haploid number of 23

What is the diploid number of corn? The haploid number?

How many PAIRS of chromosomes do humans have?

Is the chromosome number related to the complexity of the organism?

Example: Humans Diploid =46 Haploid = 23 Female gamete = egg = n = 23 Male gamete = sperm = n = 23 Zygote (fertilized egg) = 2n = 46

2n 2n mitosis 2n meiosis meiosis mitosis n zygote 2n n fertilisation

Chromosome Pairs Each diploid cell has two copies of each chromosome One copy 'donated' by the haploid egg One copy 'donated' by the haploid sperm

Chromosome Pairs genes Tetrad: - Two homologous chromosomes attached Chromosome Pairs genes Homologous Chromosomes Tetrad Matching chromosomes Carry information for the same genes Two homologous chromosomes attached

Divided into two parts: Meiosis I & Meiosis II The Phases of Meiosis Divided into two parts: Meiosis I & Meiosis II

Homologous chromosomes form 2n n interphase prophase I metaphase I anaphase I telophase I Homologous chromosomes align at equator and separate into chromosomes DNA replication

Chromosomes align at equator and separate into sister chromatids prophase II metaphase II anaphase II telophase II

Gamete Formation Gametogenesis Development of gametes Occurs in organs (gonads) of reproductive system Ovaries Testicles

Gamete Formation Oogenesis -In females - Produce eggs (ovum/ova) Spermatogenesis. -In males - Produce sperm

How Genetic Variation Occurs Random Fertilization (not part of meiosis) - picking a random mate - determines which sperm fertilizes which egg Average number of sperm cells in the ejaculate of a healthy man: 200 and 500 million an estimated 400,000 to 450,000 immature eggs residing in each ovary at puberty. Any sperm can fertilize any egg. The resulting zygote could contain anyone of more than 70 trillion possible combinations of chromosomes.

2. Crossover (Recombination) Totally Random and mistakes can occur

3. Random Line up During Metaphase I We have pairs of 23 so 223 = 8 3. Random Line up During Metaphase I We have pairs of 23 so 223 = 8.4 million combinations

4. Random Seperation During Metaphase II and Anaphase II