Meiosis and genetic variation

1. Meiosis and genetic variation Honors Biology Unit 5 2011-2012

2. Genome • Genome: Complete complement of an organism’s DNA. • Includes genes (control traits) and non-coding DNA organized in chromosomes.

3. Genes • Eukaryotic DNA is organized in chromosomes. • Genes have specific places on chromosomes.

4. Heredity • Heredity – way of transferring genetic information to offspring • Chromosome theory of heredity: chromosomes carry genes. • Gene – “unit of heredity”.

5. Reproduction • Asexual • Many single-celled organisms reproduce by splitting, budding, parthenogenesis. • Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent).

6. Sexual reproduction • Fusion of two gametes to produce a single zygote. • With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

8. In humans … • 23 chromosomes donated by each parent Total = 46 or 23 pairs • Gametes (sperm/ova): haploid (n) • Haploid= Contains a single set of chromosomes (23) • Zygote fertilized egg - now diploid (2n). • Diploid= Contains a two sets of chromosomes (23x2=46) Somatic cell: any cell other than gametes, most of the cells in the body. – diploid (2n)

9. Two kinds of Chromosomes • Autosomes: • Code for most genes in your body (not sex determining chromosomes) • In humans chromosome #1-22

10. Two kinds of Chromosomes • Sex Chromosomes: • chromosome that determines what gender you will be. • #23 in humans • Females are: XX • Males are XY

11. Sex Chromosomes • Mammals use a chromosomal method of determining sex: XX is female and XY is male. • Birds use a ZW system: ZZ is male and ZW is female. • the evolutionary origin of mammalian and bird sex chromosomes is different • Some reptiles use developmental temperature to determine sex: depends on the species, but hot is male and cold is female in some.

12. Homologues • Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). All autosomes are in homologous pairs.

13. Chromosome numbers All are even numbers – diploid (2n) sets of homologous chromosomes! Ploidy = number of copies of each chromosome. Diploidy

14. Meiosis: What is it? • Meiosis: cell division process by which the number of chromosomes per cell is cut in half • Why does it occur: Meiosis is used to produce the haploid(n) gametes.

15. Meiosis: Key differences from mitosis • Meiosis reduces the number of chromosomes by half. • Daughter cellsdiffer from parent, and each other. • Meiosis involves two divisions, Mitosis only one.

16. Meiosis 1 First division of meiosis • Prophase 1: Duplicated homologous chromosomes match up forming tetrads. Crossing-overoccurs at the chiasmata. • Metaphase 1: Tetrads align at the equator of the cell. • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

18. Meiosis II – Similar to mitosis Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. • Metaphase 2: Chromosomes line up at the equatorial. • Anaphase 2: Sister chromatids move separately to opposite ends of the cell. • Telophase 2: Cell division is complete. Four haploid(n) daughter cells are produced.

20. Animation Meiosis KM

21. Mitosis vs. meiosis Meiosis KM

22. Meiosis KM

23. Meiosis creates genetic variation • During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) • Meiosis results in genetic variationby: • Shuffling of maternal and paternal chromosomes and crossing over. • No daughter cells formed during meiosis are genetically identical to either mother or father • During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

24. Independent assortment Meiosis KM

25. Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations

26. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!

27. Crossing over Crossing over produces recombinantchromosomes, mixing the genes of the mother and father, recombining them. Chiasmata– sites of crossing over

28. Harlequin chromosomes

29. Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! Meiosis KM

30. Gamete Formation in Animals Male gamete formation - Spermatogenesis • In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

31. Gamete Formation in Animals Female gamete formation - Oogenesis • In females, most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. • The other 3 meiotic cells are small “polar bodies”, which degenerate.

32. Why Sexual Reproduction … • More genetic diversity: more potential for survival of species when environmental conditions change. • Shuffling of genes in meiosis • Crossing-over in meiosis • Fertilization: combines genes from 2 separate individuals • DNA back-up and repair. • Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. • Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.

33. Study Questions 1.What happens as homologous chromosomes pair up during prophase I of meiosis? 2. How does metaphase of mitosis differ from metaphase I of meiosis? 3. What is the sole purpose of meiosis? 4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis?

34. 5. Compare mitosis and meiosis on the following points: a. number of daughter cells produced. b. the amount of DNA in the daughter v. parent cell c. mechanism for introducing genetic variation. 6.What is a zygote and how is it formed?