MEIOSIS

1. MEIOSIS Sperm & Eggs & Variation..OH MY!

2. Sexual Reproduction requires Cells Made by Meiosis • What if a new individual was formed through mitosis?

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

4. Genes • Eukaryotic DNA is organized in chromosomes • genes have specific places on chromosomes • exon: portion of a gene that is translated into protein (more in chapter 5) • intron: non-coding segment of DNA, often found within an exon; removed before transcription

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

6. Sexual reproduction • Fusion of two gametes to produce a single zygote. • Introduces greater genetic variation, allows genetic recombination. • zygote has gametes from two different parents.

7. Chromosomes • Karyotype: • ordered display of an individual’s chromosomes • collection of chromosomes from mitotic cells • staining can reveal visible band patterns, gross anomalies • Make a Karyotype

8. Karyotyping

11. HomologOUs CHROMOSOMES • Chromosomes exist in homologous pairs in diploid (2n) cells. • One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromsosome). • Homologous chromosomes are similiar but not identical. Each carries the same genes in the same order, but the alleles (alternative form of a gene) for each trait may not be the same.  • Exception: sex chromosomes (X, Y)

12. HomologOUs CHROMOSOMES

13. In humans … • 23 chromosomes donated by each parent (total = 46 or 23 pairs). • Gametes (sperm/ova): • contain 22 autosomes and 1 sex chromosome • haploid (haploid number “n” = 23 in humans) • Fertilization results in zygote with 2 sets of chromosomes - now diploid (2n). • Most cells in the body produced by mitosis. • Only gametes are produced by meiosis.

14. Chromosome numbers All are even numbers – diploid (2n) sets of homologous chromosomes.

15. Meiosis – key differences from mitosis • Meiosis reduces the number of chromosomes by half. • Daughter cells differ from parent, and each other. • Meiosis involves two divisions, Mitosis only one. • Meiosis I involves: • synapsis • homologous chromosomes pair up • chiasmataform (crossing over of non-sister chromatids) • metaphase I: homologous pairs line up at metaphase plate • anaphase I: sister chromatids do NOT separate • separation of homologous pairs of chromosomes, not sister chromatids of individual chromosome

18. Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinantchromosomes.

20. Animation

21. Meiosis I • Prophase 1 • each chromosome duplicates and remains closely associated (sister chromatids) • crossing-over can occur during the latter part of this stage • Metaphase 1 • homologous chromosomes align at the equatorial plate

22. Meiosis I • 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

23. Meiosis II Second division of meiosis: Gamete formation • Prophase 2 • DNA does not replicate • Metaphase 2 • chromosomes align at the equatorial plate

24. MEIOSIS II • Anaphase 2 • centromeres divide • sister chromatids migrate separately to each pole • Telophase 2 • cell division is complete • 4 haploid daughter cells

25. Mitosis vs. meiosis

27. Meiosis creates genetic variation • During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) • Meiosis results in genetic variation by 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

28. Independent assortment

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

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

31. Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinantchromosomes.

32. HOW SEX IS DETERMINED IN HUMANS • Females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives.  • For this reason, we know that nothing on the Y chromosome is absolutely necessary.

33. SEX DETERMINATION IN OTHER SPECIES

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

35. Meiosis & sexual life cycles • Life cycle = sequence of stages in organisms reproductive history; conception to reproduction • Somatic cells = any cell other than gametes, most of the cells in the body • Gametes produced by meiosis

36. Meiosis & sexual life cycles Generalized animal life cycle

37. Sex is costly! • Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required • Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.) • Genetic costs: in sex, we pass on only half of genes to offspring. • Males are an expensive luxury- in most species they contribute little to rearing offspring.

38. But … • 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

39. NONDISJUNCTION Unequal distribution of chromosomes during meiosis Resulting gametes zero or two copies of a chromosome instead of a single copy

40. Down's Syndrome

41. TOO MANY OR TOO FEW CHROMOSOMES