Cell Division : Mitosis and Meiosis

1. Chapter 8 Cell Division : Mitosis and Meiosis

2. Asexual vs. Sexual reproduction • Asexual reproduction – new organisms/cells are genetically identical to parent cells/organisms • Sexual reproduction – offspring have a combination of genes from both parents.

3. Asexual Reproduction • Budding - plants • Vegetative propagation -plants • Binary fission -bacteria • One cell dividing to become two –mitosis • Hermaphroditic organisms are NOT asexual!!!

8. Cells only come from other cells • To make more cells they must divide • Mitosis – one cell divides to make two genetically identical cells for asexual reproduction and growth and repair. • Meiosis – One cell divides twice to create 4 cells that are not genetically identical. These cells are eggs or sperm (gametes)

9. Cell Cycle • Interphase – cell does normal job, grows, and duplicates genetic material to prep for division, 90% of cell cycle • G1, S, G2 • Mitosis – division of genetic material • Cytokinesis- division of cytoplasm, usually occurs after mitosis

11. Cell Cycle • Interphase • G1- first gap, growth, normal function, makes proteins etc • S phase- synthesis, cell copies DNA to prepare for division • G2- second gap, growth and final preparation for division

14. Eukaryotic cells have complex genomes • DNA in a non-dividing cell is disorganized • Called chromatin • When a cell prepares to divide the chromatin (DNA and proteins called histones) coil into chromosomes • Each chromosome is made of two identical halves called sister chromatids • Sister chromatids are connected by a centromere

26. Cytokinesis • In animal cells a cleavage furrow develops. The cell pinches from the outside • In plant cells a cell plate forms. A new cell wall develops from the inside and works out to the borders

28. Factors that affect cell division • Must be attached to a surface • Will stop dividing when they touch each other- density dependent inhibition • Secretion of proteins called growth factors • Three key checkpoints in the cell cycle • G1 • G2 • M-

29. Cancer • Do not respond normally to checkpoints in the cell cycle • Excessive cell division, wasting of cellular resources, form masses called tumors • Benign- stays in original location • Malignant- spreads from original location- metastasis

30. Mitosis Summary 8.11 • Occurs in somatic or non-sex cells • Creates two genetically identical cells from one cell • The cells created are diploid (2n)– having a full set of chromosomes. • Used for repair and growth

31. Human life cycle • Diploid cells in ovaries and testes divide by meiosis to create haploid gametes. • Gametes are egg and sperm. • Haploid cells have a half set of chromosomes • Haploid gametes combine to form a diploid zygote • Diploid zygotes divide by mitosis to form Multicellular organisms

32. Homologous chromosomes 8.12 • Humans have 22 pairs of autosomes – non-sex chromosomes and one pair of sex chromosomes • Females have two X chromosomes • Males have an X and a Y chromosome

35. Meiosis- steps • Two divisions • Meiosis I; meiosis II • Major differences • Prophase I- homologous chromosomes pair into a tetrad. Sometimes the homologous pairs exchange small pieces – crossing over • Synapsis – the exchange of pieces

39. Meiosis increases genetic variation • 1. Crossing over – creates chromosomes that are mosaics of both maternal and paternal genes. • Is a random event and doesn’t happen for every meiotic cycle • Occurs in prophase I • Called genetic recombination

41. Meiosis- increases genetic variation 2. Law Of Segregation • Each haploid cell inherits only one chromosome from each parent. Homologous chromosomes carry genes for the same trait but not necessarily the same gene – Law of Segregation. • The physical process that underlies this law occurs in Anaphase I

42. Genetic variation • 3. Law of Independent assortment • Each homologous chromosome pair lines up side by side and separates randomly in metaphase I. • Creates many different random combinations of chromosomes in each egg or sperm • Different possibilities = 2 to the n power, where n= the haploid number

44. Genetic variation • 4. Random fertilization increases genetic variability in a species • Why is variation needed? • Organisms with very similar genomes have no raw material for natural selection should the environment abruptly change

45. Genes • Are carried on chromosomes • Each trait in your body is determined by at least two genes on two different homologous chromosomes – one from dad, and one from mom

46. Mistakes occur in meiosis • Crossing over • Separation in anaphase I and/or anaphase II • Nonreciprocal crossovers- exchange of pieces of DNA of different sizes • Inversion- pieces of chromosomes are reattached incorrectly • Non homologous crossovers • Failure to separate – nondisjunction

47. Crossing over mistakes • Chromosomes missing parts due to non reciprocal cross overs have deletions • Chromosomes with too much info have duplications. • Fragments reattached in the wrong sequence are inversions • Translocations occur when non-homologous chromosomes cross-over

48. Disorders caused by CO mistakes • Cri du chat- deletion on # 5, “cry of the cat” in babies • Down Syndrome can be caused when #21 attaches to another chromosomes • Chronic myelogenous Leukemia (CML)- non homologous cross over activates a cancer gene. Call it the “Philadelphia chromosome”

49. Nondisjunction • Can occur in anaphase I or anaphase II • Results in organisms with the wrong number of chromosomes for their species – aneuploid individuals • Most situations with missing or extra chromosomes lead to spontaneous miscarriage

50. Extra Autosomes • Trisomy • Trisomy #21 – Down’s Syndrome • Trisomy- # 18- Edward’s Syndrome • Trisomy #13- Patau’s Syndrome