Exam III Notes

1. Exam III Notes Powerpoint 2

2. Mitosis

3. Mitosis continued Mitosis can be divided into phases based on the appearance of the chromosomes through a compound light microscope. Interphase occurs before mitosis and consists of G1 + S + G2. During this phase the chromatin is spread out and is not visible using a compound light microscope. The nucleus and nucleolus are visible and DNA replication occurs. Cytokinesis which does not occur in all organisms follows the last phase of mitosis

4. STAGES OF MITOSIS Prophase Metaphase Anaphase Telophase PMAT

5. Prophase The chromosomes shorten and thicken and become visible using a compound light microscope. The nuclear membrane breaks down In animals, the centrioles move to the opposite ends of the cell Spindle fibers appear

6. Metaphase The chromosomes line up at the equator of the cell (note the arrangement and compare it to Metaphase I of meiosis) The spindles are complete

7. Anaphase The centromeres divide The sister chromatids separate and are pulled toward the opposite poles

8. Telophase The sister chromatids have reached the opposite ends of the cell The nuclear membranes form around each set of chromosomes In most cases, the cytoplasm begins to divide

9. Cytokinesis the parent cell divides into two daughter cells (not a stage of mitosis)

10. Mitosis PMAT

11. MITOSIS - ONE MORE TIME

12. Reading: Cell division out of control means cancer pp. 222-224.

13. MEIOSIS

14. MEIOSIS continued Like mitosis, meiosis can also be divided into stages, but it is more complex than mitosis. Meiosis involves two sets of stages or two PMATS.

15. STAGES OF MEIOSIS Meiosis I Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Meiosis II Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

16. Prophase I The chromosomes shorten and thicken and become visible using a compound light microscope The nuclear membrane breaks down In animals, the centrioles move to the opposite ends of the cell Spindle fibers appear The homologues pair up (remember these are not the same as sister chromatids which are already paired up). Crossing over occurs. This process involves the swapping of DNA between the homologues. It is an important source of variation.

17. Metaphase I The chromosomes line up at the equator of the cell, but the homologues not the sister chromatids are arranged toward the opposite ends of the cell. The spindles are attached to the homologues

18. Anaphase I The chromosomes begin to separate But, the homologues, not the sister chromatids are pulled apart

19. Telophase I Now the homologues are positioned at the opposite ends of the cell In some organisms, nuclear membranes reform Cytokinesis then occurs

20. Meiosis I Review

21. Prophase II If the chromsomes lengthened, after cytokinesis they now shorten again

22. Metaphase II Spindles form and attach the chromosomes line up. Note the arrangement

23. Anaphase II Now the sister chromatids separate

24. Telophase II Now the sister chromatids of half of the original number of chromosomes are present in each of the haploid nuclei Nuclear membranes form Cytokinesis

25. Meiosis II Review

26. Meiosis I and II

27. Mitosis and Meiosis

28. Spermatogenesis In male animals, the gametes (sperm) are formed by a process called spermatogenesis Germ cells (spermatogonia or sperm mother cells) divide and some become primary spermatocytes (in the seminiferous tubules of the testes) Meiosis occurs producing four N daughter cells Immature gametes are called spermatids Spermatids mature into sperm with a head containing the nucleus, a tail (flagellum) and mitochondria In most males, sperm are produced continuously once puberty is reached

29. Oogenesis In female animals, the gametes (ova or eggs) are formed by a process called oogenesis Germ cells divide by mitosis and some become primary oocytes that divide (Meiosis I) into a secondary oocyte and a polar body. Thus there is an unequal distribution of the organelles and cytoplasm. This polar body may not undergo Meiosis II. The secondary oocyte divides (Meiosis II) into an ovum and another polar body (both N) In many animals, meiosis begins before birth and the females are born with primary oocytes arrested in meiosis I. This is related to chromosomal abnormalities in the children of older women.

30. Chromosomal Abnormalities result from non-disjunction during meiotic divisions. This means that one gamete gets zero of a chromosome and the other gets both members of the pair. Although non-disjunction is more common in the gametes of older women, it can also occur in younger women or in males of any age. When an egg or sperm that is missing a chromosome forms a zygote, the embryo often fails to develop. When an egg or a sperm that has both members of a pair form a zygote, the embryo may fail to develop or in other cases, the offspring develops but is born with three of that chromosome (trisomy)

31. Down syndrome (trisomy 21) 1/750 births overall, but the frequency increases for older women because their eggs have been arrested in Meiosis longer and the spindles can be faulty. For women over 45, the risk is as high as 1/16

32. Non-disjunction of sex chromosomes When X chromosomes fail to separate, some gametes have 2 Xs and some have 0 Xs. Thus an offspring can be XXX (sterile but otherwise normal), X0 (sterile, short stature, webbed neck, Turner’s syndrome) or XXY (male, sterile with many female secondary sex characteristics, Klinefelter’s syndrome).

33. More non-disjunction of sex chromosomes The Y chromosome can also be present in two copies in the sperm. XYY males are fertile, but may be more aggressive or antisocial.

34. Chromosome structure changes Changes in the structure of chromosomes also occur [deletions (cri-du-chat), inversions and translocations (a form of cancer), and duplications (FMR gene on X chromosome sometimes 700 repeats causing Fragile X Syndrome).

35. Genetics

36. Some Background Evolution is the change in allele frequencies of individuals within a population over time. It occurs very slowly for the most part. Charles Darwin is given credit for generating the theory behind evolution. However, Darwin took the knowledge of others combined it with his own research and observations and suggested that natural selection is the mechanism of these changes.

37. More Background Genetics is a field of biology dealing with the study of heritable characteristics. Gregor Mendel conducted studies (breeding garden peas) and greatly increased our knowledge of inheritance (remarkably without any knowledge of DNA or Darwin’s work)

38. Genetics and evolution are interrelated in that changes occur in genes through different mechanisms. Thus, in order to understand evolution, we need to understand genetics. In addition, genetics and research in genetics could soon impact you in numerous and potentially good or bad ways.

39. Definitions I Genes are units of heredity (composed of DNA). The location of each gene on a chromosome is referred to as its locus. Each diploid individual has a pair of genes for each trait. One is inherited from the mother and the other is inherited from the father. The gene from the mom is present on one chromosome and the gene from the dad is present on that chromosome’s homologue.

40. Definitions II Alleles are one or more alternative states of a gene. If the two alleles for the trait code for the same protein, then we say that they are homozygous.

41. Definitions III If the two alleles code for different proteins, then we say that they are heterozygous. Dominant genes mask the expression of genes that are recessive. Homozygous dominant (AA), Heterozygous (Aa), or Homozygous recessive (aa) are the possible genotypes of individuals

42. Definitions IV The genotype of an individual is the actual genetic makeup. The phenotype is the observable trait that is controlled by the genotype. For example: if red is dominant to white (red and white are phenotypes), then RR (or homozygous dominant individuals) = red, Rr (heterozygotes) = red, and rr (homozygous recessives) = white. But there are exceptions (see below).

43. Definitions V One of the tools that we use to determine the possible genotypes and phenotypes of offspring is the Punnett square. In crosses, P = parental generation, F1 = the first generation of offspring, and F2 = the second generation of offspring.

44. Mendel’s Laws based on his work with garden peas (why garden peas? earlier studies had shown that hybrids could be produced, large numbers of true breeding varieties were available, they are small and easy to grow, and they have a short generation time). Law of Segregation. Diploid organisms inherit a pair of genes for each trait and these genes segregate during meiosis and end up in different gametes Law of Independent Assortment. Each gene pair tends to assort into gametes independently of other gene pairs located on other homologous pairs of chromosomes

45. Monohybrid crossone trait

46. Dihybrid crosstwo traits, different chromosomes

47. Not all alleles affect just one trait- Pleiotropy is the influence of a single gene on unrelated traits.

48. Another variation is Epistasis where two alleles of a gene mask the alleles of another gene and as a consequence, the expected phenotypes associated with the latter are not present Some Puli are e/e at MC1R and in the case of these dogs, one can not predict their K genotype. Such e/e dogs could be KB/KB or KB/ky and still not be black since the e/e genoytpe prevents black pigmentation of hairs in dogs (but not nose leather or pads). This is an example of "epistasis".

49. Sometimes more than one gene affects a phenotype - Continuous variation or Multigene inheritance occurs when multiple genes act jointly to determine a trait such as height or weight & thus it is difficult to determine the contribution of an individual gene

50. Incomplete dominance involves the ability of two alleles to produce a heterozygous phenotype that is different from either homozygous phenotype