The Chromosomal Basis of Heredity

1. 3 The Chromosomal Basis of Heredity

2. Human Chromosomes • Humans have 46 chromosomes organized as 23 pairs which are homologous because each pair contains the same genes • Humans are genetically diploid= 2 copies of each chromosome , except for the sex chromosomes (X+Y) which are non-identical

3. Mammalian Cell Cycle • Cell division cycles occur in stages: - G1 = pre-DNA synthesis - S = DNA synthesis - G2 = post-DNA synthesis - M = mitosis: cell division occurs by precise steps which distribute one set of chromosomes to each of two cells

5. Stages of Mitosis • Occurs in dividing somatic cells • Chromosome replication:exact duplicates = sister chromatidsattached atcentromere • Prophase- chromosomes are visible, spindle fibers organize and attach to chromosomes at kinetochore

7. Stages of Mitosis • Metaphase- chromosomes line up in center of cell = metaphase plate • Anaphase - sister chromatids separate: one member of each pair is pulled to either pole of the cell • Telophase - nuclei of two new cells reorganize; the cells are diploid = each contains both members of every pair of chromosomes • Mitosis is usually accompanied by cytokinesis = cytoplasmic division

8. Meiosis • Meiosisis a specialized type of cell division which occurs only in reproductive cells = germ cells • Two rounds of cell division result in the formation of gametes which are genetically haploid = contain only one copy of each pair of homologouschromosomes

9. Meiosis: First Division Meiosis occurs in specialized cells called meiocytes in stages and requires two cell division events: First Meiotic Division: - chromosomes duplicate in S phase - homologous chromosomes pair - homologous chromosomes separate and are pulled to either pole of the cell at anaphase

10. Meiosis: Second Division • Each daughter cell contains only one member of each homologous pair of chromosomes after meiosis I • Second Meiotic Division: - cell division occurs in the absence of chromosome duplication - sister chromatids separate at anaphase as in mitotic division

11. Meiotic vs. Mitotic Division • Meiosisproduces four cells, each of which contains one copy of each pair of homologous chromosomes = genetically haploid • Mitosis produces two cells which contain both members of each pair of homologous chromosomes = genetically diploid

12. Meiosis I: Stages • Prophase - unique process of genetic recombination occurs - homologous chromosomes pair = synapsis - physical exchange of genetic material occurs between homologous chromosomes - chiasmata = linkage points

14. Prophase I: Meiosis • Leptotene - chromosome condensation • Zygotene- pairing (synapsis) of homologous chromosomes=bivalent • Pachytene - crossing-overbetween homologous chromosomes occurs • Diplotene- chromosome repulsion • Diakinesis- maximum chromosome contraction

15. Independent Assortment • Random alignment of homologous chromosomes during metaphase I results in independent assortment of non-homologous chromosomes • This occurs because the genetic elements on non-homologous chromosomes are unlinked = inherited as separate physical units

17. Meiosis I • Metaphase I - homologous chromosomes line up at metaphase plate; random alignment of non-homologous chromosomes is basis of Law of Independent Assortment • Anaphase I - physical separation of homologous chromosomes to opposite poles of spindle-demonstrates Law of Segregation;

18. Meiosis I and II • Telophase I - spindle breaks down, nuclear reorganization; one homolog from each bivalent is at each pole • Second Meiotic Division occurs in absence of chromosome replication • Meiosis II = equational division as the number of chromosomes in each cell remains constant

19. Meiosis II • Meiosis II consists of prophase II, metaphase II, anaphase II and telophase II which are identical to the stages of mitosis • Anaphase II - sister chromatids of each chromosome separate • Telophase II - each cell contains haploid chromosome number, one member of each homologous pair

20. Chromosome Structure • Eukaryotic chromosomes are highly coiled complexes of DNA and protein • Chromosome size is measured in kb=kilobase pairs; 1 kb=1,000 base pairs; 1 Mb (megabase) = 1 million bp • Chromosome-sized DNA molecules can be separated by electrophoresis in which DNA molecules move in response to electric field

21. Chromatin Structure • Chromatinis a stable, ordered complex of DNA and protein • Histones = major class of basic proteins in chromatin fibers • Five major types of histones are found in chromatin: H1, H2A, H2B, H3 and H4 • Histones of different species are similar = conserved

23. Chromatin Structure • Nucleosome = basic structural unit of chromatin • Each nucleosome core contains about 200 bps of DNA wrapped around a core of histone proteins = two each of H2A, H2B, H3 and H4 • Linker regions with DNA + H1 occur between adjacent nucleosomes • Structure termed: beads on a string

24. Chromatin Structure • Nucleosomes coil to form higher order DNA structure = 30 nm fiber which is a left-handed superhelix or solenoidal supercoil; contains 6 nucleosomes per turn • 30 nm fiber condenses to compact metaphase chromosome in which DNA/histone complex is attached to scaffold of non-histone proteins

26. Chromatin Structure • Heterochromatin= compact, heavily staining chromosome regions rich in satellite DNA and low in gene content • Euchromatin= less condensed chromosome regions high in gene content • Satellite DNA = highly repeated non-coding DNA sequences

28. Chromosomes and Heredity • Chromosomes consist of linear sequences of genes = genetic information which specifies the physical expression of a phenotypic trait • Homologous chromosomes contain the same sequence of genes which may vary in expression = alleles

29. Sex Chromosomes • X and Y chromosomes = sex chromosomes which are non-identical but share some genes • Males are genetically haploid for most genes on the X chromosome which results in unique pattern of X-linked inheritance • Autosomes = non-sex chromosomes

30. X-Linked Inheritance • Genes on the X-chromosome are X-linked • Females = XX; Males = XY • Sex of progeny is determined by X or Y of sperm • Morgan discovered X-linked inheritance by identifying mutations exclusive to male fruit flies

31. Morgan’s Fruit Fly Experiments • Morgan’s studies of inheritance patterns in Drosophila melanogaster revealed important genetic principles • Fruit flies were excellent tools for research due to short generation time, large number of offspring, and ease of producing and analyzing mutations

32. Morgan’s Fruit Fly Experiments Morgan’ s genetic principles: • X-linked inheritance based on mutations observed in males only • gene linkage based on the inheritance of genes as a single unit • chromosome mappingbased on recombination frequencies between linked genes

34. X-Linked Inheritance in Humans • Many human genes are on the X-chromosome = X-linked • Males have XY genotype and only one copy of X-linked genes • Mutations = genetic changes in X-linked genes will be expressed phenotypically in males even if recessive = X-linked genetic disorder • Hemophilia A =X-linked disorder

35. Meiosis Error: Nondisjunction • Nondisjunction = chromosomes fail to separate properly during anaphase of meiosis I or II • This results in unbalanced chromosome segregation, such that one cell receives both copies of the chromosome pair • Nondisjunction of X in Drosophila = proof of Chromosome Theory

37. Nondisjunction • Nondisjunction in meiosis I produces gametes with a pair of homologous chromosomes • Nondisjunction in meiosis II produces gametes with a pair of sister chromatids • Fertilization produces a zygote with 3 copies of a single chromosome = trisomy

38. Nondisjunction: Aneuploidies • Nondisjunction can occur for any human chromosome resulting in zygotes with an abnormal number of chromosomes = aneuploidy • Trisomy of chromosome 21 is the most common autosomal aneuploidy = Down’s Syndrome • Most aneuploidies are genetic lethals

39. Mendel’s Ratios: Chi Square Data Statistical (Chi Square) analysis of Mendel’s experiments in which phenotypic frequencies used to derive phenotypic ratios are the basis for the Law of Dominance, Law of Segregation and Law of Independent Assortment show close correlation between data and predicted outcome

40. Did we get the right ratio? • “Decide” using the Chi-square (c2) test • Formula: (Observed-Expected)2 S Expected • Example -- is 72:28 a 3:1 ratio ???? • step 1: total is 72 + 28 = 100 • step 2: expected is 3/4 x 100 = 75, 1/4 x 100 = 25 • (observed - expected) = 72-75 = -3, squared is 9 • sum over all catagories! (28-25, etc.)_

41. Chi-square table

42. How we know what to “Expect” • Statistics • One tool -- Bionomial Distribution • Formula: n! s!t! psqt • S + t = n (number of trials) • p + q = 1 (probability of two alternatives) • example: probability of 2 boys in 5 sibs • answer = 10/32 • Find factorial term with Pascal’s Triangle 23

43. Pascals Triangle

44. Chi-Square Analysis • Goodness of fit = test analyzes whether observed data agree with theoretical expectation • Statistically significant = refers to the magnitude of the difference between the observed and expected data measurements