1. Chromosomal Basis of Heredity
Biology 331: Chapter 3
Chromosomal Basis of Heredity

2. Meiosis & Mitosis: Review your Intro Biology notes
If you have difficulty stop by my office

3. Meiosis...a review: Interphase: Chromosomes and centrosomes replicate
Much like mitosis

4. Meiosis I: Prophase I: Chromosomes begin to condense
Synapsis: Homolgous chromosomes come together as pairs
Tetrad: A complex of four sister chromatids (2 from each homolog chromosome)
Chiasmata: Where homologous chromatids "cross over"
The exchange of genetic material
90% of the time for meiosis

5. Meiosis Diagram

6. Crossing Over

7. Meiosis I continued Metaphase I: Anaphase I:
Paired sister chromatids of homologous chromosomes move toward opposite poles
Sister chromatids do not separate
Telophase I and cytokinesis:

8. Meiosis Diagram

9. Meiosis II: Prophase II: Metaphase II: Anaphase II:
Centromeres of sister chromatids separate
Individual chromosomes move toward opposite poles
Telophase II and cytokinesis:
Each cell is now haploid

10. Meiosis Diagram

11. Mitosis –vs- Meiosis

12. Some origins of genetic variation in sexually reproducing organisms:
Without variation there is no Evolution!

13. Independent assortment of chromosomes to gametes:
Which of the homologous chromosomes is included in a gamete is random
The chances are for each chromosome
Which of your parents chromosomes goes into a gamete
100% maternal, 100% paternal, or some combination
The number of possible of possible combinations of gamete is 2n (n is the haploid number)
For humans this is 823 or about 8 million combinations

14. Random fertilization:
Which sperm meets which egg?
Multiply the possible gametes of each parent together
Assuming 2 parents with 8 million possible gametes you get 64 trillion combinations

15. Crossing over:
Individual chromosomes in a gamete do not have to be exclusively maternal or paternal
When chiasmata form portions of chromosomes can switch places
In humans you get ~2-3 crossover events per chromosome pair!
Number of possible combinations is nearly infinite

16. Crossing Over

17. Life Cycles:
If you get chromosomes from both parents how do you avoid a doubling of genetic material?
Meiosis: The process of creating gametes with half the normal chromosome complement
Review from Intro. Bio. I & II

18. Diploid Life Cycles Common in animals
Gametes are the only haploid cells
Meiosis occurs during the production of gametes
Gametes do not undergo further division until fertilization
Diploid zygotes undergo mitosis to form a multicellular adult

19. Diploid Life Cycle

20. Haploid life cycle Most fungi and some algae:
Gametes fuse to form a transient diploid zygote
Gametes undergo meiosis to form two haploid cells
These haploid cells undergo mitosis to form a multicellular adult
The adult produces gametes via mitosis
Implications for genetics??

21. Haploid Life Cycle

22. Alternation of Generations
Plants and some algae:
Sporophyte: Multicellular diploid stage
Meiosis in the sporophyte produces haploid spores
Spores give rise to multicellular adults without fusing with another cell
Gametophyte: Multicellular haploid stage
The haploid gametophyte makes gametes via mitosis
Fertilization results in a diploid zygote
The zygote develops into the next sporophyte generation

23. Alternation of generations

24. Moss and Alternation of Generations

25. Implications for genetics??
Some genes affect the sporophyte and others the gametophyte
Height of gametophyte and color of sporophyte for example
Mendel and his peas
Why did it work out?

26. Topography of the chromosome set:

27. Chromosome number:

28. Centromere Position

29. ….and chromosome size

30. Banding patterns:

31. Heterochromatin patterns
Utilizes DNA stain (like Feulgen)
Euchromatin:
Poorly staining
Less densely packed
Implications?
Heterochromatin:
Stains well
Densely packed
Can be constitutive or facultative
Heterochromatin typically found at centromeres and telomeres
The whole drosophila Y chomosome is heterochromatic

32. G banding patterns:
Chromosomes partially digested with proteolytic enzymes
Stained with Giemsa reagent
Produces light and dark banding regions

33. G Banding

34. G-light bands: Tend to be GC rich Not densely packed Replicate early
mRNA label binds to these regions most

35. G-dark bands: Tend to be AT rich Densely packed
mRNA does not bind well to these areas
Implications??

36. Three dimensional structure of a chromosome:

37. The molecule: Each chromosome is ONE molecule of DNA
E.coli contains about 1.3mm of DNA per cell
H. sapiens contains about 2m of DNA per cell
Implications for cells and cell division ??

38. The Chromosome

39. Histone Proteins and packaging:

40. Chromatin:
The length of a chromosome is much less than that of a DNA molecule
Chromosomes made up of chromatin
Chromatin is DNA and protein

41. Condensation of a chromosome

42. Histones: Histones are the primary proteins
The structure of histones is conserved across eukaryotes
Histones form an octamer

43. The solenoid: DNA wraps two times around each octamer
This histone DNA spool is called a nucleosome
The nucleosome "beaded necklace" assumes a coiled form called a solenoid
The solenoid is stabilized by another histone protein (H1)

44. Histones

45. High order coiling: Solenoids are ~30nm across
However, chromosomes are ~700nm across!
Must have more structure

46. Scaffold protein: Non histone The scaffold forms a spiral
The nucleosomes loop on the scaffold
Loops attach at SARs (scaffold attachment points)
The DNA region where SARs occur are non-coding

47. Scaffold

48. SARs

49. Sequence organization:
How are genes arranged?
What proportion of the DNA is genes?
What is the nature of the non-coding regions?

50. Portion of the genome outside the nucleus
Organelle Genomes:
Portion of the genome outside the nucleus

51. Chloroplast DNA (cpDNA):
Chloroplasts came from endosymbiotic cyanobacteria
cpDNA is a circular double helix
Haploid
Each organelle has copies of cpDNA
Contains genes related to photosynthesis and those required for gene expression
Has few noncoding sequences compared to nDNA
Highly conserved (even the order is conserved)

52. Uniparental inheritance
Angiosperms maternal
Some gymnosperms paternal
Biparental inheritance has been found in some angiosperms
Recombination is rare in the extreme
Inherited as one allele

53. Mitochondrial DNA (mtDNA)
Came from purple bacteria endosymbiont
mtDNA is a circular double helix
Haploid
Codes for many proteins used in cellular respiration
5-10 copies in each organelle
mtDNA in plants, fungi and animals differ

54. Plant mtDNA: Great variation in genome size Structure varies
300-2,400 kb in melons
Structure varies
Can be circular or linear for example
Recombination is frequent
Sequence divergence is low however
Evolution is slow

55. Fungal mtDNA: Genome size variable Coding sequences include introns
kb in one group
Coding sequences include introns
Modification of the "standard" DNA code
UGA = tryptophan not "stop"
CUN = threonine not leucine
AUA = methionine not isoleucine
In some cases

56. Animal mtDNA: Very Compact Genome size and gene order conserved
No introns and noncoding regions are limited
In the mouse 94% of the mtDNA genome is coding
Genome size and gene order conserved
Mutation rate is high
Sequence divergence is high
Evolutionary rate 5-10 times that of nDNA
AUA = methionine not isoleucine
UGA = tryptophan not "stop"
Maternal Inheritance (Normally)
No Recombination