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LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I

LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I. how do we know that genes are parts of chromosomes? how are they arranged on chromosomes? are chromosomes only genes? how is chromosome number maintained through the generations?. LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I.

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LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I

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  1. LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I • how do we know that genes are parts of chromosomes? • how are they arranged on chromosomes? • are chromosomes only genes? • how is chromosome number maintained through the generations?

  2. LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I • what is the chromosomal basis for Mendel’s 1st law - equal segregation? • what is the chromosomal basis for Mendel’s 2nd law - independent assortment? • how does all the DNA fit into a tiny nucleus?

  3. LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I • CH3 key concepts • historical development of the chromosome theory... genetics + cell biology • chromosomes • mitosis & meiosis • chromosome behavior & inheritance patterns • organelle chromosomes

  4. CHAPTER 3: KEY CONCEPTS • genes are parts of chromosomes • mitosis = 1 nuclear division  2 daughter nuclei identical to original nucleus • meiosis = 2 nuclear divisions  4 genetically distinct daughter nuclei with one set of chromosomes • Mendel’s laws of (1) equal segregation & (2) independent assortment are based on (1) separation of each member of a chromosome pair & (2) the independent movement of chromosome pairs

  5. CHAPTER 3: KEY CONCEPTS • chromosomes can be identified microscopically using various visible landmarks • a chromosome (chromatid) contains a single DNA molecule • DNA winds around protein spools, spooled units then coil, loop & supercoil to form a chromosome • much of eukaryotic DNA is present in multiple copies • most multiple copy DNA has no known function

  6. OVERVIEW

  7. GENETICS + CELL BIOLOGY • mechanisms of character transmission (so far studied) unknown until chromosomes were accessible by more advanced microscopy • behavior of Mendel’s particles parallel chromosome movement – merger of cytology and genetics – Sutton-Boveri Theory (1902)

  8. GENETICS + CELL BIOLOGY • cell biology findings leading to understanding of remarkable correlation with transmission: • nuclear divisions – initially ‘aberrations’ in cell physiology • 2 gametes fusion during fertilization • discovery of chromosomes • discovery that somatic chromosome # constant for given species; # did not seem to correlate with species complexity (e.g., ant with 1pr) • chromosomes present in pairs* – diploid number • gametes formed have half as many – haploid number • each member of diploid pair are derived from each parent; each parent contributes a haploid complement to zygote

  9. GENETICS + CELL BIOLOGY • Sutton-Boveri theory: genes are located on chromosomes • Mendelian genes and observed chromosomes... • occur in pairs • segregate equally into gametes • assort independently into gametes • evidence is convincing... but correlative only...

  10. GENETICS + CELL BIOLOGY • Sutton-Boveri theory: genes are located on chromosomes • problems? • do chromosomes retain their physical integrity through interphase? • some chromosome pairs look the same... how can you tell that they assort independently?

  11. CHROMOSOME THEORY - SEX LINKAGE • a grasshopper sp. has unpaired & heteromorphic pair • observed 2 patterns with = frequency (counted) • non-homologous chromosomes assort independently • ... but not random... mechnanism?

  12. CHROMOSOME THEORY - SEX LINKAGE • a thistle sp. has 12 chromosome pairs • 12 variants identified • each variant had a 3rd copy of one chromosome • chromosomes all carry genetically distinct material

  13. CHROMOSOME THEORY - SEX LINKAGE • meiosis in a beetle Tenebrio • segregation of heteromorphic chromosomes X and Y in  • gametes receive different material • s don’t have Y

  14. CHROMOSOME THEORY - SEX LINKAGE • chromosome complement in 2 insect spp. • gender correlates with 2 patterns, s have 2 Xs, no Y • s have 1X and either 0 or 1 Y

  15. CHROMOSOME THEORY - SEX LINKAGE • X and Y segregate like homologues in s • s make 2 kinds of gametes in equal proportions • responsible for determination of gender

  16. CHROMOSOME THEORY - SEX LINKAGE • in some species... • are heterogametic WZ • are homogametic ZZ • distinguish system from XY • found in some birds, reptiles, fish, and some insects (moths and butterflies) • same rules apply as in XY but in reverse

  17. CHROMOSOME THEORY - SEX LINKAGE • compare this pattern with X-linked inheritance in flies (discussed in lecture 03)

  18. CHROMOSOME THEORY - SEX LINKAGE • compare this pattern with X-linked inheritance in flies (discussed in lecture 03)

  19. CHROMOSOME THEORY - SEGREGATION

  20. CHROMOSOME THEORY - SEGREGATION • in flies... • X:autosome ratio  1,  mRNA on • X:autosome rato < 1,  mRNA on • XXY =  • X0 =  • fertility is determined by Y , X0 = sterile  • in humans... • gender determined by Y

  21. CHROMOSOME THEORY - SEGREGATION • sometimes observe “exceptional” progeny

  22. CHROMOSOME THEORY - SEGREGATION • proof of (... ok, strong evidence for) the chromosomal theory of inheritance

  23. CHROMOSOME THEORY • 2 important points to remember... • not all genes on sex chromosomes have to do with gender or sexual differentiation • many autosomal genes are important for gender

  24. CHROMOSOMES 2n = 6 • levels of genetic organization: 1. ploidy – chromosome sets 2.n – how many of each type 3.size – arbitrary / relative 4.centromere–position 5. landmarks – chromomeres, puffs, abnormalities

  25. medium chromosome topography small chromosome structure very small DNA very large genome    medium chromosome topography small chromosome structure very large genome very small DNA    CHROMOSOMES text order of ideas for this section of chapter 3 my order of ideas for this section of chapter 3

  26. n CHROMOSOMES - n

  27. CHROMOSOMES - CENTROMERE • also... (d) acentric and (e) dicentric ... later!

  28. CHROMOSOMES - n,SIZE, CENTROMERE

  29. CHROMOSOMES - n,SIZE, CENTROMERE

  30. CHROMOSOMES - TOPOGRAPHY • human nucleus, metaphase, n = 23 • centromeres - note positions • chromomeres - bead-like thickenings

  31. CHROMOSOMES - TOPOGRAPHY • tomato nucleus, prophase I, n = 12 • centromeres - orange • heterochromatin - green: dense, note position... later • euchromatin - white: less dense, active genes... later

  32. CHROMOSOMES - TOPOGRAPHY • Drosophila melanogaster chromosomes, n = 4 • heterochromatin - blue: dense, note position... later • euchromatin - gold: less dense, active genes... later

  33. CHROMOSOMES - TOPOGRAPHY • mouse nucleus, satelite DNA, different G+C content • multiple tandem DNA sequence repeats • much of it in centromeric heterochromatic regions

  34. CHROMOSOMES - TOPOGRAPHY • tomato chromosome-2 • nucleoli: • nuclear organelles • containing rRNA • 1 or more / nucleus (spp.) • nucleolar organizers (NO): • genes encoding rRNA • redundant - high copy

  35. CHROMOSOMES - TOPOGRAPHY • telomeres • no visible “structure” • FISH (fluorescent in situ hybridization) • telomere-specific DNA • tandem arrays of non-coding sequence • overcomes replication problem... more on this later

  36. CHROMOSOMES - TOPOGRAPHY • human nucleus, metaphase, n = 23, Giemsa stain • banding patterns - consistent, chromosome-specific • G-dark & G-light... significance of bands?

  37. CHROMOSOMES - TOPOGRAPHY • Drosophila chromosomes • centromeres • telomeres • euchromatin • heterochromatin • polytene chromosomes • chromocenter

  38. CHROMOSOMES - TOPOGRAPHY why is this significant?

  39. CHROMOSOMES - TOPOGRAPHY • Zea maize, n = 10

  40. CHROMOSOMES - GENES

  41. CHROMOSOMES - GENES • single gene ID with FISH

  42. CHROMOSOMES - GENES • meaningful relationships ?

  43. CHROMOSOMES - GENES • meaningful relationships ?

  44. CHROMOSOMES - GENES • repetative genes - duplication ?

  45. CHROMOSOMES - STRUCTURE • honeybee metaphase chromosomes • continuous “fiber” 30 nm wide

  46. CHROMOSOMES - STRUCTURE • D. melanogaster DNA molecule from 1 chromosome* • single continuous “fiber” 15 mm (5 x longer than the fly!) • a chromosome contains only 1 DNA molecule

  47. CHROMOSOMES - STRUCTURE

  48. CHROMOSOMES - STRUCTURE • nucleosome (10 nm) • solonoid (30 nm)

  49. CHROMOSOMES - STRUCTURE • scaffold loop (? nm) • supercoil (chromatid)

  50. CHROMOSOMES - STRUCTURE • where should the genes be ?

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