LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I
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Transcript of 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
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?
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
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
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
OVERVIEW
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)
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
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...
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?
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?
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
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
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
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
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
CHROMOSOME THEORY - SEX LINKAGE
compare this pattern with X-linked inheritance in flies (discussed in lecture 03)
CHROMOSOME THEORY - SEX LINKAGE
compare this pattern with X-linked inheritance in flies (discussed in lecture 03)
CHROMOSOME THEORY - SEGREGATION
XX XY XXY X0 Flies
Mammals
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
CHROMOSOME THEORY - SEGREGATION
sometimes observe “exceptional” progeny
CHROMOSOME THEORY - SEGREGATION
proof of (... ok, strong evidence for) the chromosomal theory of inheritance
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
CHROMOSOMES
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
2n = 6
CHROMOSOMES
text order of ideas for this section of chapter 3
very smallDNA
very largegenome
mediumchromosometopography
smallchromosome
structure
my order of ideas for this section of chapter 3
very smallDNA
very largegenome
mediumchromosometopography
smallchromosome
structure
CHROMOSOMES - n
n
CHROMOSOMES - CENTROMERE
also... (d) acentric and (e) dicentric ... later!
CHROMOSOMES - n, SIZE, CENTROMERE
CHROMOSOMES - n, SIZE, CENTROMERE
CHROMOSOMES - TOPOGRAPHY
human nucleus, metaphase, n = 23 centromeres - note positions chromomeres - bead-like thickenings
CHROMOSOMES - TOPOGRAPHY
tomato nucleus, prophase I, n = 12 centromeres - orange heterochromatin - green: dense, note position... later euchromatin - white: less dense, active genes... later
CHROMOSOMES - TOPOGRAPHY
Drosophila melanogaster chromosomes, n = 4 heterochromatin - blue: dense, note position... later euchromatin - gold: less dense, active genes... later
CHROMOSOMES - TOPOGRAPHY
mouse nucleus, satelite DNA, different G+C content multiple tandem DNA sequence repeats much of it in centromeric heterochromatic regions
CHROMOSOMES - TOPOGRAPHY
tomato chromosome-2 nucleoli:
nuclear organelles containing rRNA 1 or more / nucleus (spp.)
nucleolar organizers (NO): genes encoding rRNA redundant - high copy
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
CHROMOSOMES - TOPOGRAPHY
human nucleus, metaphase, n = 23, Giemsa stain banding patterns - consistent, chromosome-specific G-dark & G-light... significance of bands?
CHROMOSOMES - TOPOGRAPHY
Drosophila chromosomes centromeres telomeres euchromatin heterochromatin polytene chromosomes chromocenter
CHROMOSOMES - TOPOGRAPHY
why is this why is this significant?significant?
CHROMOSOMES - TOPOGRAPHY
Zea maize, n = 10
CHROMOSOMES - GENES
CHROMOSOMES - GENES
single gene ID with FISH
CHROMOSOMES - GENES
meaningful relationships ?
CHROMOSOMES - GENES
meaningful relationships ?
CHROMOSOMES - GENES
repetative genes - duplication ?
CHROMOSOMES - STRUCTURE
honeybee metaphase chromosomes
continuous “fiber” 30 nm wide
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
CHROMOSOMES - STRUCTURE
CHROMOSOMES - STRUCTURE
1. nucleosome (10 nm)
2. solonoid (30 nm)
CHROMOSOMES - STRUCTURE
3. scaffold loop (? nm)4. supercoil
(chromatid)
CHROMOSOMES - STRUCTURE
where should the genes be ?
TUTORIAL #1: T.9.19 or R.9.21
go to the TUTORIAL page on the genetics web site download the file tut1-06F.pdf follow instructions and attempt as many of the
problems as possible you will continue to work on these in tutorial #1, with
the assistance of your TA and your peers