Week 1 Flashcards
Define Chromosome banding ?
Technique for the differential staining of mitotic and meiotic chromosomes to produce a characteristic banding pattern
selective staining of certain chromosomal regions such as
centromeres
NOR (nucleolus organizer region)
GC or AT rich regions.
What are the two different types of stains produced by Chromosome banding and explain the essence of the each technique?
Differential staining
- uses Giemsa (fluorescent) staining
- light and dark bands along entire chromosome
- q,g,r bands
Selective staining
- different regions of DNA are subject to variable degrees of staining
- c bands (NOR)
What is a karyotype
The chromosome complement of a cell or individual.
Used to refer to the arrangement of prometaphase chromosomes in a sequence according to length and centromere position.
What is an idiogram
Diagrammatic representation of a karyotype
Pictorial reference point useful for locating the positions of individual genes on chromosomes & identifying abnormalities associated with chromosome disorders.
What is the difference between G, R, Q and C banding?
G- banding
- uses proteolysis with trypsin
- Giemsa dye
Dark regions: AT rich
Light regions: GC rich
R-banding
-heat denatured
Giemsa stained (reverse)
Teleomere stained
Dark regions: GC rich
Light regions AT rich
Q-banding
-quinacrine mustard dye
Dark regions:AT rich
Light regions: GC rich
C-banding
Denatures with barium hydroxide
Giemsa dye
Dark bands: constitutive Hererochromatin
Centromere stained
What are banding patterns used for ?
Distinguish chromosomes & regions of chromosomes
• Identify various abnormalities
• Accurate gene mapping
• Compare banding of related species – evolution
• Identify parental origin of different chromosomes
List all present Human chromosome categories?
Learn from slide
Name the criteria for a successful mapping cross
The genotype of the organism producing the crossover gametes must be HETEROZYGOUS at all loci under consideration.
– PHENOTYPE of offspring must reflect GENOTYPE
– A sufficient number of offspring must be produced in the mapping experiment
Explain how drosophila can have white eyes?
• White eye locus
• The protein for this locus is an ATP-binding cassette (ABC) transporter
• Transports precursors of the red and brown eye colour pigments, guanine and tryptophan, into developing eyes during pupation
• Mutations that inactivate the white gene produce flies with white eyes
• There are other genotypes that can also produce white eyes
– Enzymes for the pigments are produced by genes that are epistatic to one another.
Explain how drosophila gets echinus eye color?
- Echinus locus
- Programmed cell death is used to remove excess cells between ommatidia (750-800 individual unit eyes) in the Drosophila pupal retina
- Echinus functions in the pupal retina to promote interommatidal cell death
- Mutants for the echinus (ec) gene are devoid of this pupal-stage apoptosis and extra cells are left between the ommatidia to create a rough eye in the adult fly.
What is genetic distance
The genetic distance between loci in a chromosome is measured in map units , or recombination units
• These are a measure of the % of crossing over between them
• % of recombination
– total number of recombinants/total number of
offspring X 100
• By convention, 1% crossing over between 2 loci equals a distance of 1m.u. or 1 cM
Explain the phenomenon of interference
• If the 2 single crossovers occur independently of each other, we can calculate the expected frequency of
DCOs
• Usually in mapping experiments ,the observed DCO frequency is less than the expected number of DCOs
• This phenomenon is called INTERFERENCE (I)
• I is when a crossover event in one region of a chromosome inhibits a second event in nearby regions.
Explain the coefficient of coincidence (C)
To quantify the disparities that result from interference, we calculate the coefficient of coincidence (C)
• C = DCO (obs)/DCO (exp)
• Then we can quantify interference
• I = 1-C
– If interference is complete and no DCO occur, then I = 1
– If fewer DCOs than expected occur I is positive and positive interference occurred
– If more DCOs than expected occur, I is negative and negative interference has occurred.
