18 - Cytogenetic Techniques and Chromosome Function Flashcards
Roles of chromosomes
- Transmit genetic information (mitosis and meiosis)
- Express genetic information (protein and non-coding RNA)
C
Amount of DNA
n
Number of Chromosomes
ploidy
- Number of copies of chromosomes. Varies with cell type
Haploid
Gametes (n chromosomes, C DNA) n=23
Diploid
Most human and mammalian cells (2n chromosomes, 2C DNA) n=46
Polyploid
non-mammalian cells (4n, 8n, etc)
Karyotype
Species specific chromosome number, size and shape at metaphase
3 elements required for a chromosome to be copied
- A centromere (narrowest part of chromosome, where spindle fibres attach)
- Replication origins (initiation)
- Telomeres (ends of linear chromosomes)
What does meiosis produce
haploid cells. Haploid sperm (23) + haploid egg (23) = diploid zygote (46)
What does mitosis produce
diploid cell (2n) producing diploid cell (2n)
Tightly packed DNA
No gene expression, can be seen under light microscope
Cytogenetics
Study of how chromosomes relate to cell behaviour
Preparation of sample for cytogenetic analysis
- Blood sample taken, isolate white blood cells
- Culture cells in medium
- Stimulate to divide with mitogen
- treat with spindle disrupting agent (can enter metaphase but not progress)
What are chromosome numbers 1-22 based on
Size
Metacentric
Centromere close to middle of chromosome
Submetacentric
Centromere in between middle of chromosome and telomeres
Acrocentric
Centromere close to telomere
p arm
Short arm (petite)
q arm
Long arm
G banding
- Chromosomes treated with trypsin and stained with Giemsa
- Preferentially binds AT rich regions
- Produces alternating dark and light bands
- Genes are associated with GC rich sequences
- Dark bands are gene poor, light bands are gene rich
Q banding
Chromosomes stained with fluorescent dye, binds to AT rich DNA
R banding
Reverse of G banding. Heat denatures AT rich DNA, GC specific dyes
T banding
Identifies subset of R bands concentrated close to telomeres
C banding
Constitutive heterochromatin, mainly at centromeres
How can resolution be increased
By using more elongated chromosomes (prometaphase)
Proximal
Close to centromere
Distal
more distant from centromere
Karyogram
Graphical representation of individuals full set of chromosomes
FISH fluorescence labelling
- Fluorophore (fluorescent dye) is bound to the probe
- Fluorophore is associated with target sequence
Interphase FISH
Interphase chromosomes are much less condensed than in metaphase. Detects translocations
