Evolution - Karyotypes Flashcards
Karyotype
The number and visualisation of the set of chromosomes of individuals
How can karyotype be visualised?
Chromosome mapping
Chromosome Mapping
Hybridization with flurescent dyes label each chromosome a different colour
How are autosomes arranged in CM?
Longest to shortest(1 to 22)
Cytogenetic
The study of inheritance in relation to structure/function of chromosomes
Alternative way of staining chromosomes…
Giemsa reagant exhibit G-bands specific to each pair of homologs.
How are bands indicated in G-band staining?
G-bands replicated in S phase. being gene poor and R bands with are gene rich.
How are bands arranged in G-band staining?
Numbered from centromere to telomere.
What would 1p34 indicate?
First chromosome, on the short arm and 34th band.
What are examples of genetic landmarks?
Rhesus is 1p34
ABO blood-group is 9q34
Red-green color blindness at Xq28
How are chromosomes obtained for chromosome mapping?
Microdissection followed by PCR amplification, then labelling with fluoresnce, made to hybridize with complement strands, observed under confocal microscope.
Types of chromosome based on centromere…
Metacentric (In the middle)
Submetacentric (Off-centred)
Acrocentric (Localised to one end)
Acentric
Chromosomes without centromere, lost due to poor division.
Dicentric
Chromosomes containing two centromeres.
Why do majorty of chromosomes have single centromere?
Only ones reliably segregated in mitosis.
Dicentric segregation…
Bridge between each daughter cell, breaking leaving broken chromosome in either cell
Example of dicentrism?
Human chromosome 2, derived from chimps where there’s two medium-sized acrocentric chromosomes, G-bands indicate chromo 2 formed by telomeric fusion between the short arms.
Chromosomes in great apes?
48 chromosomes
Gene dosage
Number of copies of a particular gene present in a genome
Why is gene dosage a problem in sex-determination in chromosomes?
Y chromosomes are heterochromatic with less genes.
Dosage Compensation
An evolved mechanism either increasing male X chromosome activity or decreases female X chromosome activity
X-Inactivation
This is inactivation of a random X chromosome in early embryo development.
Where does X-inactivation occur?
At the XIC site near centromere on the long-arm.
Lyon Hypothesis
This results in equalisation of number of active copies of X-linked genes, both having same active x-chromosomes
Mosaicm
The presence of DNA alteration in only some of the bodys cells
What does mosaicsm result from?
X-Inactivation
Why does mosaiscm occur?
Because in each cell, X-inactivation can occur in either chromosome, if heterozygous for enzyme A or B, progeny produce half and half of the alleles
Pseudoautosomal Regions
This is the resembelence of functional genes in Y and X chromosomes
Where do pseudoautosomal regions exist?
Ends of short and long arms.
Why aren’t Y nor X linked despite pseudoautosomal regions?
They are indistinguishable and can undergo crossover.
Function of centromeres…
Allow sister chromatid segregation
Centromere structure…
Establish of alpha sattelites
Alpha Sattelites
Tandemly repeat sequences at noram human chromosomes
What causes X-Inactivation?
Expression of the Xist/Tsix genes and differential regulation of active/inactivity
What does Xist and Tsix do?q
Sist silences/inactivates one X chromosome whilst Tsix preveents Xist accumulation.
How is Xist/Tsix balance controlled?
DNA methylaiton
How is a karyotype done?
Taken blood for nucleated white blood cells
Mitotic stimulation by phytohaemagglutinin.
Incubation
Condensed chromosomes frozen in metaphase mitosis by colcemia
Tube centrifugation then cell lysis by hypotonic solution.
Giemsa staining.
Point of giemsa staining?
Seperates the chromosomes into bands allowing characterisation.
What ways are chromosomes identified?
Length
Banding patterns
Centromere placement.
