What are chromosomes made up off?
Chromosomes are made up of chromatin Chromatin is made up of:
- Non-histone proteins
- Histones (H1, H2A, H2B, H3, H4)
What components make up chromatin?
Histones, RNA, DNA and non-histone protein
List the types of histone and their characteristics
H1, H2A, H2B, H3, H4 H1 - Varies between species
H2A, H2B, H3 and H4 directly interact with the DNA H3 and H4 are highly conserved and are responsible for bead on a string structure
What is euchromatin?
Lightly packed chromatin, often under active transcription
What is heterochromatin?
Tightly packed chromatin and not under active transcription
What are numerical chromosomal abnormalities?
Number of chromosomes other than 46
What are the two types of numerical chromosomal abnormalities?
Polyploidy - cells containing more than two paired (homologous) sets of all chromosomes. Caused by polyspermy
Aneuploidy - (an abnormal number that is not a multiple of the haploid number) Monosomy is a loss of one chromosome i.e one chromosome pair missing Trisomy is a gain of one chromosome i.e one “chromosome pair” exists as a triplet
What are structural chromosomal abnormalities?
Physical changes to one or more chromosome
What are two types of structure abnormalities?
Balanced - Change does not result in missing or extra genetic information
Unbalanced - Change results in missing or extra genetic information
List all the mutations that can occur within one chromosome
4) Ring chromosome
5) Isochromosome; balanced - the arms are mirror images
How does a ring chromosome form?
Loss of the telomeres at ends of both arms forming a ring
List the mutations that occur between two chromosomes?
2) Reciprocal Translocation
3) Robertsonian Translocation
What is an inversion mutation?
No loss of genetic material, but as rearrangement of genetic material to a non-homologous chromosome
What is reciprocal translocation (chromosome abnormality)?
No loss of genetic material, but an exchange of genetic material between two non-homologous chromosomes
What is a robertsonian translocation?
Rearrangement of genetic material between two chromosomes; the q- arms (long arms) of two afrocentric chromosomes combine to form one 'super-chromosome' with the loss of both p-arms
What is the difference between reciprocal and robertsonian translocation
Reciprocal is the exchange of genetic material between two non-homologous chromosomes whereas Robertsonian involves the exchange of genetic material produced two q arms from two acrocentric chromosomes to form a super chromosome and loss of the two small p arms
What is a karyotype?
Picture of the systematic assortment of the complete set of chromosomes of an individual, starting with chromosome 1 and ending with the sex chromosomes
How is a karyotype produced?
Produced by ‘cut and paste’ of chromosome pictures by hand or computer to make organised set of metaphase chromosomes
Describe chromosome nomenclature
Number of chromosomes-comma-X chromosomes-Y chromosomes + or – for loss or gain of genetic material State whether in p or q arm
E.g. Normal Female = 46,XX and a Normal Male is 46,XY
E.g. 5p- means ‘missing a segment of the p-arm on chromosome 5
Why would a patient be referred for karyotyping?
Congenital: - Prenatal screening e.g. family history
- Recurrent foetal loss
- Birth defects e.g. mental retardation
- Abnormal sexual development e.g. Klienfelter’s Syndrome
- Acquired Abnormalities - Leukaemia and related diseases
What can be identified from FISH?
Rearranged chromosomes, marker chromosomes and microduplications and microdeletions
What is microarray used for?
High resolution of 5-10 megabases means that microarray can be used to test for submicroscopic chromosome abnormalities Used for individuals with suspected unbalanced chromosomal abnormalities
What is uniparental disomy? (UPD)
Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or of part of a chromosome, from one parent and no copy from the other paren
Why is imprinting significant to UPD?
Imprinted chromosomes show differential expression of specific genes depending on the parental origin of the chromosome If chromosome involved is not imprinted then UDP has no phenotypic effect
Name a UPD syndrome
What is the most common mechanism to generate UDP?
Describe non-invasive pre-natal testing (NIPT)
Cell free DNA is present in plasma fraction of maternal blood
What can NIPT be used to test for?
Chromosome aneuploidy, foetal sex, single gene disorders
What is the advantage of NIPT compared to invasive pre-natal testing
Removes miscarriage risk