5.3 Brittle Bones Flashcards
What is osteogenesis imperfecta and what causes it?
Disease where patients are prone to repeated fractures of long bones and malformed bones
Caused by mutation to glycine residue, producing a defective structural assembly
What is the affect of the point mutation which causes osteogenesis imperfecta?
The larger cysteine residue produces more steric hindrance, which generates a kink in the triple helix, causing defective assembly of fibres
What does the SDS-PAGE method allow for in gel electrophoresis of collagen?
Separation of the proteins in the collagen depending on size and not charge
Which is heavier, cysteine or glycine?
Cysteine
What would a change from glycine in collagen to cysteine do in gel electrophoresis with SDS?
Cysteine forms disulfide bonds between the two alpha(I) chains in the helix
This produces crosslinked polypeptide chains that migrate more slowly in gel electrophoresis
What is 2-mercaptoethanol?
A reagent which breaks the disulphide bonds between the cystine residues in proteins, allowing chains to migrate according to molecular weight
In OI, why are only some of the childs collagen chains affected?
The child may be heterozygous, so only one allele is mutated and therefore only some collagen molecules will carry the mutation
Why is the pattern of inheritance basically dominant for OI?
Because the collagen triple helix contains 2 alpha 1 chains, and is disrupted if even only one chain is affected
Why does abnormal collagen production result in the symptoms and signs seen in osteogenesis imperfecta?
Abnormal collagen structure leads to a defect in bone mineralisation, as hydroxyapatite cannot be laid down on collagen
This leads to skeletal abnormalities and bone weakness
Suggest a suitable prenatal diagnostic test to identify a foetus who may be at risk of OI?
Foetal DNA from amniocentesis or chorionic villus sampling
Amplified by PCR
What is restriction fragment length polymorphism?
When specific probes thta are complenentary to the part with the mutation are set under thr right conditions of temperature and ionic strength - only becomes hybridised if the sequence is exactly complementary which allows mutant and normal sections to be identified
What does cysteine contain in its side chain?
A reactive sulphydryl group
What is the effect of cysteine having a reactive sulphydryl group in its side chain?
Inappropriate disulphide bonds between the two alpha(I) chains in the helix, resulting in a cross linked polypeptide chain
What effect does a cross-linked polypeptide chain have on the speed of migration on an agar plate?
Polypeptide chains will migrate much more slowly than the individual chains when examined by gel electrophoresis in the presence of SDS
Why is directly sampling the patients collagen protein not a suitable pre-natal testing method?
Sampling collagen from a foetus would be impractical and risky
