Brittle Bones

Question 1

what is the function of collagen?

Answer 1

• major part of the extracellular matrix surrounding cells and giving mechanical strength and rigidity to tissues and organs.
• provide the tensile strength of skeletal tissues including bone, cartilage, tendons and ligaments.

Question 2

where is collagen secreted from?

Answer 2

fibroblasts and other cells

Question 3

what is common between all type of collagen

Answer 3

they all have the same triple helix structure

Question 4

what is unusual about the the AA sequence of collagen?

Answer 4

has high concentration of glycine

Question 5

What is osteogenesis imperfecta and what causes it?

Answer 5

A disease where you are prone to repeated fractures of long bones and also malformed bones

Whole range of genetic disorders cause it- most likely is mutation in glycine residues producing defective structural assembly

Question 6

which condition is OI easily misdiagnosed as?

Answer 6

child abuse

Question 7

what happens in OI biochemically?

Answer 7

• single gene point mutation ⇒ glycine swapped with cysteine
• leads to defective assemble of collagen as protein larger ⇒ partial loss of function
• disulphide bonds formbetween 2 alpha chains in the helix
• 2-ME can cleave these bonds

Question 8

how would the electropheresis differ between px with + without OI?

Answer 8

Question 9

What effect does SDS have on the electrophoresis of collagen?

Answer 9

The speed of migration of the proteins in the collagen depend on their size not charge

Question 10

What is 2-mercaptoethanol?

Answer 10

A reagent which reduces the disulphide bond between the cystine residues in proteins

Question 11

What is the effect on the protein of a different, larger amino acid being coded for in a protein sequence?

Answer 11

Thr larger amino acid will cause steric hinderance - this generates a kink in the normally straight chain resulting in defective protein forming

Question 12

Which is heavier cystine or glycine?

Answer 12

Cysteine

Question 13

In OI, why are only some of the childs collagen chains affected?

Answer 13

The child may be heterozygous, so only one allele is mutation and therefore only some collagen molecules will carry the mutation

Question 14

Why is the pattern of inheritance basically dominant for OI?

Answer 14

Because the collagen triple helix contains 2 alpha 1 chains, and is disrupted if even only one chain is affected

Question 15

Why does abnormal collagen production result in the symptoms and signs seen in OI?

Answer 15

Abnormal collagen structure leads to defects in the mineralisation process of bone as hydroxyapetite has nothing to be layed onto

Question 16

Suggest a suitable prenatal diagnostic test to identify a foetus who may be at risk of OI?

Answer 16

Amniocentesis or genetic screening

Question 17

What is restriction fragment length polymorphism?

Answer 17

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

Question 18

Why does a substitution of glycine for cysteine result in defective assembly of collagen fibres?

Answer 18

The larger amino acid in the mutant molecule will cause steric hindrance which generates a kink in the normally straight triple helix, with a resulting defect in the assembly into fibres.

Question 19

What does cysteine contain in its side chain?

Answer 19

a reactive sulphydryl group

Question 20

What is the effect of cysteine having a reactive sulphydryl group in its side chain?

Answer 20

inappropriate disulphide bonds between the two α1(I) chains in the helix, resulting in a cross linked polypeptide chain

Question 21

What effect does a cross-linked polypeptide chain have on the speed of migration on an agar plate?

Answer 21

polypeptide chains will migrate much more slowly than the individual chains when examined by gel electrophoresis in the presence of SDS

Question 22

Why is directly sampling the patients collagen protein not a suitable pre-natal testing method?

Answer 22

Sampling collagen from a foetus would be impractical and risky