mutations

14

1. About one in 20 000 humans has the condition, albinism. This is caused by the absence of melanin, the dark brown pigment normally present in the human skin, hair, and eyes. Albinism arises from a gene mutation that causes skin cells to produce a different version of the enzyme tyrosinase. This different version of tyrosinase is faulty.

(a) Explain how a gene mutation may result in a different version of tyrosinase.

(b) The faulty tyrosinase does not produce melanin.
Suggest an explanation for this.

Reply 1

Iscariot

11

a) - If there is an error in the DNA coding of the person then the incorrect amino acid sequence will be produced. This will result in the wrong kind of protein being produced, producing the wrong kind of enzyme. Hence the enzyme does not function as it should so does not serve it's intended purpose.

b) - The active site may be incorrectly configured (due to the incorrect amino acid sequence) meaning it cannot catalyse the reaction which produces melanin. If the active site is disfigured, the substrates cannot bind to it as they are supposed to so no reaction takes place.

Reply 2

nikk

15

Iscariot

a) - If there is an error in the DNA coding of the person then the incorrect amino acid sequence will be produced. This will result in the wrong kind of protein being produced, producing the wrong kind of enzyme. Hence the enzyme does not function as it should so does not serve it's intended purpose.

A frameshift mutation may also inserts a premature stop codon, leaving you with a truncated protein. Alteratively, the protein may not be encoded at all from mutations-
- In 5'UTR or 3'UTR which may adversely affect mRNA stability.
- May be a translocation chromosomal mutation which moves the gene to a transcriptionally inert region.
- Mutation may effect the promoter or enhancer regions of the gene.

Iscariot

b) - The active site may be incorrectly configured (due to the incorrect amino acid sequence) meaning it cannot catalyse the reaction which produces melanin. If the active site is disfigured, the substrates cannot bind to it as they are supposed to so no reaction takes place.

Or the mutation may affect a regulatory site on the enzyme. If the mutation occured in another gene, that faulty gene product may adversely affect the functioning of the enzyme (dominant negative mutation).
- Or the mutation may affect a regulatory site on the enzyme.

Reply 3

Revenged

15

Iscariot

a) - If there is an error in the DNA coding of the person then the incorrect amino acid sequence will be produced. This will result in the wrong kind of protein being produced, producing the wrong kind of enzyme. Hence the enzyme does not function as it should so does not serve it's intended purpose.

An important point that nikk didn't mention is that if you have a mutation then it may not make no difference to the protein... This is because there is more than one type of DNA anticodon for an mRNA codon... So changing the genetic code may not affect the protein...

But more interestingly even with a change in protein there still may be no difference... A typical example may be a membrane protein... If a hydrophobic protein was replaced by a different hydrophobic protein then it is very unlikely to make any difference... And lets say you had a protein then changing one protein is probably not likely make much difference unless it is in an important part of the protein such as the active site...

I know it can seem completely irrelevent at this stage... but there are loads of different polymorphism of proteins found in the body... For example, a polymophism in a membrane protein may cause a drug to be inactive...

When I did my project I can across this...

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1399-6576.2004.00517.x?cookieSet=1&journalCode=aas

It's a relatively new area of research but some different structures of proteins (in this case morphine acting on the mu opioid receptor) are present in people and this may explain why the doses of morphine vary so much for people who want pain relief (i.e. not completely psychological)... So basically one day it may be possible to look at your genes to decide what drugs would be most effective / drug dosage... but it's a hell of a long way off...

mutations

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