Science Questions

How do mutations happen?

Fri, 14th Sep 2012

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Question

Kashefa Farooqui asked:

Hi Chris

 

 

Love your show on Five Live. I have a question thats been puzzling me recently. At the molecular level how does mutation occur in the gene? How does say exposure to radiation cause a point mutation on the chromosome?

 

 

Regards

Kashefa

Answer

Louise - This monthís question is being answered by Dr. Philip Zegerman from the Gurdon Institute in Cambridge. Kashefa Farooqi emailed us to ask, ďAt the molecular level, how does mutation occur in the genome and how does exposure to radiation for example cause a point in mutation in the

Philip - And so, our genome is really just a long organic molecule of repeating units which are called bases and itís the order and composition of these bases that really determines your genome, what your genes do. And if you do get changes to the base composition or the organisation of these As, Cs, Gs, and Ts, thatís a process thatís called mutation.

So this polymer, this organic polymer that is your genome is actually sensitive to damage by chemicals or high energy radiation. What UV does, for example, is it causes these bases that your genome is made out of to stick together really in a process thatís crosslinking. And x-rays have sufficient energy that actually make your DNA helix, your double helix to actually cause it to break. So it can actually cause the backbone of the DNA to be nicked or broken, and these are called strand breaks.

Iíve told you that the genome is essentially a long string of bases, 3.2 billion of them in the case of humans, but the important point is that every cell in the body has a perfect copy of your genome which means that every time your cell divides, it has to make a perfect copy of this 3.2 million bases. This essentially involves a process where your DNA is duplicated by a special group of enzymes which are called DNA polymerases. And these polymerases travel along your DNA, copying it as faithfully as they can, but if they hit a problem, for example, a strand break, a lesion or perhaps two bases that have been stuck together by UV light, then your DNA polymerases canít replicate very well, and they have a high propensity to cause errors.

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Kashefa Farooqui asked the Naked Scientists: Hi Chris Love your show on Five Live. I have a question that's been puzzling me recently. At the molecular level how does mutation occur in the gene? How does say exposure to radiation cause a point mutation on the chromosome? Regards Kashefa What do you think? Kashefa Farooqui , Wed, 24th Nov 2010

A mutation occurs when the nucleotide sequence along a chromosome is permanently broken or altered by radiation with high enough energy (ultraviolet light or greater) to break covalent bonds, by a mutagenic chemical, or by some natural cellular processes. SteveFish, Thu, 25th Nov 2010

Viral mutation should not be ignored;- as these 'buggers' are simply most adept @ the re-sequencing act. 5nutjob, Fri, 26th Nov 2010

A typical chromosome has between 50,000,000 and 250,000,000 base pairs.

DNA Transcription errors can spontaneously occur during replication.  While your body is constantly subjected to "background radiation", it is quite possible that these errors will crop up independent of the radiation.

The cells have mechanisms to repair such transcription errors, but they are not perfect, so mutations will periodically spontaneously occur.

Another problem is that the proteins can not transcribe to the end of the genes...  so telomeres of essentially null base pairs are appended to the ends.  If these telomeres aren't maintained, then with each successive replication the DNA gets shorter until genetic regions start getting gobbled up and mutations start occurring.

As mentioned above, viruses also have to replicate their DNA or RNA genes.

A special class of viruses called retroviruses carry RNA genes which they transcribe into DNA genes, and then have them transcribed back into RNA.  Their retro transcription mechanism is notoriously error prone which causes a very high spontaneous mutation rate within a single host.  This is one of the reasons that HIV/AIDS is so difficult to combat.  The gene transcription errors and spontaneous mutations becomes a virulence mechanism.

CliffordK, Wed, 1st Dec 2010

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