How can a single base change in a single gene cause so many effects in an organism?

Question

How can a single base change in a single gene cause so many effects in an organism?

i don't know if it helps, but the article that the group of questions is for is about progeria...

Answer 1

A base change in the DNA changes the amino acid that the DNA triplet sequence (codon) codes for. For example, the codon GTA might code for one amino acid, while GTC codes for another. This new (mutated) amino acid no changes the structure of the protien that it is a part of, which can have wide ranging effects depending on the function of that protien.

An example of this is the disease Phenylketonuria (PKU) where the enzyme for converting phenylalanin to tyrosine is mutated. This leads to high levels of toxic metabolites of phenylalanin which can cause mental retardation, characteristic facial features and kidney damage.

Answer 2

It is pretty amazing, isn't it?  As a matter of fact, this is the basis of all evolution--small mutations that may or may not benefit the organism.  If they do have a benefit, they will tend to make that organism more successful.  If they're not, the organism will either not survive or will tend to be less successful at reproduction.

One thing to realize is that a single base change doesn't always make any difference at all.  The genetic "code" is not quite that rigid.  For one thing, there is more than one triplet code ("codon") for most of the amino acids, so changing one base to another may not change which amino acid is placed there at all.

Also, there are many proteins that are made in slightly different ways in different people and it doesn't cause a problem at all.  Genes have slightly different forms, called "alleles," and as long as the resultant protein can do its assigned job, no harm is done.  

Sometimes the change is a deletion of that base, and it is easier to understand how that can cause problems--that causes a shift of every "letter" that follows it and usually results in a protein that is substantially different from the normal one.

But you are right--there are quite a number of genetic disorders that can be traced to a single base change.  I actually wasn't aware that they had pinned down any definite gene for progeria (so thank you for prompting me to look that up), but probably the most famous one of all is sickle cell anemia.  That is caused by one single base mutation (alanine to thymine) that changes a glutamate to a valine on the beta-hemoglobin molecule at a key spot.

Something that's also interesting to note, though, is that the reason that this mutation has probably survived is that people who have one copy of this alternate form of beta-hemoglobin are much more resistant to malaria.  It's only when you have two copies of the aberrant gene that you have full-blown sickle cell anemia.  So sometimes there is an evolutionary reason that these things survive.

Isn't genetics fascinating?