|
| Point Mutations |
Mutations: What They Are, Their Causes and Effects - an Overview.
There are three types of point mutations, each
arising from the change in a single nucleotide base in DNA. This
error may come about when the DNA is replicating if DNA polymerase
makes but does not catch its own error. More likely though is
that something in the environment causes the point mutation.
Environmental factors such as X-rays, UV radiation or toxic
chemicals can all cause point mutations by disrupting chemical bonding
within DNA. The result is an absent or misplaced nucleotide.
Each of the three point mutations, substitution, insertion and
deletion, has its own effects on the process of protein synthesis.
The littlest of changes in the DNA code can have enormous effects
- in the protein hemoglobin, when a single valine (amino acid) is
changed to another amino acid, glutamic acid, the result is sickle cell
anemia.
DNA is composed of a long string
of nucleotide base pairs. It's housed in the nucleus, but
proteins are assembled out in the cytoplasm at the ribosomes. To
take the message coded on a gene, a section of DNA, it must first be
transcribed into mRNA. To make this mRNA the gene is read
three bases at a time, called triplets. Each triplet on DNA and
the corresponding codon on mRNA code for a specific amino acid.
Point mutations result in a change in the way that the DNA is
read, the amino acid for which it codes and ultimately, a potential
change in the protein for which the gene codes.
The first, and least harmful type of point mutation is substitution. Substitutions only affect a single
triplet and its corresponding amino acid, leaving the rest of the
protein unchanged. There are three outcomes from a substitution
mutation:
- Nonsense - the amino acid that should have been coded for is
changed to a stop code and the protein terminates at this point.
This will likely lead to a useless protein.
- Missense - the change in nucleotide leads for a different amino
acid. This may have little effect, or it may greatly change the
resulting protein.
- Silent - this type of point mutation is the least harmful as
there is no noticeable effect on the protein. As there is
redundancy in the amino acid coding scheme from mRNA this change in
nucleotide base ends up not changing the amino acid coding at all.
Of the three types of point mutations the most
harmful are insertions and deletions. They result in what is
known as a frameshift mutation. When a nucleotide base is either
inserted or deleted it disrupts the amino acids coded for from the site
of the mutation on to the end of the gene.
If the original code was:
- TAC CGG AAT TAT ACG GCA AAA TTC (the spaces added to the code are only to emphasize that DNA is read in triplets)
and a deletion is made at the fifth base (seen above in red), in the second triplet, the resulting frameshift would look like this:
- TAC CGA ATT ATA CGG CAA AAT TC
Likewise, if an insertion were made at this site (in red) instead of a deletion the frameshift might look like this:
- TAC CGA GAA TTA TAC GGC AAA ATT C
Both of these mutations greatly affect each triplet
that follows the point of mutation. For all three lines of DNA
the code remains the same prior to the mutation. Each differs downstream from that nucleotide base. With
the deletion, only the first triplet after the mutation yields the same
amino acid. It is of little consequence that all of the triplets
that follow code for a different amino acid than in the original strand
because the second triplet past the mutation now yields a termination
sequence. The protein, were it translated, would terminate at
this point. Now, with the insertion, the two triplets following the
mutation happen to code for the same amino acids as in the original
strand. However, not only are none of the others correct but the
fourth triplet past the mutation has become a start sequence, coding for
methionine. Such mistakes in the translation of DNA generally
lead to a useless protein. It is rare that such a mutation is
conserved within the code.
Alignment
Tutorial Main
Greengenes Main |
|
|
