In a silent mutation, a change in one base pair has no effect on the protein produced by the gene.
This is allowed for by the redundancy in the genetic code.
Example (as shown in picture):
Both GGC and GGU code for the amino acid glycine.
Thus, the mutation is “silent,” i.e. causes no change in the final protein product.
Part of figure 17.22, page 313, Campbell's Biology, 5th Edition
1.2. Missense Mutation
In a missense mutation, a change in one base pair causes a single amino acid to be changed in the resulting protein.
The result is called “missense” since the code is now different.
In the following example, GGC has been changed to AGC, resulting in a different amino acid.
Part of figure 17.22, page 313, Campbell's Biology, 5th Edition
The effect of a missense mutation on the protein is unpredictable.
A missense mutation is the cause of the disease, sickle cell anemia.
The effects of a change in one base pair alters one amino acid in the resulting hemoglobin protein, causing red blood cells to take on a strange shape.
The strange-shaped red blood cells cause a variety of phenotypic effects in the individual: this is an example of pleiotropy.
Figures 17.21, 5.19, and 14.5, pages 72, 312, and 254, Campbell's Biology, 5th Edition
Please note: a missense mutation is not always bad.
It could create a protein that improves reproductive success or survivability.
Natural selection could then help increase the frequency of this new allele in the population.
1.3. Nonsense Mutation
In a nonsense mutation, a change in a single base pair creates a stop codon.
Because this kind of mutation creates a stop signal in the middle of a normally functional gene, the resulting protein is almost always nonfunctional, hence the term “nonsense” mutation.
Part of figure 17.22, page 313, Campbell's Biology, 5th Edition
2. Base-Pair Insertions or Deletions (Frameshift Mutations)
Insertions and deletions are additions or losses of one or more nucleotide pairs in a gene.
As you might imagine, these kinds of mutations have a more disastrous effect on the resulting protein than substitutions do.
Because the genetic code is read as a triplet, an insertion or deletion of a number of nucleotide bases that is not a multiple of three can cause a shift in the reading frame.
Insertions and deletions are therefore called frameshift mutations.
Everything downstream from the mutation will be improperly grouped into incorrect codons.
Such mutations can result in an entirely new set of amino acids (extensive missense) and probably also premature termination (nonsense).
Causing Extensive Missense
Part of figure 17.22, page 313, Campbell's Biology, 5th Edition
Causing Immediate Nonsense
Part of figure 17.22, page 313, Campbell's Biology, 5th Edition
Insertion or Deletion of Three Nucleotides
If the insertion or deletion is of a multiple of three, then there will be insertion or deletion of amino acids, but there will not be a frameshift.
The resulting protein can therefore be relatively functional depending on the location of the amino acid in the protein.
Part of figure 17.22, page 313, Campbell's Biology, 5th Edition
What Causes Mutations?
High-energy electromagnetic radiation
Ultraviolet radiation (you should protect yourself from the sun!)
X-rays
Mutagenic chemicals
We can tell if chemicals are mutagenic by using the Ames test.
The Ames test was developed by Bruce Ames at UC Berkeley in the 1970s.
Transcribed but not Translated
No more than 3% of the genome actually codes for proteins. Therefore what is going on with the other 97% of our DNA? Is it just "junk?"
The ENCODE project has discovered that most "junk DNA" is transcribed, or copied, into more RNA molecules than can be accounted for by most prevailing theories has led scientists to the tantalizing possibility that organisms may be defined more by differences in their "junk" than in their actual genes.
The number of different RNAs performing some sort of labor in cells is about 37,000, compared with only 22,000 genes. Instead of running errands, RNA appears to be running the whole show. RNA may be the secret puppetmaster of genes; RNA may represent a command and control system for cells. Perhaps the majority of the human genome is operated by a hidden RNA regulatory system that directs differentiation and development.
