Mutations are a random occurrence during DNA replication and the rate of mutation is influenced by external factors such as UV radiation. Mutation occurs spontaneously.
There are different types of mutation:
1. Deletion where one or more nucleotide bases are deleted. AGTCA becomes AGCA
2. Substitution where one or more nucleotide bases are replaced by others. AGTCA becomes AGTCG
3. Addition where one or more nucleotide bases are added as extra. AGTCA becomes ATGTCA
4. Inversion where a section of DNA is inverted e.g. AGTTCATTCCAGG becomes AGTTCCCTTAAGG
5. Duplication where one or more nucleotide bases are repeated e.g. AAGTCG becomes AAGTCGAAGTCG
6. Translocation where one or more nucleotide bases are moved between non-homologous chromosomes e.g. AAGCTT on human chromosome 1 is moved and becomes AAGCTT on human chromosome 3
Since the genetic code is degenerate, it’s possible that a mutation won’t have any effect whatsoever! This represents silent mutations. If 2 different triplet codes translate into the same amino acid, the polypeptide chain will remain unchanged. This of course only applies to substitutions.
Another scenario where a mutation may cause no effect is if it arises in an intron. Since these are removed before mRNA is translated, no mutations would be carried along.
If mutation occurs in a region of DNA that regulates transcription, it could interfere with the transcription process and prevent it, when it should be initiating it.
What happens if a base is deleted or added? The genetic code is non-overlapping, so the error cannot simply be overlooked and the following triplets read correctly. The entire subsequent code will be shifted. This is called a frameshift.
Deletion: AGT GGC TTA… –> lose the first G –> ATG GCT TA…
Insertion: AGT GGC TTA… –> insert an A after the first A –> AAG TGG CTT A…
The code is affected significantly!!! In fact, it may be totally ruined. One way this can happen is by a nonsense mutation which by a frameshift causes the code to arrive at a stop codon earlier than it’s supposed to. This will result in a shorter polypeptide and therefore truncated protein which may malfunction.
A missense mutation is when a substitution changes the amino acid encoded. This does not necessarily impact the overall protein, but it may result in a protein with an altered binding site and therefore affect its activity.
Mutations and Cancer
Cell division is kept in check by two kinds of genes: proto-oncogenes which trigger division, and tumour suppressor genes which inhibit division. Cancer is caused by mutated genes involved in cell division. Mutated proto-oncogenes, called oncogenes, trigger cell division at a far greater rate than normal, thus allowing cells to divide out of control.
Mutated tumour suppressor genes fail to inhibit division any longer, therefore contributing to the growth of cancerous tissue by indefinite division.
Polyploidy and evolution
It is possible that during cell division whole chromosome sets don’t get separated properly. In an event called nondisjunction, chromosome numbers are doubled, resulting in an organism with twice the number of chromosomes it should have.
While plants are very commonly polyploid, it is a rarer occurrence in animals. This phenomenon occurs spontaneously, as well as by design in agriculture. Having multiple sets of chromosomes can bring its advantages and disadvantages. Often polyploids are larger, and in the case of fruit and vegetables get selected by humans for cultivation.
Plants can propagate asexually, so the sterility that results from being polyploid is not necessarily as severe a side effect as it appears. Plants can be cultured clonally because any part of a plant can be used to create more plants, not necessarily just the sexually reproducing parts if present at all.
This is useful because the seeds of polyploid plants e.g. apples will not give rise to the same original apples. They might instead be smaller and more sour. Another upside of polyploid sterility is the possible absence of seeds as seen in many types of fruit. This makes them more marketable for human consumption.
In the history of agriculture, key crops such as wheat and cotton have undergone hybridisation and therefore become polyploid. Through the merger of multiple species of plant, the offspring kept all the genomes of its contributing parents. Therefore, present bread wheat is hexaploid (no less than six sets of chromosomes) while present cotton is tetraploid (four sets of chromosomes).
Hybridisation can produce new kinds of crops with “best of both” properties, such as the large yield of one species plus the pest resistance of another species. This is the case with triticale which is a wheat and rye hybrid.
A potential advantage of being polyploid is that many more copies of the same genes are available for mutation. This means that rounds of experimenting with new outcomes can take place without jeopardising the safe, current function of the gene.
All mutation events, whether at a DNA nucleotide base level, a chromosome level or indeed a whole genome level, are the source of diversity upon which different kinds of evolution can take place. Many mutations are inconsequential as they occur. Over time, those that result in meaningful variations in outcomes at cell, organism or population levels, whether for better or worse, create a shift in the prevalence of said mutations.
Many factors contribute to evolution, and often the explanations for certain alleles (versions of genes created through mutation) being absent or present are not straightforward. Gene products can have different effects as the environment changes, as well as at different times. For example, both susceptibility to AIDS and sickle-cell anaemia are bad on their own, as they cause suboptimal health outcomes. However, when the two diseases (the former being infectious while the latter is genetic) are present in the same individual, the sickle-cell anaemia phenotype confers resistance against AIDS. This is an example of how variables in the biological world of evolution do not necessarily follow a simplified model of idealised organisms evolving in idealised environments.