Role Of Dna Mutations In Production Of Genetic Diversity Biology Essay


DNA, the fundamental blueprint of all life is coded in the precise sequence of its nucleotide pairs; a mutation is a change in this sequence. Many new phenotypes result from new genes being produced by structural changes (mutations) of existing genes (Mayr, E. 1976). It is a species ability to have genetic diversity in a population that drives its evolutionary success, and mutations play a vital role in creating that diversity, (Levitan, M., 1988). Without this diversity organisms would be at a great disadvantage. If for instance, a deadly virus was introduced into a population without diversity, all individuals would die. However since each individual in a population is genetically different at least some should survive (Devlin, T. 2006). Generally mutations are the result of a failure of the cells mechanisms to repair DNA damage due to replication errors or environmental mutagens (Huxley, J., Hardy, A., Ford, E., 1954). Most of these errors have no effect as they change non-coding introns however some (those that change the coding exons) can have a negative effect on the organism and very few can yield a positive mutation (Campbell, N., Reece, J. 2005). Since it takes so long for evolution based on natural mutations there have been ways to speed up the production of genetic diversity by changing existing known gene sequences in organisms (Murray, R. et. Al. 2003). Organisms with artificially altered DNA are referred to as genetically modified organisms, which have great potential even with their ethical issues (Allan, R. 2004).

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Some mutations occur due to failure of repair mechanisms to correct mistakes, or if wrong nucleic acid tautomers are added which can't be recognized by the repair mechanisms as wrong, but can cause mistakes in replication. (Levitan, M. 1988). The primary repair mechanism lies in DNA polymerase. It can only add a nucleotide to a growing strand if the previous nucleotide is properly base paired, it will stall until it removes the mismatched nucleotide (Hedrick, W. 1997). The accuracy of DNA replication in E. coli by DNA polymerase III holoenzyme is one pairing mistake in every one hundred thousand base pairs, so with 3 million base pairs this results in a significant percentage of genes which have errors every division. So it has its own proofreading activity which identifies copying errors and corrects them. It is called 3'to 5' exonuclase and makes on average 1 error in every 10 billion nucleotides polymerized (Hine, R., Martin, E., 2004). Even an efficient repair system will lead to an accumulation of mutations from replication errors which is why there is genetic diversity. This leads to new genes being formed some of which are beneficial and will be passed on to increase the genetic diversity of a population. Even with numerous repair systems working to maintain an identical code mistakes are still made (mutations) (Campbell, N., Reece, J. 2005).

Most mutations that are not caused by replication errors are caused by substances produced in normal metabolism. These are known as spontaneous mutations. Most are due to substitutions or deletions of one or more nucleotides; which are known as point mutations (Hedrick, W. 1997). They can either result in the incorporation of the wrong amino acid in the chain and consequently produce a faulty protein (missense mutation); convert an amino acid coded codon into a stop codon which prematurely stopping synthesis, this has a dramatic effect as it shortens the polypeptide so forms a different protein (nonsense mutation); or have no effect on the chain as the substituted nucleotide still codes for the same amino acid (Hine, R., Martin, E., 2004). In some cases during meiosis there can be chromosomal mutations which involve rearrangements of whole blocks of genes which results in a change in number and or sequence of whole gene sets. These can be in the form of: Deletion where there is a break at 2 points and the middle bit falls out or the end can fall off; inversion where the middle piece falls out, rotates 180 degrees then rejoins; translocation which is when genes move between different chromosomes; and duplication where a segment is lost from 1 chromosome and is added to its homologue. All of these cause dramatic changes in the DNA of the organism, some chromosomes are deficient, some have too many genes, but in all of these causes the sequence and/or number of genes change (Allan, R. 2004).

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The rarity of any favorable mutation is 1 in 100,000 mutations so as you can guess just waiting for this to occur naturally is very time consuming especially considering the favorable mutation probably wont be the one you want (Huxley, J., Hardy, a., Ford, E., 1954). So scientists have found ways to artificially insert genes into an organism so they can display desired characteristics without waiting or biological wastage. These organisms are known as GMOs (Genetically modified Organism's) and are modified in 1 of 3 ways: 1. A foreign gene is inserted into the organism so it can express that trait (these are known as transgenic animals). 2. A gene is altered to be expressed at a higher level or in a different way (for instance in a tissue that wouldn't normally express it). 3. A gene is deleted or turned off to prevent its expression (like deactivating the ripening gene in a tomato) (Allan, R. 2004).

These techniques raise the possibility of many wonderful things such as engineering strong, insect resistant strains of crops or to produce transgenic animals which may give vital insight into correcting genetic diseases and many other wonderful things (Devlin, T. 2006). Humankind has been improving crops by selective breeding for millennia its time consuming, you don't always get the combinations you want and undesirable genes are transferred along with the advantageous ones. With GMOs you can simply identify useful genes, isolate and separate them and then insert the genes directly into a host instantly creating genetic diversity in a fraction of the time (Hedrick, W. 1997).

Many, if not most mutations have only slight effects on an organism, however the build up of these effects are the fundamental basis of genetic diversity (Mayr, E. 1976). Even though only a tiny fraction of these random mistakes are beneficial, they still provide essential genetic variation a species needs in an ever changing world. Mutations will inevitably cause different degrees of biological advantages or disadvantage on their possessors but without them evolution would be impossible. This is why even thought there are many repair mechanisms in place for DNA to stay the same it is evolutionary beneficial to maintain a small amount of mistakes or mutations to perpetuate diversity within a species (Huxley, J., Hardy, a., Ford, E., 1954). It is with this view that genetic engineers work to artificially alter organisms in hopes of giving instant variations of stronger organisms with hopes of helping many people which would not be possible otherwise. Overall DNA mutations are fundamentally the mechanism by which genetic diversity is produced in a population. This diversity means that even with intense selection pressures at least some of the species should survive to pass their DNA to future generations (Devlin, T. 2006).