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Mutation versus polymorphism
Although both mutations and polymorphism occur due to changes in DNA sequence, mutations are rare and lead to abnormal alleles. In other words, a mutation can best be understood and defined as a change in the DNA sequence that occurs in a small portion of the gene population i.e. less than 1% and is normally induced by an external factor outside the cell (Weatherbee et. al., 2005). Conversely, polymorphism is a change in the DNA sequence that occurs in a wider portion of the population i.e. more than 1%. Usually, polymorphisms are associated with healthy species while mutations are often disease-causing (Pfohl-Leszkowicz & Manderville, 2007). Since diseases compromise a persons ability to survive then they will often be rare and hence will be classified as mutations. Therefore, the main distinction between these two terms is in their prevalence.
Types of mutations
Mutations are classified based on different premises. Sometimes this can be based on a structure where they can be: gene point mutations (nucleotides exchanged for others) (Ira et. al., 2009), gene insertions ( nucleotides are added onto DNA through frameshifts or alteration of the splice mutation site), gene deletions(nucleotides are removed DNA) (Thornton & Orengo, 2005), chromosomal amplification (increased numbers of genes caused by greater duplication of genes at different parts of the chromosome) (Wang et. al., 2003), chromosomal translocations (exchange of gene parts between chromosomes which are not homologous), chromosomal deletions (loss of chromosomal parts hence gene loss), chromosomal inversions (a part of a chromosome gets reversed), interstitial deletions (A chromosome loses a part of its DNA thus bringing together different genes). Sometimes mutations may be classified based on function such that anti morphic mutations refer to mutations that go against natural alleles (Coluzzi & Ayala, 2005). Lethal mutations cause death, reversion mutations restore original phenotypes, amorphic mutations lead to loss of functions in the allele but are recessive and neomorphic mutations cause abnormal functions but are dominant (Keightley & EyreWalker, 2007), (Whitcomb & Pfutzer, 1999).
Causes of mutations
Mutations occur when mistakes occur in DNA duplication. All cells need to divide and this can only occur when the DNA sequence is replicated into the new cell. Since all DNA is double-stranded, then cell division starts by the separation of the two strands. A DNA polymerase will copy the sequences and thus create two separate DNA molecules. However, when the polymerase goes wrong then a mutation can occur (Howand & Drake, 1999), (Hurles, 2004). The other major cause of mutations is environmental. Chemicals and ultraviolet radiations can change nucleotide bases so that they resemble other nucleotides thus pairing with wrong bases and causing mutations. These external agents can also lead to the breakage of phosphate bonds which lead to different proteins. Examples of such environmental agents include excessive sunlight, smoking, chemicals, and other radiation-emitting sources (Bertam, 2000).
Somatic versus germline cells and clinical significance
Germinal mutations occur in germ cells and are not expressed phenotypically in the person that has the mutation but is passed to the next generation through reproduction. Somatic cell mutations occur in body tissues and are not perpetuated to the next generation (Shibata et. al., 1993). In this case, they may lead to the development of daughter cells with the same mutations thus leading to the appearance of an extension of the concerned individual. One such instance is cancerous tumors. Clinically, this will illustrate whether the mutation will be carried forward or will die with the individual who had the mutation (Royal et. al., 1986).
Importance of phenotypic mutations
It is crucial to know the phenotypic expression of mutation because this determines the prevalence of disease or variation in appearance. It is usually through the phenotypic expression that one can see that genetic mutations have taken place (Sawyer et. al., 2007). Furthermore, these phenotypic expressions allow one to predict inheritance patterns since individuals with mutations may not phenotypically express changes in their genotype but these may be seen in their offspring. The clinical significance allows practitioners to trace inherited diseases such as Huntingtington disease which is inherited and dominant and others like cystic fibrosis which are recessive (Gerstein & Harrison, 2002). Therefore offspring can receive medical assistance easily if a family history of diseases is well known. Suspected diseases can be detected more accurately if such phenotypic information is known about parents.
References
Bertam, J. (2000). Molecular biology of cancer. Molecular Aspects Medical Journal, 21(6), 167-223.
Coluzzi, M. & Ayala, F. (2005). Chromosome speciation: humans, mosquitoes and drosophila. Proc Natl Acad Sci, 102(1), 6535-42.
Gerstein, M. & Harrison, P. (2002). Studying genomes through the aeons: protein families and pseudogenes. Molecular biology Journal, 318(5), 1155-1174.
Keightley, P. & EyreWalker, A. (2007). The distribution of fitness effects of new mutations. Genetics, 8(8), 610-618.
Howand, J. & Drake, J. (1999). Mutation rates among RNA viruses. Proc Natl Acad Sci, 96(24), 13910-3.
Hurles, M. (2004). Gene duplication: the genomic trade in spare parts. PLOS biology, 2(7), 206.
Ira, G., Rosenberg, S., Lupski, J., Hastings, P. (2009). Mechanisms of change in gene copy number. Nature reviews genetics, 10(8), 551-564.
Pfohl-Leszkowicz, A. & Manderville, R. (2007). An overview on toxicity and carcinogenicity in animals and humans. Mol Nutr Food Jnl, 51(1), 61-99.
Royal, A., Langelier, Y. & Pilon, L. (1986). Herpes simplex virus type 2 mutagenesis. Molecular Cell Biology, 6(8), 2977-2983.
Sawyer, S., Zhang, Z., Hartl, D. & Parsch, J. (2007). Prevalence of positive selection among neutral amino acid replacements in Drosophila. Proc Natl Acad Sci, 104(16), 6504-6510.
Shibata, D., Perucho, M., Malkhosyan, S., Peinado, M. & Ionov, Y. (1993). Ubiquitous somatic mutations in simple repeated sequences reveal colonical carcinogenesis. Nature Journal, 363(6429), 558-561.
Thornton, J. & Orengo, C. (2005). Protein families and their evolution. Annual Review Biochemistry, 74(867-900).
Weatherbee, S., Genier, J., Carroll, S. (2005). From DNA to diversity: molecular genetics and the evolution of animal design. Oxford: Blackwell.
Wang, W., Thornton, K., Betran, E. & Long, M. (2003). The origin of new genes. Natural Review Genetics, 4(11), 865-875.
Whitcomb, D. & Pfutzer, R. (1999). Trypsinogen mutations in chronic pancreatitis gastroenterology, Mol Biol,117, 1507-1508.
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