By Dr Ananya Mandal, MD
The genetic code undergoes several changes during the process of evolution. However, despite such changes, there is preservation of the majority of characteristics of the species.
Mutations are the basis of evolution as well as genetic disorders. Evolution depends on mutations because this is the only way that new alleles are formed. Mutations may be the cause of genetic disorders as well.
During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These are termed mutations. Error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of DNA polymerases.
Types of genetic mutation
Types of mutations include:-
Silent mutations - Here mutations may take place in the vast amounts of DNA that lie between genes or they may occur by encoding for the same amino acid at the site of the mutation.
Point mutations - These affect a single amino acid or short sequence. These are also called single-base substitutions. Missense mutations may change the code for a particular protein. Nonsense mutations may make a change so that the whole chain stops. This mutation codes for a Stop codon.
Snips or SNPs (Single nucleotide polymorphisms) - These are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA base for example nucleotide cytosine (C) replaced with the nucleotide thymine (T) in a certain stretch of DNA.
Deletions of vital segments.
Inversions of segments or exchange of one segment with its consecutive one.
Duplication or triplication of segments of DNA.
Some chemicals may increase the risk of mutations in the genes. These are termed mutagenic. Mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing. UV radiations may also cause mutations.
Mutations in evolution
Mutations are important in evolution as well. Most mutations have little effect on an organism's characteristics or phenotype. The genetic makeup is called the genotype while the actual traits and physical makeup and expression of the genes are called phenotype.
Evolution has been studied in various animals on the basis of genetic changes. Experimental genetic studies in the fruit fly or Drosophila melanogaster have shown that if a mutation changes a protein produced by a gene, about 70 percent of these mutations will be harmful with the remainder being either neutral or weakly beneficial.
Over many generations genetic codes of organisms can change significantly, resulting in the phenomenon of evolution. There is a selection of a beneficial mutations and this is how a species evolves.
New species are also formed through the process of speciation. This means the species are separated from their original ancestors by geographical demarcations and are thus unable to exchange genes with each other. These then branch out into different species.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)