Mutation

Insertion and deletion mutations add or remove one or more DNA bases. Insertion and deletion mutations cause frameshift mutations which change the grouping of nucleotide bases into codons. This results in a shift of reading frame during protein translation.

A single nucleotide pair alteration is the cause of sickle cell anemia: the template strand of mutated sickle cell DNA has an A rather than a T. U takes the place of A in the mRNA. A valine amino acid is made in the sickle cell haemoglobin instead of a glutamic acid. GAG which codes for the amino acid glutamic acid is changed to GTG for valine as a result of the mutation in the position of second nucleotide of the codon GAG.

A substitution is a mutation that exchanges one base for another that is a change in a single chemical letter such as switching an A to a G. Such a substitution could change a codon to one that encodes a different amino acid and cause a small change in the protein produced. For example sickle cell anemia is caused by a substitution in the beta-haemoglobin gene which alters a single amino acid in the protein produced. Sickle cell anemia is a disease in which your body produces abnormally shaped red blood cells. The cells are shaped like a crescent or sickle. They do not last as long as normal round red blood cells which lead to anemia.

A frameshift mutation is generally much more serious and will cause a change all the way down a DNA sequence making each codon a different sequence not just in one point or base like a point mutation that would only slightly change the sequence of a single codon. For example consider our DNA sequence is GTA CCT AGG. In a frameshift mutation a whole base would be added somewhere in that sequence making it look something like GTAT CCT AGG. Since it is impossible though to have 4 bases in a codon our body would automatically shift every letter down one even to the next codon in the sequence. In the end the final product would look something like GTA TCC TAG (with the left over G being the first base in the next codon and so on).

It occurs when an organism is exposed to a physical or chemical agent known as mutagen that interacts with DNA to cause a mutation. Induced mutations typically occur at a much higher frequency than spontaneous mutations. The first induced mutations were created by treating Drosophila with X-rays.

Chromosomal mutation is involving the change in chromosome structure. The changes in structure will change the composition or location allele. Polyploidy is an example of chromosomal mutation. Polyploidy is a condition wherein organisms have three or more sets of genetic variation (3n or more).

Mutation is affected by external influences. Mutations can be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural even in the most isolated and pristine environments DNA breaks down.

It is a point mutation involving substitution of one base pair for another by replacement of one purine by another purine and of one pyrimidine by another pyrimidine but without change in the purine-pyrimidine orientation. Here the GC base pair is changed to AT that is the purine G is substituted by another purine adenine and the pyrimidine C is substituted by another pyrimidine thymine. So it is a transition type of mutation. Hydroxylating agents can make such type of transition mutation.

Methylnitrosoguanidine is a biochemical tool used experimentally as a carcinogen and mutagen. Specific mispairing is caused when a mutagen changes a base's structure and therefore alters its base pair characteristics. An example of this type of mutagen is methylnitrosoguanidine which preferentially reacts with some bases and produce a specific kind of DNA damage. Methylnitrosoguanidine is an alkylating agent that adds methyl groups to guanine causing it to mispair with thymine.

This type of mutation is originated from lesions in DNA as well as from replication errors. For example it is possible for purine nucleotide to be depurinated that is to lose their base. This results in the formation of an apurinic site which will not base pair normally and may cause transition type mutation after the next round of replication. Cytosine can be deaminated to uracil which is then removed to form an apyrimidinic site.

Ultraviolet light induces the formation of covalent linkages by reactions localized on the C=C double bonds. These radiations with wavelength above 260 nm are absorbed strongly by bases producing pyrimidine dimers which can cause error in replication if left uncorrected. Two common UV products are cyclobutane pyrimidine dimers (CPDs including thymine dimers) and 6, 4 photoproducts. A cyclobutane pyrimidine dimer (CPD) contains a four membered ring arising from the coupling of the C=C double bonds of pyrimidines. Such dimers interfere with base pairing during DNA replication leading to mutations.

A transition is a point mutation that changes a purine nucleotide to another purine (A ↔ G) or a pyrimidine nucleotide to another pyrimidine (C ↔ T). So here in the given DNA sequence the transition mutation makes changes in the third position where the pyrimidine cytosine is present. As a result this cytosine is substituted by another pyrimidine that is thymine. The DNA base sequence GTC which codes for the amino acid valine is changed to GTT that also codes for the same amino acid valine.

A base analog is a chemical that can substitute for a normal nucleobase in nucleic acids. A common example would be 5-bromouracil (5BU), the abnormal base found in the mutagenic nucleotide analog BrdU. When a nucleotide containing 5-bromouracil is incorporated into the DNA and it is most likely to pair with adenine. It is because 5BU is a base analog of thymine it base pair with cytosine as thymine does.

Moss rose or rose moss, Portulaca grandiflora produces flowers of two colours that is purple and orange as a result of a mutation. The orange is probably the mutant as it is closer to the purple wild type. This is a somatic mutation that may also be passed on in the germ line. Somatic mutation is a genetic mutation that occurs in a somatic cell after conception.

A strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesize an essential compound. This technique distinguishes between mutants and the wild type strain based on their ability to grow in the absence of a particular biosynthetic end product. For example a lysine auxotroph will grow on lysine supplemented media but not on a medium lacking an adequate supply of lysine because it cannot synthesize this amino acid.

A lethal mutation refers to any and all mutations that prevent an organism from reproducing in any way. Lethal mutations do not have to kill the organism to classify as lethal. An example of a lethal mutation in humans is Turner syndrome which is a disorder whereby the 23rd chromosome pair only contains an X chromosome. The person lives but is infertile. Turner syndrome is caused by a missing or incomplete X chromosome. People who have Turner syndrome develop as females.

In genetics a nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon or a nonsense codon in the transcribed mRNA and in a truncated, incomplete and usually nonfunctional protein product. It differs from a missense mutation which is a point mutation where a single nucleotide is changed to cause substitution of a different amino acid. Some genetic disorders such as thalassemia and DMD result from nonsense mutations.

Auxotrophs would be the mutant cells that are not capable of utilizing certain nutrients or synthesizing certain compounds. Auxotroph is a mutant that requires nutritive(s) not needed by the wild type (prototroph). Auxotrophic mutations have been extensively used for the study of biochemical pathways and for the identification enzymes catalyzing particular metabolic steps. In the example of bacteria capable of utilizing lactose as a nutrient source, the ability to lactose is denoted by lac⁺ and the inability to utilize lactose is denoted lac^-.

Induced mutation on the molecular level can be caused by ultraviolet radiation. Two nucleotide bases in DNA that is cytosine and thymine are most vulnerable to radiation that can change their properties. UV light can induce adjacent pyrimidine bases in a DNA strand to become covalently joined as a pyrimidine dimer.

Conditional mutation is a mutation that has wild type or less severe phenotype under certain permissive environmental conditions and a mutant phenotype under certain restrictive conditions. For example a temperature sensitive mutation can cause cell death at high temperature (restrictive condition) but might have no deleterious consequences at a lower temperature (permissive condition).