Transversions
Tranversions are mutations that result in a change from a pyrimidine to a purine, or from a purine to a pyrimidine in deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
DNA nucleotides, each contain one of four nitrogenous (nitrogen containing) bases: adenine (A), thymine (T), guanine (G), and cytosine (C). RNA molecules contain uracil (U) in the place of thymine. In DNA, the pyrimidines, single-ring nitrogenous basic compounds, are cytosine and thymine. Because uracil substitutes for thymine in RNA, the pyrimidines in RNA are cytosine and uracil. For both DNA and RNA the purines, nitrogenous double-ringed basic compounds are adenine and guanine.
The bonding between DNA strands, or between an RNA molecule and a DNA strand, or between two RNA molecules is very specific. Adenine always bonds with thymine in DNA strands (A-T base pairing) or, in the alternative, adenine bonds with the substituting uracil in RNA molecules to form a stable A-U bond. In both DNA and RNA, cytosine always bonds with guanine (C-G). Such pairs are pairing between a purine and a pyrimidine. This is critical to the structural integrity of the DNA helix because only a bond between a purine and a pyrimidine is chemically stable enough and structurally the right size to create the double helix.
Mutations are changes nucleotide sequence of the genome. If the normal sequence of a DNA triplet (a set of three bases upon which codons are constructed that ultimately direct the order of insertion of amino acids in the polypeptide chain or direct other processes during translation) was ATC the normal codon for that sequence would have a UAG sequence. The UAG sequence normally instructs the termination of protein synthesis. For example, if a transversion mutation were to take place in the second position of the ATC sequence, either the purine, adenine, or guanine would be substituted for the pyrimidine thymine. Such a transversion mutation would result in either a sequence of AAC or AGC. Instead of creating a stop codon, these mutated sequences would, respectively, produce UUG and UCG sequences. Instead of ordering the end of translation, the UUG sequence would order the insertion of the amino acid leucine and the UCG codon would order the insertion of amino acid serine. In either case, the protein structure would be altered. The alteration of one amino acid for another, or the production of a protein chain that is too long or too short can have a dramatic effect on the functional ability of the protein product.
Transitions (mutations involving the substitution of one purine for another purine, e.g., thymine substituting for cytosine in a DNA molecule or adenine substituting for guanine) are far more common than are transversions.
This is the complete article, containing 442 words
(approx. 1 page at 300 words per page).
