Pyrimidine-purine transversions

A transversion is a point mutation that replaces a purine-pyrimidine base pair with a pyriinidine-purine base pair. For example, an A-T base pair becomes a T-A or a C-G base pair. 

TRANSVERSION MUTATION A base-pair substitution mutation in which the purine pyrimidine base-pair orientation Is reversed, as in adenine thymine - thymine adenine. 

There are two mutation processes where mispairing due to enol/imino tautomeric forms could be involved. In one, transition, a purine is replaced by another purine or a pyrimidine by another pyrimidine in the other, transversion, a purine/pyrimidine or pyrimidine/purine exchange takes place. In these mutations, formation of base pairs of the type illustrated in Fig. 20.7 is postulated. 

Point mutations are changes to a single base. They are classified as either transition mutations or transversion mutations (Fig. 65.1). Transitions are an interchange of a purine for a purine or a pyrimidine for a pyrimidine. Transversions are an interchange of a purine for a pyrimidine. 

Resistance to certain antibiotics can arise as a consequence of mutations to chromosomal genes because of changes in the DNA sequence. Mutations can occin due to single base pair changes. Transitions involve the substitution of one purine (A or G) for another and therefore one pyrimidine (C or T) for another. Transversions involve a change from a pyrimidine to a purine and vice versa. Frameshift mutations occin when one or... 

Mutations may be produced in many ways. Bases may be deleted or new ones may be inserted more frequently an existing base may be chemically modified so that on replication, improper base pairing will cause a different base to appear at the modified position. The latter type of mutation is called a replacement. When a purine is replaced by another purine or a pyrimidine by a different pyrimidine, the change is called a transition. A transversion is a change from pyrimidine to purine or purine to pyrimidine. 

Point mutations can occur when one base is substituted for another (base substitution). Substitution of another purine for a purine base or of another pyrimidine for pyrimidine is called a transition, while substitutions of purine for pyrimidine or pyrimidine for purine are called transversions. Both types of base substitution have been identified within mutated genes. These changes lead to a codon change which can cause the wrong amino acid to be inserted into the relevant polypeptide and are known as mis-sense mutations. Such polypeptides may have dramatically altered properties if the new amino acid is close to the active center of an enzyme or affects the three-dimensional makeup of an enzyme or a structural protein. These changes, in turn, can lead to change or reduction in function, which can be detected as a change in phenotype of the affected cells. 

The base substitutions can be either a change from one purine or pyrimidine to another, which is a transition or a change of a purine for a pyrimidine and vice versa, which is a transversion. A chemical change in the base or formation of an adduct, which changes the nature of the base(s), could cause a substitution at replication. The position of the adduct on the particular DNA base would determine the type of mutation. 

In a transversion a purine in one chain is replaced by a pyrimidine, while the pyrimidine in the complementary chain is replaced by a purine ... 

Transversion mutations convert purines to pyrimidines or vice-versa. For example, conversion of an A-T base pair to a T-A or C-G base pair is a transversion. Ultraviolet light can cause transversions, although not exclusively. Frameshift mutations result from the insertion or deletion of a single base pair. These are often due to planar intercalating mutagens, which insert into the double helix. More extensive deletions and insertions are common events, as shown in Figure 8-13. 

What happens if the following transversion mutations (change of a pyrimidine to a purine or the reverse) occur in the coding strand of a DNA molecule ... 

The 12 possible types of nucleotide substitution can be treated differently (assuming nonsymmetry of change, e.g., the frequency of A to C does not equal that for C to A) or treated equally, or any combination of these substitutions can be grouped. One obvious division of base substitutions is to treat transitions (changes of purine to purine or pyrimidine to pyrimidine) separately from transversions (change of purine to pyrimidine or vice versa). Insertion/deletion events can also be treated as a separate type of mutation. Additionally, nucleotide substitutions can be preferentially treated by a combination of position and mutation (e.g., transversions occurring in the first and second codon positions). 

Mutations have been found in primary untreated, local and metastatic hormone-refractory prostate cancer as well as in LNCaP cells. However, mutations seem to be more common in advanced and hormone-refractory prostate cancers (C25, E5, L4, N3, S30, S31, Tl, T5). This point is still debated. Transition mutations, where a purine is replaced by a purine or a pyrimidine by a another pyrimidine, seems to be more common in prostate cancer than transversion mutations, where a purine is converted to a pyrimidine or vice versa. It was proposed that this may be due to endogenous carcinogens (HI). 

The substitution of one base pair for another is the a common type of mutation. Two types of substitutions are possible. A transition is the replacement of one purine by the other or that of one pyrimidine by the other. In contrast, a transversion is the replacement of a purine by a pyrimidine or that of a pyrimidine by a purine. 

In this type of change, a base, either a purine or a pyrimidine, may be replaced by another. If the replacement base is of the same type, i.e. if a purine is replaced by another purine, this is known as a base-pair transition. If a purine replaces a pyrimidine this is known as a base-pair transversion. 

If the transformation involves the same type of base, that is a purine or pyrimidine is transformed into another purine or pyrimidine, this is termed a transition whereas if a purine is transformed into a pyrimidine or vice versa, this is termed a transversion. 

---