Purines transition mutations

TRANSITION MUTATION A base-pair substitution mutation in which the purine pyrimidine base-pair orientation is preserved, as in adenine thymine -+ guanine cytosine. 

Transition Mutation. Occurs when one purine is mutated to another purine or one pyrimidine is mutated to another pyrimidine, that is, A G, G - A, C - T, and T —> C. 

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). 

Transition Mutation - A mutation that changes a purine-pyrimidine base pair to a different purine-pyrimidine base pair. An example is a transition from G-C to A-T. 

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. 

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

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. 

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). 

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. 

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. 

The answer is c. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 245-257. Wilson, pp 151-180.) Point mutations that are frame-shift mutations put the normal reading frame out of register by one base pair. The insertion of an extra base pair or the deletion of one or more base pairs falls into this category. Transitions and transversions are not frame-shift mutations they are substitutions of one base pair for another. Substitutions are the most common type of mutation. In transitions, a purine is replaced by... 

Fig. 4 Percentage of tumor mutations at p53 base pairs with purine on the nontranscribed strand. The strand bias of several types of mutations affecting purines is shown in several common human cancers. A strong strand bias is indicative of a possible perturbation of the transcription-repair complex at an adducted DNA base. CpG transitions show almost equal distribution on both strands. GC to TA transversions show a strong strand bias in most cancers. The strong bias of AT to GC transitions in lung and bladder cancer is a clue to the involvement of carcinogens in the genesis of these mutations.

Transition replacement of a purine by a different purine, or a pyrimidine by a different pyrimidine in the polynucleotide chain of DNA. T. results in a gene mutation. It may occur spontaneously, or it may be promoted experimentally with mutagens. 

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