Mutations can be forward and reverse. Forward mutations represent
lead to hereditary changes in the genes of naturally existing species, and
the reverse — to restoring the initial form which can be full or partial. If the
mutations are connected with a loss of genetic material, then a reverse
mutation is impossible.
Also, there exist other classifications of the mutations — generative
and somatic, dominant and recessive, vital and lethal, fertile and sterile,
etc. They have a rather formal character.
Molecular Bases and Mechanisms of the Mutation Changes
This problem is the essence of the mutation process. That is why it attracts
the attention of many investigators. Its elucidation will answer to a number
of questions about the mechanisms of realizing the changes in living
organisms.
Though suddenly arising, the mutation changes are linked with some
processes that prepare their appearance. Undoubtedly these processes are
related to the hereditary material, since the mutations are inherited. The data
show that a considerable part of the mutations are a result of changes in the
base sequence of DNA-molecule. Most of them are obtained as a result of
investigations on phages and bacteria, but it is considered that their
mechanisms are universal in the living nature. For the present, best studied
are the molecular processes leading to the appearance of point mutations.
That is why, they will be considered in more detail.
Point mutations, as it was already mentioned, are due to changes of
single nucleotides in the molecule of DNA. The theoretical analysis of Frees
(1959 a, b) has shown that two types of nucleotide substitutions can exist.
In one of them there is realized a substitution of pyrimidine for another
pyrimidine or of purine for another purine. This type of substitutions are
called transitions. They represent a substitutions of the nucleotide pair A
In the other type of nucleotide substitutions a pyrimidine is substituted
for a purine, or a purine is substituted for pyrimidine. This type of
substitutions are called transversions. Here, the possible substitutions are
These substitutions can lead to arising of so-called “prohibited” or
“wrong” nucleotide base pairs. If some base is in a rarely met tautomeric
form, it can make “incorrect” hydrogen bonds with another base: T—G,
C—A, A—C and G—T (Fig. 2–77).
The appearance of “incorrectly” bound base pairs in DNA that leading
to replication and mutation changes of the genetic material, can be caused