Mutation

Heritance is based on genes that are faithfully transmitted from parent to offspring during
reproduction. The genes are located on chromosome which are duplicated and passed to progeny
through gamete. On very rare occasion, there are sudden heritable change in the genetics materials
that is the chromosomal DNA such changes are called mutation.
Mutation is therefore any chemical or physical change that alter the sequence of bases in a DNA
molecule. Organism exhibiting a novel phenotype as a result of presence of mutation is refer to as
mutant. Mutation can also be defined as any sudden heritable change in the genotype of an
organism that is not explainable by recombination of pre-existing genetic variability. It is the
ultimate source of all genetic variation and it provide the raw materials for evolution. Evolution is
said to occur by natural selection of mutants and not by direct mutation. Majority of mutation are
changes of individual genes (gene mutation) but some are gross structural alteration of
chromosome (inversion, translocation) or changes in number of whole chromosome per nucleus
(polyploidy) majority of mutation are deleterious upsetting the balance mechanism of embryonic
development of the organism. It should however be noted that without mutation all genes will exist
in only one form. Organism will not be able to evolved and adapt to environmental changes.
Therefore, some level of mutation is essential to provide new genetic variability to allow organism
adapt to new environment.
General causes of Mutation
Chromosomes undergo complex reaction during their duplication, segregation and differentiation.
Their structure can therefore be altered by many physical and chemical reaction since the
information strand in the chromosome consist mostly of DNA. All reagent which can react with
DNA will most likely cause alteration in the information carried by that DNA. Example are
a) Physical: this includes mechanical tearing apart of DNA, cutting by ionizing radiation or
32 p decay, non-disjunction of chromosome and high temperature.
b) Chemical: this include alteration or removal of DNA bases, incorporation of altered bases,
intercalation of oligo cyclic aromatic compound and alteration of DNA backbone.
c) Enzymic: this involves production of chemical affecting DNA replicating system, mistake
in recombination or repair.

Molecular basis of Mutation

Structure of bases in DNA are not static, hydrogen can and does move from one position to
another e. g from an amino group to a ring N such behavior is called tautomeric shift. They are
very rare but are of considerable importance in DNA metabolism since they altered the base
pairing potential of the bases. The more stable form of the base-pairs may infrequently undergo
tautomeric shift to the less stable (enol or imino). It should be noted that however that the
unstable exist only for a very short period.
 If bases exist in the rare form at the moment that it is been incorporated into a nascent DNA chain,
a mutation might result. When the bases are present in their rare form, they can form A-C and G-
T base pair instead of A-T and G-C.
Instead of G-C
The net effect of such an event as shown above and subsequent replication require to segregate the
mismatched base pair is an A-T to GC or GC to AT base pair substitution.
Note that mutation rate is increase by any perturbation of DNA replication apparatus or DNA
repair system.

Types of mutation
1. Point mutation: if a mutant behaves as if one biochemical function has been altered and
if it can recombine with all (but one) others, it is called point mutation. They can usually
revert to original phenotype although with a low frequency. Therefore, point mutation is
defined as an alteration of a single nucleotide pair in DNA. Some point mutation may occur
spontaneously while others result from UV-light e. g X-ray and radioisotopes or a variety
of chemical compound. Point mutation causes the synthesis of proteins with defect in their
amino acid sequence. It can arise as result of any of the following.
a. Insertion: this involves the addition of one or more extra nucleotide besides those
already present in a DNA gene e.g
 b. Deletion: it is the opposite of insertion i. e one or more nucleotide is missing from
the normal sequence in DNA

c. Substitution: it is a situation where one more of the original base pair is replace by
another base pair.

The changes in a sequence cause by insertion or deletion occurs during the translation of mRNA
because the codon of mRNA are read without interruption deletion or insertion causes a shift in
the code. This result in frame shift mutation because they altered the reading frame of all base pair
triplet in mRNA e.g
Frame shift mutation usually leads to a large number of change in the amino acid sequence of
protein. Consequently, the protein no longer carried out its biological function. This is usually fatal
to the organism.
Note that point mutation are present among spontaneously occurring mutation. The key factors to
note are:
i. Accuracy of DNA replication machinery
ii. Efficiency of the numerous mechanism that had evolved for repair of damage DNA
iii. Degree of exposure of mutagenic agents present in the environment

2. Transition and Transversion: mutation resulting from tautomeric shift in the bases of
DNA involved the replacement of a purine in one strand of DNA with the other purine and
the replacement of pyrimidine on the complementary strand with the other pyrimidine such
base pair substitution is called transition. Example
The one involving substation of a purine for a pyrimidine and vice versa is called transversion
Example

Transition and transversion are the most frequent spontaneous mutation and are not often recognize
phenotypically. This is due to two reasons
i. For many coding triplets the third base can be replaced by another base without changing
the coding properties of the triplet, the amino acid remains the same.
 ii. Many transition cause amino acid interchange e. g among non-polar amino acid (Ala, Gly,
Ile, Leu, Thr and Val) which do not significantly alter the tertiary folding of the
corresponding protein.

Mutagens
Mutagens are mutation producing agents whose effect was first demonstrated in 1927 by Muller
who use a very large dose of X-ray on Drosophila flies. Other mutagens have since been known
and they include chemicals such as nitrous acid, ethyl methane, 2-aminopurine, 2,6-
diaminopurine, 5-bromo uracil, 5-bromo deoxycytidine. Mutation can also be cause by high energy
radiation like UV and X-ray. Few examples of mechanism of action of mutagen are describe below

1. Oxygen: the effect of mutation process is brought about by non-metabolize oxygen, this
situation occurs for example when there is pretreatment with cyanide which inhibit cellular
oxygen respiration and thereby permit the non-metabolize oxygen to become involved in
mutation process. The effects of oxygen on mutation arises from its utilization in the
formation of peroxide during irradiation. Irradiation ionize water to hydrogen and hydroxyl
radical.

H2O H+ + OH-
irradiation

in presence of O2, H+ may easily produce peroxide

i.e
H + O2 HO2 hydroperoxyl radical
HO2 + H H2O2
2HO2 H2O2 + O2

Peroxide are highly reactive molecules and may be responsible for some observe mutation. The
mutagenic activity of hydrogen peroxide has been shown to be affected by enzyme like catalase
which are produce within the cell so that if an enzyme poison e.g sodium azide or potassium
cyanide are added the mutation rate increase.
2. UV-irradiation: mutagenic effect of UV light was first demonstrated by Alterburg,
although it causes chromosomal abrasion. Its effect is considerab
3.
4. ly milder than X-rays, because it has a wavelength that is too long to produce ions UV
appears to act by affecting only those compounds that absorbed it directly. In the cell direct
 absorption of UV is mainly confined to compound with organic ring structure e.g purines
and pyrimidines.

In vitro studies indicate that thymine dimerization may be the primary mutagenic effect produce
by UV such dimers will distort the DNA helix and interfere with proper replication. Generally,
UV affect system that enable the cell to repair DNA damage.
5. Chemicals: most of chemical mutagen also cause severe skin irritation in mammals and
many are cancer producing, example are nitrous acid, Sulphur mustered, formaldehyde,
diazomethane e.t c.
Note that prediction of mutagenic activity cannot be made on the basis of chemical
structure alone also some compounds affect certain organism but no other while some are
restricted in action to specific development stages or to specific sexes (e.g formaldehyde).
By the 1950s when DNA was fondly identified as genetic material. Agents of mutation that
have direct effect on nucleotide structure were study. Watson and Crick suggested that
mutation could be as a result of occasional changes in the H-bonding properties of the
 nucleotide bases as we have seen this lead to Adenine binding to cytosine instead of its
normal thymine. This is thus a base pair change as illustrated below

Similarly, tautomeric shift may occur in T changing from normal keto form to the rare enol
form. T will now bind with G instead of normal A.

Note that the pairing error arise through the transition of the pairing relationship of one purine base
(e .g Adenine) into the pair form of another purine (e.g Guanine) or of a pyrimidine (e.g thymine)
into that of another pyrimidine (e.g cytosine) because of this they are called Transitional errors.
There can also be Transversional error in which a nucleotide position occupied by a purine (e.g
Adenine) may change into a pyrimidine (e.g cytosine). For both transition and transversion a base
substitution is produce which lead to a new nucleotide sequence. Replication of DNA is necessary
if the errors produce by tautomeric shift are to appear in DNA molecules they are thus classified
as copy error. Similar mistake in copying mechanism also take place when base analogue is
incorporated whose pairing relationship are occasionally ambiguous, a good example is 5-
bromouracil (BU). 5-bromouracil is analogues to thymine and is usually in the keto form, it can
undergo occasional tautomeric shift to enol form that enable it pair with Guanine thus BU may be
incorporated as a pairing mate to A and then produce a G-C substitution for the original A-T or it
may occasionally be incorporated in the enol form as a pairing mate with G and then revert to its
keto form to produce A-T substitution to the original G-C.
Another base analogue 2-amino purine (AP) show similar mutational properties enabling it to be
incorporated as a substitute of A but to pair subsequently with C or to pair initially with C and then
subsequently with T.
In contrast to copy error direct changes in nucleotide structure may be produced by agents such as
nitrous acid and nitrogen mustered. Schuster et al show that nitrous acid act primarily through the
removal of the amino group (i.e deamination) from Adenine and cytosine thereby converting them
to hypoxanthine (H) and uracil (U) respectively. i. e
Hypoxanthine is capable of binding with C thereby substituting a G-C pair for A-T
Similarly, deamination of cytosine

Substitution of T-A pair for C-G

DYES
Acradine dyes like proflavine and acradine orange are mutagen that seems to produce direct effect
on DNA molecules. They act by inserting themselves between two neighboring purine bases in a
single DNA strand. The consequence of such insertion is to causes either the insertion or deletion
of a single nucleotide thereby causing frame shift mutation that when translated into amino acid
sequence result insignificantly altered protein.