103
MOLECULAR BASIS OF INHERITANCE
in some viruses, RNA is the genetic material (for example, Tobacco Mosaic
viruses, QB bacteriophage, etc.). Answer to some of the questions such as,
why DNA is the predominant genetic material, whereas RNA performs
dynamic functions of messenger and adapter has to be found from the
differences between chemical structures of the two nucleic acid molecules.
Can you recall the two chemical differences between DNA and RNA?
A molecule that can act as a genetic material must fulfill the following
criteria:
(i) It should be able to generate its replica (Replication).
(ii) It should be stable chemically and structurally.
(iii) It should provide the scope for slow changes (mutation) that
are required for evolution.
(iv) It should be able to express itself in the form of 'Mendelian
Characters’.
If one examines each requirement one by one, because of rule of base
pairing and complementarity, both the nucleic acids (DNA and RNA) have
the ability to direct their duplications. The other molecules in the living
system, such as proteins fail to fulfill first criteria itself.
The genetic material should be stable enough not to change with
different stages of life cycle, age or with change in physiology of the
organism. Stability as one of the properties of genetic material was very
evident in Griffith’s ‘transforming principle’ itself that heat, which killed
the bacteria, at least did not destroy some of the properties of genetic
material. This now can easily be explained in light of the DNA that the
two strands being complementary if separated by heating come together,
when appropriate conditions are provided. Further, 2
'
-OH group present
at every nucleotide in RNA is a reactive group and makes RNA labile and
easily degradable. RNA is also now known to be catalytic, hence reactive.
Therefore, DNA chemically is less reactive and structurally more stable
when compared to RNA. Therefore, among the two nucleic acids, the DNA
is a better genetic material.
In fact, the presence of thymine at the place of uracil also confers
additional stability to DNA. (Detailed discussion about this requires
understanding of the process of repair in DNA, and you will study these
processes in higher classes.)
Both DNA and RNA are able to mutate. In fact, RNA being unstable,
mutate at a faster rate. Consequently, viruses having RNA genome and
having shorter life span mutate and evolve faster.
RNA can directly code for the synthesis of proteins, hence can easily
express the characters. DNA, however, is dependent on RNA for synthesis
of proteins. The protein synthesising machinery has evolved around RNA.
The above discussion indicate that both RNA and DNA can function as