(iii) For a particular amino acid more than one codon can
be used. This is called degeneracy of codon.
(iv) AUG is the initiation codon and UAG does not code for
any amino acid hence, is the stop codon which signals
termination of polypeptide synthesis.(b) Three types of RNA involved in the process of protein
synthesis are mRNA, tRNA and rRNA.
(i) mRNA brings coded information from DNA and takes
part in its translation by bringing amino acids in a
particular sequence during the synthesis of polypeptide.
Codons of mRNA are recognised by anticodons of their
adapter molecules (tRNAs → aa-tRNAs). Translation
occurs over the ribosomes. The same mRNA may be
reused time and again. In the form of polysome, it can
help synthesise a number of copies simultaneously.
(ii) tRNAs pick up particular amino acids (at CCA or 3′
end) in the process called charging. The charged
tRNAs take the same to mRNA over particular codons
corresponding to their anticodons.
(iii) rRNA binds with protein and give rise to ribosomes.
Protein synthesis occurs over the ribosomes. Ribosomes
are, therefore, also called protein factories.
OR
(a) Differences between adaptive radiation and adaptive
convergence are as follows :
Adaptive radiation Adaptive
convergence
(i) Origin of a variety
of species from a
common ancestral
form.Independent
development of similar
adaptive functional
structures in unrelated
organisms usually
as an adaptation to
similar environment at
conditions.
(ii) Homologous organs
exhibit adaptive
radiation. E.g.,
evolution of placental
mammals.Analogous organs exhibit
adaptive convergence.
E.g., similar streamlined
body form, fins in fish
and whale.(b) Reproductive isolation is the mechanism which checks the
populations of two different species from interbreeding.
Two main subtypes may be considered under reproductive
isolation : premating isolation and postmating isolation.
I. Premating or prezygotic isolation: The principal factors
operating under this sub-types are:
(i) Mechanical isolation: The morphology of genitalia, or
reproductive organs (of male and female) of the two
populations may be very complicated and unlike; with
the result, copulation between males of one population
and females of another, fails to occur.
(ii) Psychological isolation: The behavioural differences
restrict random mating of male and female individuals
of different species.
(iii) Seasonal isolation : The breeding period of mating
individuals is different for different species.
(iv) Gametic isolation : In free living aquatic forms, where
the fertilisation is external, the gametes produced by
different species usually do not attract each other and
this kind of barrier is known as gametic isolation.
II. Post-mating or postzygotic isolation : The main factors
operating under this sub-type are:
(i) Incompatibility: In some instances, mating takes place
between populations, but fertilisation may not take
place; or even fertilisation may occur, but no hybrid
progeny will be formed.
(ii) Hybrid inviability: Here, normal fertilisation occurs,
and hybrid offspring is also formed, but the hybrid has
reduced viability. The hybrid inviability may appear at
any stage of development.
(iii) Hybrid sterility : In many cases, hybrids may be vigorous
and live to sexual maturity, but are sterile.
(iv) Hybrid breakdown: In some instances, not only
vigorous F 1 hybrids are produced, but also, these
hybrids produce F 2 individuals of backcross
progeny. Unfortunately, hybrid breakdown results
in the F 2 and backcross generations, as these
individuals have reduced vigour of fertility or both.
It is very significant as it is the main mechanism to give
rise to a new species.
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