Thursday , 24 April 2014

KSP Sub Inspector PSI Constable Recruitment 2014 Notification Syllabus Pattern

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Polypeptides would have played only a limited role early in the evolution of life because their structures are not suited to
self-replication in the way that nucleic acid structures are. However, polypeptides could have been included in
evolutionary processes indirectly. For example, if the properties of a particular polypeptide favored the survival and
replication of a class of RNA molecules, then these RNA molecules could have evolved ribozyme activities that
promoted the synthesis of that polypeptide. This method of producing polypeptides with specific amino acid sequences
has several limitations. First, it seems likely that only relatively short specific polypeptides could have been produced in
this manner. Second, it would have been difficult to accurately link the particular amino acids in the polypeptide in a
reproducible manner. Finally, a different ribozyme would have been required for each polypeptide. A critical point in
evolution was reached when an apparatus for polypeptide synthesis developed that allowed the sequence of bases in an
RNA molecule to directly dictate the sequence of amino acids in a polypeptide. A code evolved that established a relation
between a specific sequence of three bases in RNA and an amino acid. We now call this set of three-base combinations,
each encoding an amino acid, the genetic code. A decoding, or translation, system exists today as the ribosome and
associated factors that are responsible for essentially all polypeptide synthesis from RNA templates in modern
organisms. The essence of this mode of polypeptide synthesis is illustrated in Figure 2.8.
An RNA molecule (messenger RNA, or mRNA), containing in its base sequence the information that specifies a particular
protein, acts as a template to direct the synthesis of the polypeptide. Each amino acid is brought to the template attached
to an adapter molecule specific to that amino acid. These adapters are specialized RNA molecules (called transfer RNAs
or tRNAs). After initiation of the polypeptide chain, a tRNA molecule with its associated amino acid binds to the
template through specific Watson-Crick base-pairing interactions. Two such molecules bind to the ribosome and peptidebond
formation is catalyzed by an RNA component (called ribosomal RNA or rRNA) of the ribosome. The first RNA
departs (with neither the polypeptide chain nor an amino acid attached) and another tRNA with its associated amino acid
bonds to the ribosome. The growing polypeptide chain is transferred to this newly bound amino acid with the formation
of a new peptide bond. This cycle then repeats itself. This scheme allows the sequence of the RNA template to encode
the sequence of the polypeptide and thereby makes possible the production of long polypeptides with specified
sequences. The mechanism of protein synthesis will be discussed in Chapter 29. Importantly, the ribosome is composed
largely of RNA and is a highly sophisticated ribozyme, suggesting that it might be a surviving relic of the RNA world.
2.2.5. The Genetic Code Elucidates the Mechanisms of Evolution
The sequence of bases that encodes a functional protein molecule is called a gene. The genetic code that is, the relation
between the base sequence of a gene and the amino acid sequence of the polypeptide whose synthesis the gene
directs applies to all modern organisms with only very minor exceptions. This universality reveals that the genetic
code was fixed early in the course of evolution and has been maintained to the present day.
We can now examine the mechanisms of evolution. Earlier, we considered how variation is required for evolution. We
can now see that such variations in living systems are changes that alter the meaning of the genetic message. These
variations are called mutations. A mutation can be as simple as a change in a single nucleotide (called a point mutation),
such that a sequence of bases that encoded a particular amino acid may now encode another (Figure 2.9A). A mutation
can also be the insertion or deletion of several nucleotides.

KSP Sub Inspector PSI Constable Recruitment 2014

Other types of alteration permit the more rapid evolution of new biochemical activities. For instance, entire sections of
the coding material can be duplicated, a process called gene duplication (Figure 2.9B). One of the duplication products
may accumulate mutations and eventually evolve into a gene with a different, but related, function. Furthermore, parts of
a gene may be duplicated and added to parts of another to give rise to a completely new gene, which encodes a protein
with properties associated with each parent gene. Higher organisms contain many large families of enzymes and other
macromolecules that are clearly related to one another in the same manner. Thus, gene duplication followed by
specialization has been a crucial process in evolution. It allows the generation of macromolecules having particular
functions without the need to start from scratch. The accumulation of genes with subtle and large differences allows for
the generation of more complex biochemical processes and pathways and thus more complex organisms


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