DNA Replication
Now that we have
seen how genetic information encoded in the
nucleotide sequences of DNA is translated into
the structures of proteins that perform most
cell functions, we can appreciate the necessity
of the precise copying of DNA sequences during
DNA replication.
The regular
pairing of bases in the double-helical DNA
structure suggested to Watson and Crick that new
DNA strands are synthesized by using the
existing (parental) strands as templates in the
formation of new, daughter strands complementary
to the parental strands.
This base-pairing
template model theoretically could proceed
either by a conservative or a semiconservative
mechanism. In a conservative mechanism, the two
daughter strands would form a new
double-stranded (duplex) DNA molecule and the
parental duplex would remain intact. In a
semiconservative mechanism, the parental strands
are permanently separated and each forms a
duplex molecule with the daughter strand
base-paired to it. Definitive evidence that
duplex DNA is replicated by a semiconservative
mechanism came from a now classic experiment
conducted by M. Meselson and W. F. Stahl.
Copying of a DNA
template strand into a complementary strand thus
is a common feature of DNA replication and
transcription of DNA into RNA. In both cases,
the information in the template is preserved. In
some viruses, singlestranded RNA molecules
function as templates for synthesis of
complementary RNA or DNA strands. However, the
vast preponderance of RNA and DNA in cells is
synthesized from preexisting duplex DNA.
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