Genetic Material Replication
A
crucial characteristic of genetic material is its ability to undergo
replication. The DNA model proposed by Watson and Crick immediately hinted at a
replication mechanism based on complementary base pairing. During replication,
each original DNA strand from the parent molecule serves as a template for the
creation of a new strand in the daughter molecule.
This
process of DNA replication is termed semi-conservative replication because each
daughter molecule retains one of the original strands. The replication process
involves the following steps:
Unwinding: The parental DNA strands are unwound and separated, breaking the
weak hydrogen bonds between the paired bases. The enzyme helicase is
responsible for this unwinding.
Complementary
Base Pairing: New complementary
nucleotides, available within the nucleus, are precisely positioned through
complementary base pairing.
Joining: Complementary nucleotides link together to form new strands. Each
daughter DNA molecule consists of one original strand and one newly synthesized
strand. The enzyme complex DNA polymerase facilitates both complementary base
pairing and strand joining.
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| Semiconservative replication of DNA (simplified) |
Meselson and Stahl's DNA Replication
Experiment
The
concept of semi-conservative replication was empirically validated by Matthew
Meselson and Franklin Stahl at the California Institute of Technology in 1958.
They employed centrifugation to separate particles from the suspending fluid
within tubes. Meselson and Stahl exploited the principle that DNA molecules
with varying densities could be separated through centrifugation. DNA
containing heavy nitrogen (15N), with an atomic weight of 15, is the densest,
while DNA containing light nitrogen (14N, with an atomic weight of 14) is the
least dense. Hybrid DNA molecules, with one heavy and one light strand, exhibit
intermediate density.
The
experimental procedure involved growing bacteria in a medium containing 15N to
ensure the presence of exclusively heavy DNA molecules. The bacteria were then
transitioned to a medium containing 14N. After one division, the cells
contained only hybrid DNA molecules. Following two divisions, the DNA molecules
split into halves – half light and half hybrid. These results precisely matched
the expectations for semi-conservative DNA replication.
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| Meselson and Stahl's DNA Replication Experiment |
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