Ans. Watson and Crick (1953) proposed the double helical structure for DNA.
They were also aware about its nature
for replication. During replication, the
two strands of DNA would separate
and act as a template for the synthesis
of new complementary strands.
At the end of replication each daughter molecule of DNA would have one strand
of parental and one newly synthesised strand. Hence, the process was termed
as semiconservative DNA replication.
To prove experimentally that DNA replicates semiconservatively, Matthew Messelson and Franklin Stahl (1958) demon-strated the replication of DNA
in Escherichia coli.
They used isotopes of nitrogen, the normal lighter 14N and heavier N. The heavy DNA molecule with 15N could be differentiated from the normal DNA having 14N by centrifugation in a cesium chloride (CSCI) density gradient.
Messelson and Stahl grew E. coli in a culture medium having 15NH4C1 so
that all lhe cells of E. coli became
labelled with heavy nitrogen (15N).
They then transferred the E. coli cells
into the medium with normal 14NH4Cl. The DNA was extracted from E. coli cells after 20 minutes, because E. coli divides within 20 minutes.
This DNA had a hybrid or intermediate density. Another sample of DNA was also extracted from the E. coli after 40 minutes that is from second generation.
This DNA was composed of equal amounts of hybrid DNA and of light DNA. This proved the DNA replication is semiconservative.
.The code is nearly universal. For example, codon UUU would code for Phenylalamine (phe) from bacteria to human beings.
However, some exceptions to this rule have been recorded in mitochondrial codons and in some protozoans.
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