6 23 Genes Made of RNA

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Chapter 2 / The Molecular Nature of Genes
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Replicate
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Replicate
Figure 2.15 Replication of DNA. (a) For simplicity, the two
parental DNA strands (blue) are represented as parallel lines.
Step 1: During replication these parental strands separate, or unwind.
Step 2: New strands (pink) are built with bases complementary to
those of the separated parental strands. Step 3: Replication is
finished, with the parental strands totally separated and the new
strands completed. The end result is two double-stranded DNA
duplexes identical to the original. Therefore, each daughter duplex
for DNA. Because one strand is the complement of the
other, the two strands can be separated, and each can then
serve as the template for building a new partner. Figure
2.15 shows schematically how this is accomplished. Notice
how this mechanism, known as semiconservative replication, ensures that the two daughter DNA duplexes will be
exactly the same as the parent, preserving the integrity of
the genes as cells divide. In 1958, Matthew Meselson and
Franklin Stahl demonstrated that this really is how DNA
replicates (Chapter 20).
SUMMARY The DNA molecule is a double helix,
with sugar–phosphate backbones on the outside
and base pairs on the inside. The bases pair in a
specific way: adenine (A) with thymine (T), and
guanine (G) with cytosine (C). The replication of
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gets one parental strand (blue) and one new strand (pink). Because
only one parental strand is conserved in each of the daughter
duplexes, this mechanism of replication is called semiconservative.
(b) A more realistic portrayal of the same process. Here the strands
are shown in a double helix instead of as parallel lines. Notice again
that two daughter duplexes are generated, each with one parental
strand (blue) and one new strand (pink).
DNA is semiconservative, with each strand serving as the template for building a complementary
partner.
2.3
Genes Made of RNA
The genetic system explored by Hershey and Chase was a
phage, a bacterial virus. A virus particle by itself is essentially just a package of genes. It has no life of its own, no
metabolic activity; it is inert. But when the virus infects a
host cell, it seems to come to life. Suddenly the host cell
begins making viral proteins. Then the viral genes are replicated and the newly made genes, together with viral coat
proteins, assemble into progeny virus particles. Because of