Biology 12

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 233

daughter DNA molecules were each made up of
one parental strand and one new strand. A third
hypothesis, the dispersive theory, proposed that
the parental DNA molecules were broken into
fragments and that both strands of DNA in each
of the daughter molecules were made up of an
assortment of parental and new DNA.

The issue was resolved in 1957 as the result of

an experiment conducted by Matthew Meselson

and Franklin Stahl. The steps in the experiment are

illustrated in Figure 7.22. The two scientists grew a

colony of bacteria in a medium containing^15 N,

a form of nitrogen that has a higher molecular

mass than regular nitrogen (^14 N). As the bacteria

developed, their DNA incorporated the heavier

Figure 7.21The three main theories advanced for DNA
replication. According to the conservative theory (A), the
parent molecule is re-established intact after replication. In
the semi-conservative theory (B), the individual strands of
the parent molecule remain intact but are separated, one

forming half of each of the two daughter molecules. In the

dispersive theory (C), both strands of the parent molecule

are broken into fragments, copied, and then reassembled,

with the fragments shared among parent and daughter

molecules.

A conservative B semi-conservative C dispersive

Figure 7.22The experiments conducted by Meselson and
Stahl tested all three hypotheses for DNA replication. First,
the scientists cultured a colony of E. colion a medium
containing the heavy nitrogen isotope^15 N. As a result, these
bacteria had uniformly heavy DNA. The bacteria were then
transferred to a medium containing the more common and
lighter^14 N, to allow them to incorporate this isotope during

future DNA synthesis. After one replication on the^14 N

medium, results showed a single band with a density that

indicated hybrid DNA. After two replications on the^14 N

medium, results showed two distinct bands, indicating the

presence of equal quantities of both hybrid and regular

DNA. These results supported the semi-conservative

replication hypothesis.

direction of

heavy DNA sedimentation

hybrid DNA

regular DNA

regular DNA

DNA contained

one regular and

one heavy strand

two hybrid DNAs

and two regular

DNAs were formed

hybrid DNA after

one replication

DNA after two

replications

results analysis

regular DNA has

two light strands

(^14) N
heavy DNA has
two heavy strands
(^15) N