Page 36 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

6.4 DNA Replication 195

3. Summarize the key factors DNA polymerase requires to Figure 6.18 The model of DNA replication postulated by
replicate DNA. Watson and Crick. Unwinding of the double helix allows each of
4. Outline the steps in the process of DNA replication and the two parental strands to serve as a template for the synthesis of
how they relate to the requirements of DNA a new strand by complementary base pairing. The end result: A
polymerase. single double helix is transformed into two identical daughter
double helixes.
5. Discuss three ways cells preserve the accuracy and
integrity of the genetic information in DNA.
A T
T A
G C
In one of the most famous understatements in the scien- CG
tific literature, Watson and Crick wrote at the end of 1. Original double helix
their 1953 paper proposing the double helix model: “It A T
has not escaped our notice that the specific pairing we A T
have postulated immediately suggests a possible copying T G A C
mechanism for the genetic material.” This copying, as C G
we saw in Chapter 4, must precede the transmission of Template T A Template
chromosomes from one generation to the next via meio- 2. Strands separate. T A
sis, and it is also the basis of the chromosome duplica- A T A T
tion prior to each mitosis that allows two daughter cells 3. Complementary T A T A
bases align opposite
to receive a complete copy of the genetic information in templates. C G C G
a progenitor cell. G C G C
G C G C
T A T A
Overview: Complementary Base 4. Enzymes link T A T A
sugar-phosphate
Pairing Ensures Semiconservative backbone of aligned CG Daughter CG
Replication nucleotides into a G C helixes G C
continuous new strand.
G C G C
In the process of replication postulated by Watson and Crick, C G C G
the double helix unwinds to expose the bases in each strand A T A T
of DNA. Each of the two separated strands then acts as a
template, or molecular mold, for the synthesis of a new sec-
ond strand (Fig. 6.18). The newly replicated strands form as Templates
complementary bases align opposite the exposed bases on New strands
the two parental strands. That is, an A at one position on the
original strand signals the addition of a T at the correspond-
ing position on the newly forming strand; a T on the original
signifies the addition of an A; similarly, G calls for C, and C
calls for G, as demanded by complementary base pairing. two possible alternatives. With conservative replication,
Once the appropriate base has aligned opposite to and one of the two daughter double helixes would consist
formed hydrogen bonds with its complement, enzymes join entirely of original DNA strands, while the other helix
the base’s nucleotide to the preceding nucleotide by a phos- would consist of two newly synthesized strands. With
phodiester bond, eventually linking a whole new line of dispersive replication, both daughter double helixes
nucleotides into a continuous strand. This mechanism of would carry blocks of original DNA interspersed with
DNA strand separation and complementary base pairing blocks of newly synthesized material. These alternatives
followed by the coupling of successive nucleotides yields are less satisfactory because they do not immediately
two daughter double helixes that each contain one of the suggest a mechanism for copying the information in the
original DNA strands intact (this strand is conserved) and sequence of bases.
one completely new strand (Fig. 6.19a). For this reason,
such a pattern of double helix duplication is called semi-
conservative replication: a copying in which one strand of Experiments with Heavy Nitrogen Verify
each new double helix is conserved from the parent mole- Semiconservative Replication
cule and the other is newly synthesized.
Watson and Crick’s proposal is not the only replica- In 1958, Matthew Meselson and Franklin Stahl performed
tion mechanism imaginable. Figures 6.19b and c illustrate an experiment that confirmed the semiconservative nature