Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

240 Chapter 5: DNA Replication, Repair, and Recombination
the DNA helix exposes the hydrogen-bond donor and acceptor groups on each
DNA base for base-pairing with the appropriate incoming free nucleotide, aligning
it for its enzyme-catalyzed polymerization into a new DNA chain.
The first nucleotide-polymerizing enzyme, DNA polymerase, was discovered
in 1957. The free nucleotides that serve as substrates for this enzyme were found
to be deoxyribonucleoside triphosphates, and their polymerization into DNA
required a single-strand DNA template. Figure 5–3 and Figure 5–4 illustrate the
stepwise mechanism of this reaction.
The DNA Replication Fork Is Asymmetrical
During DNA replication inside a cell, each of the two original DNA strands serves
as a template for the formation of an entire new strand. Because each of the two
daughters of a dividing cell inherits a new DNA double helix containing one original
and one new strand (Figure 5–5), the DNA double helix is said to be replicated
“semiconservatively.” How is this feat accomplished?
3′ end of strand
5′ end of strand
O
O
P O
O
H O
2 C
_ O
C G
O
CH 2
O
P
O
_ O O
PRIMER
STRAND
O
_ O P
O
H 2 C
O
O
A
T
O
CH 2
O
P
O
_ O O
TEMPLATE
STRAND
OH
CH 2
P
O
_ O P O
O _
O
P O
O _
3′ end of strand
O
P O CH 2 O
O _
C
G
O
O
P
O
_ O O
pyrophosphate
OH
incoming deoxyribonucleoside triphosphate
A
O
CH 2
O
_ O O
O
T
O
O
CH 2
_ O
O
P
O
5′ end of strand
Figure 5–3 The chemistry of DNA synthesis. The addition of a deoxyribonucleotide to the
3ʹ end of a polynucleotide chain (the primer strand) is the fundamental reaction by which DNA is
synthesized. As shown, base-pairing between an incoming deoxyribonucleoside triphosphate and
an existing strand of DNA (the template strand) guides the formation of the new strand of DNA
and causes it to have a complementary nucleotide sequence. The way in which complementary
nucleotides base-pair is shown in Figure 4–4.