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

DNA REPLICATION MECHANISMS
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gamete
gamete
zygote
germ-line cells
zygote
germ-line cells
Figure 5–1 Germ-line cells and somatic
cells carry out fundamentally different
functions. In sexually reproducing
organisms, the germ-line cells (red)
propagate genetic information into the next
generation. Somatic cells (blue), which form
the body of the organism, are necessary
for the survival of germ-line cells but do not
themselves leave any progeny.
somatic cells
somatic cells
MOTHER
DAUGHTER
with a large number of genes (germ-cell stability) and for the prevention of cancer
resulting from mutations in somatic cells (somatic-cell stability), multicellular
organisms like ourselves depend on the remarkably high fidelity with which their
DNA sequences are replicated and maintained.
Summary
MBoC6 m5.01/5.01
In all cells, DNA sequences are maintained and replicated with high fidelity. The
mutation rate, approximately one nucleotide change per 10 10 nucleotides each time
the DNA is replicated, is roughly the same for organisms as different as bacteria and
humans. Because of this remarkable accuracy, the sequence of the human genome
(approximately 3.2 × 10 9 nucleotide pairs) is unchanged or changed by only a few
nucleotides each time a typical human cell divides. This allows most humans to
pass accurate genetic instructions from one generation to the next, and also to avoid
the changes in somatic cells that lead to cancer.
DNA REPLICATION MECHANISMS
All organisms duplicate their DNA with extraordinary accuracy before each cell
division. In this section, we explore how an elaborate “replication machine”
achieves this accuracy, while duplicating DNA at rates as high as 1000 nucleotides
per second.
Base-Pairing Underlies DNA Replication and DNA Repair
As introduced in Chapter 1, DNA templating is the mechanism the cell uses to copy
the nucleotide sequence of one DNA strand into a complementary DNA sequence
(Figure 5–2). This process requires the separation of the DNA helix into two template
strands, and entails the recognition of each nucleotide in the DNA template
strands by a free (unpolymerized) complementary nucleotide. The separation of
template S strand
5′ 3′
S strand
5′ 3′
3′ 5′
S′ strand
parent DNA double helix
3′ 5′
new S′ strand
new S strand
5′ 3′
3′ 5′
template S′ strand
Figure 5–2 The DNA double helix acts
as a template for its own duplication.
Because the nucleotide A will pair
successfully only with T, and G only with
C, each strand of DNA can serve as
a template to specify the sequence of
nucleotides in its complementary strand by
DNA base-pairing. In this way, a doublehelical
DNA molecule can be copied
precisely.