Human Physiology, 14th edition (2016)

(Tina Sui) #1
Cell Structure and Genetic Control 73

nucleotides, guanine bases pair with cytosine-containing nucle-
otides. Enzymes called DNA polymerases join the nucleotides
together to form a second polynucleotide chain in each DNA
that is complementary to the first DNA strand. In this way, two
new molecules of DNA, each containing two complementary
strands, are formed. Thus, two new double-helix DNA mol-
ecules are produced that contain the same base sequence as the
parent molecule ( fig. 3.24 ).
When DNA replicates, therefore, each copy is composed
of one new strand and one strand from the original DNA mol-
ecule. Replication is said to be semiconservative (half of the
original DNA is “conserved” in each of the new DNA mol-
ecules). Through this mechanism, the sequence of bases in

produced is distributed to the two daughter cells. Replication
of DNA requires the action of a complex composed of many
enzymes and proteins. As this complex moves along the
DNA molecule, certain enzymes ( DNA helicases ) break the
weak hydrogen bonds between complementary bases to pro-
duce two free strands at a fork in the double-stranded mol-
ecule. As a result, the bases of each of the two freed DNA
strands can bond with new complementary bases (which are
part of nucleotides) that are available in the surrounding
environment.
According to the rules of complementary base pairing, the
bases of each original strand will bond with the appropriate
free nucleotides—adenine bases pair with thymine-containing


Figure 3.24 The
replication of DNA.
Each new double helix
is composed of one old
and one new strand. The
base sequence of each
of the new molecules is
identical to that of the
parent DNA because of
complementary base
pairing.

A T

A

A

A

A

A

T

T

T

A

C G

C G

G
G

C
C

T

T

T

A
G

G

G

C
C

A

A

T

T

A

C G

C G

A T
G
G

C
C

C G

T A

A

C
C

C

C

G

C

G

G

G

Region of replication. Parental
DNA is unzipped and new
nucleotides are pairing with
those in parental strands.

Region of completed replication.
Each double helix is composed
of an old parental strand (light
purple) and a new daughter
strand (blue). The two DNA
molecules formed are
identical to the original
DNA helix and to one another.

Region of parental DNA helix.
(Both backbones are light.)

T

G C

G C
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