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664 Chapter 20 NEL

Separating the DNA Strands

The two strands of the DNA helix cannot simply pull apart because they are tightly held

together by the hydrogen bonds between bases and by the twists of the helix. The enzyme

DNA helicaseunwinds the helix by breaking the hydrogen bonds between the com-

plementary bases. As this happens, the bonds between bases tend to reform. To prevent

this, proteins bind to the separated DNA strands, helping to hold them apart. The two

strands are now separated along part of the DNA molecule and are the template strands

for the next step in replication. The point at which the two template strands are separating

is called the replication fork. One template strand runs in the 3
to 5
direction in rela-

tion to the replication fork, while the other runs in the 5
to 3
direction (Figure 4).

old strand

semiconservative

replication

Key

new strand

Figure 3
DNA replicates semiconservatively.
Each daughter molecule receives
one strand from the parent
molecule plus one newly
synthesized strand.

DNA helicasethe enzyme that
unwinds double-helical DNA by
disrupting hydrogen bonds

3 


3 
 5 

3 

3 

3 

5 

5 

5 

5 

leading s

trand

2. proteins


3. DNA helicase


4. primer
   replication fork


5. DNA polymerase III

direction of replication

lagging

strand

Figure 4

1. DNA helicase opens the double helix. 2. Proteins bind to the DNA to keep the two strands
   separate. 3. RNA primers are attached to the template strands. 4. DNA polymerase synthesizes
   the new DNA strands. The leading strand is synthesized continuously, and the lagging strand
   is synthesized in short fragments. DNA polymerase III adds complementary nucleotides in the
   5
   to 3
   direction, using single-stranded primers as starting points. One nucleotide is attached
   to the next by bonding the phosphate on the 5
   end of the new nucleotide to the hydroxyl
   group on the 3
   end of the last nucleotide.

Building the Complementary Strands

The next stage of DNA replication synthesizes two new DNA strands on the template

strands through complementary base pairing. The new strands are synthesized by an

enzyme called DNA polymerase III. This DNA polymerase builds a new strand by

linking together free nucleotides that have bases complementary to the bases in the tem-

plate. A short piece of single-stranded ribonucleic acid, called a primer, is attached to the

template strand. This gives DNA polymerase III a starting point to begin synthesizing the

DID YOU KNOW??

DNA Polymerases
There are several DNA polymerases
in a cell, all with their own role.
Each has a unique name, created
by adding a roman numeral after
"DNA polymerase." The main DNA
polymerase involved in DNA
replication is DNA polymerase III. It
adds the 5
phosphate group of a
free nucleotide to the 3
carbon of
the sugar in the last nucleotide.

DNA polymerase IIIthe enzyme
that synthesizes complementary
strands of DNA during DNA
replication