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NEL Molecular Genetics 665

new DNA strand. DNA polymerase III adds nucleotides to a growing strand in only one

direction—the 5
to 3
direction. The phosphate group at the 5
end of a free nucleotide

is connected to the hydroxyl group on the 3
carbon of the sugar on the last nucleotide

in the strand. As a result, one of the new strands will be synthesized continuously as

DNA polymerase III moves in the 5
to 3
direction toward the replication fork. This

strand is called the leading strand.

The other new strand, the lagging strand, is synthesized in short fragments. This

allows the lagging strand to be synthesized in the 5
to 3
direction. RNA primers are

required. To complete the replication of the DNA, the primers are cut out from the lag-

ging strand and are replaced by DNA nucleotides by a different enzyme called

DNA polymerase I.

Another enzyme,DNA ligase, links the sugar–phosphate backbone of the DNA frag-

ments together (Figure 5).

Section20.1

leading strandthe new strand

of DNA that is synthesized towards

the replication fork and

continuously during DNA

replication

lagging strandthe new strand

of DNA that is synthesized away

from the replication fork and in

short fragments, which are later

joined together

DNA polymerase Ian enzyme that

removes RNA primers and replaces

them with the appropriate

nucleotides during DNA replication

DNA ligasean enzyme that joins

DNA fragments together

DNA Repair

As complementary strands of DNA are synthesized, both DNA polymerase I and III act

as quality control checkers by proofreading the newly synthesized strands. When a mis-

take occurs, the DNA polymerases backtrack to the incorrect nucleotide, cut it out, and

then continue adding nucleotides to the complementary strand. The repair must be

made immediately to avoid the mistake from being copied in later replications. Other DNA

repair mechanisms can correct any errors that were missed during proofreading.

DNA polymerase I

direction of synthesis DNA polymerase III

DNA polymerase III adds

nucleotides to the

primers to form short

fragments of DNA.

DNA polymerase I

removes the RNA

primers and replaces

them with DNA

nucleotides. A nick

is left between

fragments.

DNA ligase joins the

fragments together.

RNA primer

5 

3 

5 

3 

5 

3 

3 

5 

3 

5 

3 

5 

RNA primer DNA fragment

DNA ligase

Figure 5

Building the lagging strand

Okazaki Fragments

The short fragments that are

synthesized to form the lagging

strand during DNA replication are

called Okazaki fragments. They

were named after Reiji Okazaki,

who first described them in the

1960s.

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