5 Steps to a 5 AP Biology, 2014-2015 Edition

120 ❯ STEP 4. Review the Knowledge You Need to Score High

bonds between the nucleotides and producing the replication fork.Each strand then func-

tions as a template for production of a new double-stranded DNA molecule. Specific

regions along each DNA strand serve as primer sitesthat signal where replication should

originate. Primase binds to the primer, and DNA polymerase,the superstar enzyme of this

process, attaches to the primer region and adds nucleotides to the growing DNA chain in

a 5′-to-3′direction. DNA polymerase is restricted in that it can only add nucleotides to the

3 ′end of a parent strand. This creates a problem because, as you can see in Figure 11.4,

+ + + + Conservative model

+ + + + Semi-conservative model

+ + + + Dispersive model

= DNA not containing DNA of original parent.

= DNA containing DNA of original parent.

Figure 11.3 Three DNA replication models.

Figure 11.4 Semi-conservative DNA replication.

Replication fork Leading strand Lagging strand

3 ′ 5 ′ 3 ′ 5 ′ 3 ′ 5 ′ 3 ′ 5 ′ 3 ′ 5 ′ 3 ′ 5 ′

5 ′ 3 ′ 5 ′ 3 ′ 5 ′ 3 ′ 5 ′ 3 ′ 5 ′ 3 ′