1. Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

2. First Proposal: Watson and Crick’s semiconservative model of replication predicted that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand
Competing models were the conservative model and the dispersive model

3. Figure 16.10 First Second replication replication Parent cell
Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix.
Semiconservative model. The two strands of the parental
molecule
separate, and each functions as a template for synthesis of a new, comple-mentary strand.
Dispersive model. Each strand of both daughter molecules contains
a mixture of
old and newly synthesized
DNA.

4. How complementary strands are arranged will dictate how replication occurs3’ OH 5’ 5’ 3’
DNA strands are antiparallel
One strand is oriented
5’ --> 3’ with the
Complementary strand
oriented in the opposite
direction (3’ --> 5’)
5’ end has a “free” phosphate group
and 3’ end has a “free” hydroxyl group 3’ 5’ 5’ 3’ OH

5. dNTP (in this instance, dTTP)
Synthesis of nucleic acid chain occurs in the 5’ to 3’ Direction
New strand
Template strand
5¢ end
3¢ end
5¢ end
3¢ end
Sugar
Base
Phosphate
3¢ hydroxyl :
DNA polymerase
5¢ phosphate
3¢ end
Pyrophosphate
dNTP (in this instance, dTTP)
5¢ end
5¢ end

6. DNA Replication Begins at the Origin of Replication
Parental (template) strand
0.25 µm
Origin of replication
Daughter (new) strand
Bubble
Replication fork
Two daughter DNA molecules
In eukaryotes, DNA replication begins at may sites
along the giant DNA molecule of each chromosome.
In this micrograph, three replication
bubbles are visible along the DNA
of a cultured Chinese hamster cell
(TEM).

7. Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork
Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate
The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells