DNA replication: DNA making DNA
 Why must DNA replicate?

› So that when a cell divides, each new cell
gets a copy of the DNA

When does DNA replication take place?
› During the S phase of interphase
› When chromosomes go from individual
chromosomes to double chromosomes

DNA replication
where the replicated
DNA consists of one
old strand and one
newly made strand

Origin of replication
› Sites where
replication of DNA
begin

Replication
fork/bubbles
› Openings in DNA
where replication
takes place

Occurs in the Nucleus
1. Helicase binds to the origin of replication to unwind
and unzip DNA; Breaking what bond?

Hydrogen Bonds – helicase breaks hydrogen bonds
2. DNA polymerase brings in complementary
nucleotides attaching them in a 5’ to 3’ direction
(phosphate of new incoming nucleotide attaches
to the 3’ last nucleotide in the strand
3. Ligase – joins sugars and phosphates of newly
formed strands of DNA
4. DNA polymerase – proofreads DNA, checking
of spelling errors
 Replication ends with 2 identical molecules of
DNA; each with 1 old strand and 1 new stand

Direction of Replication
› 5’ to 3’ – DNA polymerase needs to have an open
3’ carbon on deoxyribose so that it can add a
new nucleotide
› Leading vs Lagging
 Leading strand is “continuously” replicated into the
the replication fork runs 5’ to 3’
 Lagging strand is “discountinuosly” replicated; out of
the replication fork runs 3’ to 5’

Okazaki Fragments - short stretches of
nucleotides formed on the lagging strand of
DNA
› Why do we need Okazaki Fragments?
› They are needed to get replication going in the
correct direction on the lagging strand