10 DNA replication

DNA replication

• occurs during the S phase

• each strand is used as a template to construct a new strand

• 2 identical double helices

DNA replication steps

1. DNA unzips at the origin of replication by the enzyme Helicase.

2. The primase enzyme attached RNA nucleotides to form an RNA primer (5’ → 3’) to initiate DNA synthesis.

3. The RNA primer is removed and the gap is filled by DNA nucleotides

4. The DNA polymerase III attaches DNA nucleotides in the 5’ → 3’. Since DNA is antiparallel, one strand replicates continuously (leading), while the other replicates discontinuously (Okazaki; lagging).

5. The end of the strands are “glued” by DNA ligase 

What does it mean for DNA to be “semi-conservative”?

it means that every helix of an organism consists of one complete “old'“ strand and a complete “new” strand

DNA strands serve as templates. What does this mean?

• each polynucleotide acts as a template for a new strand

What does it mean for DNA to “unzip”

helicase unwinds the double helix creating 2 replication forks

Why is DNA replication bidirectional?

replication proceeds in both directions until the entire molecule is copied

“Replication bubble”

• one origin of replication 

• hundreds of origins 

• replication begins here, the origins of replication

List enzymes important in DNA replication

helicase, binding protein, primase, DNA polymerase, Ligase

Helicase 

unwinds parental double helix

Binding Proteins

stabilize separate strands

Primase

adds short primer to template strand

DNA polymerase

binds nucleotides to form new strand

Ligase

joins Okazaki fragments and seals other nicks in sugar-phosphate backbone

Leading vs Lagging strands

Leading: DNA strand that replicated continuously

Lagging: replicates discontinuously which forms Okazaki fragments