DNA Replication

BIOL113 Unit 2

How is DNA replicated?

1. DNA is double stranded, but each strand is copied separately.

2. So the double strands of DNA have to be separated from each other to be copied.

3. Each parental strand serves as a template for a new daughter strand.

Areas of DNA replication

Replication bubble-forms during DNA

synthesis.

Replication fork-Y-shaped region where the

parental strands are separated. This is

where active DNA synthesis takes place.

DNA Replication Step 1

DNA helicase breaks the hydrogen bonds between the nucleotides in that location &

opens the double helix so that the two strands separate at the replication fork.

The unwinding of DNA strands should create twists further down the helix, but

topoisomerase binds to relieve twisting forces

DNA Replication Step 2

Single-strand DNA-binding proteins attach to the separated strands to prevent
them from snapping back into a double helix.

DNA Replication Step 3

Primase lays down primer on both strands:

An RNA strand about 10 ribonucleotides long.

Forms complementary base pairs with the DNA template strand.

DNA Replication Step 4

DNA polymerase α or ε synthesizes the leading strand by attaching nucleotides

to the 3’ OH ends of the primer and extending it.

DNA polymerase δ synthesizes the lagging strand and replaces primers with

deoxyribonucleotides.

Why primase adding primer is necessary:

DNA polymerase can only add nucleotides to a free OH group, it can only extend a
pre-existing strand but when DNA strands are
separated there are no free OH groups

Primase 

Primase solves this by making the primer, small sequence of RNA, thereby providing the free 3’ OH group that DNA polymerase

needs to start adding nucleotides.

Proofreading

Fixes mistakes in DNA synthesis.
DNA polymerase fixes its own
mistakes.
A newly added base that is not
paired correctly creates
misalignment.

DNA polymerase’s active site can

identify misalignment. Once

detected, DNA polymerase will pause.

DNA polymerase has exonuclease

activity – it will remove the

mismatched nucleotide. Then replace it

with the correct one.

Mismatch repair

Fixes mistakes in DNA synthesis,

not repaired by proofreading.

A mismatch is detected

immediately after DNA synthesis

is finished.

Proteins and enzymes cut out the

mismatch and then DNA

polymerase returns to add the

correct nucleotides.

DNA ligase seals the breaks.

Repair

Nucleotide excision repair

Fixes DNA damaged by UV light.

UV light from the sun or tanning

beds can cause a covalent bond

to form between adjacent

pyrimidine bases within the same

strand. This causes a kink.

Protein complexes detect the

kink, remove the section of DNA

containing the kink.

DNA polymerase adds in the

nucleotides and ligase seals the

breaks.