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DNA replication
1.
2. DNA replication is a mean to produce new
molecules that have the same base sequence.
Occurs during interphase of the cell cycle.
DNA replication is semi-conservative;
◦ The parent DNA strand is separated into two.
◦ Each strand serves as a template for new
complementary strands.
3.
4. Occurs in a 5’→3’ direction.
◦ The 5’ end of the ‘new’ nucleotide attached to the 3’ end
of the nucleotide before it
DNA is unwind and unzipped by the enzyme
helicase .
DNA polymerase III creates links between the
nucleotides.
◦ (It creates a strand that is complementary to the original
strand)
5. Replication takes place at several places on one
double helix at the same time.
Once the double helix is unwind and unzipped the
two parent strands become the leading and
lagging strands.
This happens because the strands are
antiparallel.
3’
5’ 3’
5’
6. The lagging strand runs from 3’ to 5’.
1. Before DNA replication begins there must be
RNA primer.
2. The RNA primer is made by adding
complimentary RNA nucleotides to the lagging
DNA strand by hydrogen bonding of the bases.
◦ RNA has uracil instead of thymine
3. RNA primase (an enzyme) bonds the
RNA nucleotides together.
3’ 5’
RNA primase
RNA primer
7. 4. After RNA primer is in place, DNA nucleotides
are added by DNA polymerase III.
5. Eventually, the segment of DNA will run into
another RNA primer.
6. The DNA segments are called Okazaki
fragments.
3’ 5’
RNA primer
DNA polymerase III
Okazaki fragment
8. 7. Once Okazaki fragments are formed, DNA
polymerase I replaces the RNA primer with DNA
nucleotides.
8. DNA ligase links the fragments together.
3’ 5’
3’5’
Parent strand
New strand
9. In the lagging strand DNA replication is
discontinuous.
DNA is replicated in segments that become linked
together.
10. Replication is continuous.
There are no fragments.
Helicase unwinds and unzips the double helix.
DNA polymerase III adds nucleotides in the direction
of 5’→3’.
N.B:
◦ DNA polymerase only works in the direction of
5’→3’.
11. Lagging strand:
◦ Formed in short segments of 100-200 nucleotides (these are the
Okazaki fragments).
◦ Grows in direction of 5’→3’ because DNA polymerase III only
works in the 5’→3’.
◦ 1. Helicase unwinds and unzips the parent DNA.
◦ 2.RNA primer is formed by RNA nucleotides that are joined
together by RNA primase.
◦ 3. DNA polymerase III bonds DNA nucleotides to the RNA
primer.
◦ 4. DNA polymerase I replaces the RNA primer with DNA
nucleotides.
◦ 5. DNA ligase links the Okazaki fragments together.
Leading strand:
◦ 1. Helicase unwinds and unzips.
◦ 2. DNA polymerase III adds complimentary DNA nucleotides.
