Umm AL Qura University
DNA REPLICATION
Dr Neda M Bogari
CELL CYCLE
Phase
Description
Gap 0
A resting phase where the cell has left the cycle and has stopped dividing.
Gap 1
Cells increase in size in Gap 1. The G1 checkpoint control mechanism ensures that
everything is ready for DNA synthesis.
Synthesis (S) DNA replication (8 hr) occurs during this phase.
Gap 2
During the gap between DNA synthesis and mitosis, the cell will continue to grow.
The G2 checkpoint control mechanism ensures that everything is ready to enter the
M (mitosis) phase and divide.
Mitosis (M)
Cell growth stops at this stage and cellular energy is focused on the orderly division
into two daughter cells. A checkpoint in the middle of mitosis (Metaphase
Checkpoint) ensures that the cell is ready to complete cell division.
24hr
8hr
DNA REPLICATION
Antiparallel
3
5
5
3
9
3X10 bp
3
5
5
DNA helicase
3
3
5
DNA helicase
5
3
DNA polymerase
Primer
Origins of replication
5
3
Polymerase
3
5
A CGA
T G CT
3
Polymerase
C CCA
G G GT
5
150Kp
3
5
5
DNA ligase
3
3
5
A CGA
T G CT
3
1.DNA helicase
2.DNA polymerase
3.DNA ligase
Antiparallel
Primer
Origins of replication
5
3
Polymerase
3
5
A CGA
T G CT
3
Polymerase
C CCA
G G GT
5
150Kp
3
5
How genetic information is transmitted from one generation to
the next?
The process of DNA replication provides
an answer to this question.
• During nuclear division the two strands of the DNA helix
separate through the action of enzyme DNA helicase.
• Each DNA strand directing the synthesis of a complementary
DNA.
DNA synthesis occurs in the 5`è3` direction by enzymes called
DNA polymerase which takes place at multiple points
known as origins of replication, forming Y-shaped structure
known as replication forks
The two strands joined together by the enzyme DNA ligase.
Replication is said to be semi-conservative. This means that
each copied DNA molecule contains one strand derived
from the parent molecule and one newly
synthesized daughter strand
Summary