FOR FSc BSc MSc BS

1. 1
DNA
REPLICATION
In
EUKARYOTES

2. DNA
Deoxyribonucleic Acid
• Two strands coiled =
double helix
• Sides = pentose sugar
Deoxyribose bonded to
phosphate (PO4)
• nitrogen bases bonded
together by weak
hydrogen bonds

3. DNA:Deoxyribonucleic acid
•Made up of subunits
called nucleotides
•Nucleotide made of:
1. Phosphate group
2. 5-carbon sugar
3. Nitrogenous base

4. Nitrogenous Bases
•Double ring PURINES
Adenine (A)
Guanine (G)
•Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
Purines always pair with
Pyrimidines
4
T or C
A or G

5. 5

6. TYPES OF REPLICATION
6
Hypothetically, there could be three possible ways that
DNA replication occur:
Conservative replication:
Both parental strands stay together after DNA replication.
• Semiconservative:
The double-stranded DNA contains one parental and one
daughter strand.
– Dispersive:
The daughter helices are mixes of old and new.

7. Figure 11.2
11-6

8. 8
Mechanism Of DNA Replication
May start at any point
Different steps involved in replication….
1. Unwinding of DNA Duplex:
 Helicase enzyme
 Topoisomerase
 Single Stranded Binding Proteins
1. Role of DNA polymerase:
 DNA polymerase I
 DNA polymerase II
 DNA polymerase III
 Synthesis of Leading and Lagging strand

9. 9
DNA Polymerase I
Replication occurs 5' to 3‘
Relatively small enzyme
Acts at NICK
Replaces RNA nucleotides in RNA primer by DNA nucleotide
Acts as proof reader
DNA Polymerase II

10. Main enzyme involved in replication
DIMER..Having two subunits
Can add nucleotides to already existing strand
Can add nucleotides in 5 prime to 3 prime.
DNA Polymerase III

11. Antiparallel Strands
•One strand of DNA
goes from 5’ to 3’.
•The other strand is
opposite in direction
going 3’ to 5’.
11

12. 12

13. 13
opposite to the direction of replication fork.
Synthesized in short, separated segments
called Okazaki Fragments

15. DNA Replication
•Enzyme Helicase
unwinds and separates
the 2 DNA strands by
breaking the weak
hydrogen bonds
•DNA polymerase can
then add the new
nucleotides
15

16. DNA Replication
16

17. DNA Replication
•Begins at Origins of Replication
•Two strands open forming Replication Forks (Y-
shaped region)
•New strands grow at the forks
17
Replication
Fork
Parental DNA Molecule
3’
5’
3’
5’

18. The origins of replication found in eukaryotes have
some similarities to those of bacteria.
 Origins of replication in Saccharomyces cerevisiae
are termed….
 ARS elements (Autonomously Replicating
Sequence)
 They are 100-150 bp in length
 They have a high percentage of A and T
Multiple Origins of Replication

20.  Origin recognition complex (ORC)
 Acts as the initiator of eukaryotic DNA
replication.
 It appears to be found in all eukaryotes.
Multiple Origins of Replication

22. Telomeres and DNA Replication
Telomeres:
The ends of eukaryotic chromosomes (chromosomes
are linear)
Needed for chromosomal integrity and stability.
(protect ends from degradation).
Prevents progressive shortening of lagging strands.
Telomerase
 Is a ribonucleo protein complex consisting of
proteins and an RNA that acts as a template for telomere
addition to chromosome ends.