DNA replication in eukaryotes occurs semi-conservatively, with one parental strand and one new daughter strand produced. It begins at origins of replication where the DNA unwinds and separates. DNA polymerase adds nucleotides to the new strands growing from the replication fork in the 5' to 3' direction. In eukaryotes, the lagging strand is synthesized in fragments called Okazaki fragments opposite to the replication fork's direction. Telomeres and the telomerase enzyme prevent progressive shortening of chromosomes during replication.

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, therecouldbethreepossible waysthat
DNAreplication occur:
Conservative replication:
Bothparental strands stay together afterDNAreplication.
• Semiconservative:
Thedouble-stranded DNAcontains oneparental andone
daughter strand.
– Dispersive:
Thedaughterhelices aremixesof old andnew.

7. Figure 11.2
11-6

8. 8
MechanismOf DNAReplication
Maystart at anypoint
Different stepsinvolved in replication….
1. Unwindingof DNADuplex:
 Helicase enzyme
 Topoisomerase
 Single Stranded Binding Proteins
1. Roleof DNApolymerase:
 DNA polymerase I
 DNA polymerase II
 DNA polymerase III
 Synthesisof LeadingandLaggingstrand

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
DNA Polymerase II
Acts as proof reader

10. DNA Polymerase III
Mainenzymeinvolvedinreplication
DIMER..Having twosubunits
Canaddnucleotides toalreadyexistingstrand
Canaddnucleotides in5primeto3prime.

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
oppositetothedirectionofreplicationfork.
Synthesizedinshort,separated
segmentscalledO
k
a
z
a
k
iFragments

15. DNA Replication
•EnzymeHelicase
unwinds and
separates the2DNA
strandsby breaking
theweak hydrogen
bonds
•DNApolymerasecan
thenaddthenew
nucleotides
15

16. DNA Replication
16

17. DNA Replication
•Begins at Originsof Replication
•TwostrandsopenformingReplicationForks(Y
-
shapedregion)
•Newstrandsgrowat theforks
3’
Replication
Fork
5’Parental DNA Molecule
3’
5’
17

18. Theorigins of replicationfoundin eukaryoteshave
somesimilarities tothoseof bacteria.
 Origins of replicationin Saccharomycescerevisiae
aretermed….
 ARSelements (AutonomouslyReplicating
Sequence)
 Theyare100-150bpinlength
 TheyhaveahighpercentageofAandT
Multiple Origins of Replication

20.  Originrecognitioncomplex(ORC)

 Actsasthe initiator of eukaryotic
DNA replication.
It appearstobefoundinall eukaryotes.
Multiple Origins of Replication

22. Telomeres and DNA Replication
Telomeres:
Theendsof eukaryoticchromosomes (chromosomes
arelinear)
Neededfor chromosomal integrity andstability.
(protectendsfromdegradation).
Preventsprogressiveshorteningof laggingstrands.
Telomerase
Isaribonucleoproteincomplex consistingof proteins
andanRNAthat actsasatemplate for telomereaddition
tochromosomeends.