dna_replication2

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REPLICATION FORK

RNAprimer



ELONGATION PROCESS ±DNA polymerase

Reason for synthesis of 2 type of strands:� The two strands of DNA are antiparallel. [they run

in opposite direction]From the initiation site [ORI], one parental strandruns in a 5¶ 3¶ while the other strand is running in the

3¶ 5¶ direction

� DNA synthesis is always in the 5¶ 3¶ direction.

Due to this 5¶ 3¶ polarity and antiparallel nature of DNA 2 different strands are formed.(1) Leading strand(2) Lagging strand



LEADING AND LAGGING STRAND

� Leading strand uses 3¶ 5¶ parent strand as thetemplate

� The synthesis is continuous.� The direction of the synthesis coincides with the

direction of replication.

� Lagging strand uses 5¶ 3¶ parent strand as thetemplate

� The synthesis of the lagging strand is infragments called the OKAZAKI fragments

� These fragments are synthesized as the DNAopens up [separated].



DNA POLYMERASES� DNA polymerase III -- main replicating enzyme in

bacteria. [ Pol III ]

� DNA polymerase II -- repair and proof reading. [PolII ]

� DNA Polymerase I --remove the RNA primers andreplaces them with DNA.

-- has proof reading activity. [Pol I ]

Pol I has both 3¶ to 5¶ and 5¶ to 3¶ exonuclease activity



Mammalian polymerases [eukaryotic]

DNApolymerase

Alpha Delta Epsilon Beta Gamma

Location Nucleus Nucleus Nucleus Nucleus Mitochondria

Function Lagging

strandsynthesis

Leading

strandsynthesis

Repair of

laggingstrand

Repair of

leadingstrand

Replicatio

n

Primase

activity

Yes No No No Yes

3 5¶exonuclease

No Yes Yes No Yes

Proof

readingactivity

Main

enzyme

- - 15% 15%



ROLE OF LIGASE

� The DNA Ligases---- connects DNA Okazaki fragments during replication,

---- during repair and Recombination.

� Newly replicated DNA is rapidly assembled intonucleosomes.



TELOMERE

� For initiating replication a RNA primer is required.

� The RNA primer is removed by Pol I.� DNA Polymerase cannot fill the gap so a portion of

template (3¶ end ) is not replicated.� Replication ± 5¶ - 3¶. DNA POLYMERASE ± not

able to synthesize the new strand at 5¶.� Small portion ± 300 nucleotides ± in 3¶ends ± could

not be replicated.� End piece of the chromosome not replicated ---

TELOMERE.� Stability is lost If there is no mechanism to

replicate telomere.



TELOMERASE

� Human telomere end consists of (TTAGGG) repeats which

is highly conserved.

� The shortening of telomere end is prevented by an enzymeTelomerase.

� Telomerase acts like a reverse transcriptase.[ RNA]

� In old age telomerase activity is lost leading tochromosome instability.

� In cancer cells there is continued presence of telomeraseactivity leading to continued cell division.

.



At the end of the lecture, student should be ableto:� Explain the initiation of DNA replication� Describe the formation of new DNA strands

� Explain what is semi conservative mechanismof DNA replication

� Compare and contrast DNA replication inprokaryotic and eukaryotic cells

� Explain what are telomeres.

LEARNING OUTCOMES



1.Harper¶s Illustrated Biochemistry 26th edn,

2.Lippincott¶s Illustrated Reviews ±

Biochemistry 3rd edn,3.Textbook of Biochemistry (Vasudevan andSreekumari) 5th edn

References

http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

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