DNA REPLICATION:
FRAMEWORK: [WHAT ARE WE GOING TO STUDY IN THIS]
–INTRODUCTION
–OCCURANCE IN CELL
–STRUCTURE OF DNA
–SEMI CONSERVATIVE REPLICATION
–REQUIREMENTS OF DNA REPLICATION:
(i)ENZYMES (I a) COMMON ENZYMES IN PROKARYOTES & EUKARYOTES
(I b)DIFFERENT ENZYMES IN PROKARYOTES IN EUKARYOTES
(ii)OTHER MOLECULES
–PROCESS (a) FORMATION OF REPLICATION FORK
(b) INITIATION
(c) ELONGATION
(d) TERMINATION
–CHROMATIN REORGANISATION
–REGULATION OF DNA REPLICATION
–SIGNIFICANCE OF DNA REPLICATION
–CONCLUSION:
INTRODUCTION:
REPLICATION MEANS FORMATION OF NEW DNA FROM OLD DNA IN A SEMI
CONSERVATIVE WAY(described later).IT REQUIRES A SET OF ENZYMES AND
OTHER MOLECULES.THE PROCESS INVOLVES INITIATION,ELONGATION
AND TERMINATION WHICH IS A WELL REGULATED
PHENOMENON.REPLICATION SERVES NUMBER OF BIOMEDICAL PURPOSE
OCCURANCE:
LOCATION WISE IT OCCURS IN THE NUCLEUS.
DURING CELL CYCLE IT OCCURS IN S-PHASE OF INTERPHASE OF CELL
CYCLE (described later)]
FIGURE: DIFFERENT PHASES OF CELL CYCLE
DNA STRUCTURE:
DNA STRUCTURE FACILITATE THE PROCESS OF REPLICATION.IT IS AS
FOLLOWS:
FIGURE: STRUCTURE OF DNA
SEMI CONSERVATIVE REPLICATION :
IT MEANS WHEN ONE STRAND IS CONSERVED AND ACTS AS TEMPLATE
FOR NEW STRAND SYNTHESIS.
NEW STRAND IS COMPLETELY COMPLEMENTARY TO ITS TEMPLATE
STRAND.
EXPERIMENTAL EVIDENCES FOR EXAMPLE MESELSON AND STAHL
EXPERIMENT SUGGEST THAT REPLICATION TAKES PLACE IN SEMICONSERVATICE MANER WHICH IS GIVEN BELOW
FIGURE: SEMI-CONSERVATIVE MODE OF REPLICATION
REQUIREMENTS OF DNA REPLICATION :
DNA REPLICATION REQUIRES
-DNA TEMPLATE
-RNA/DNA PRIMER
- 4 DEOXYNUCLEOSIDE TRIPHOSPHATE: DEOXY ATP,GTP,CTP,TTP
- SET OF ENZMES
COMMON ENZYMES IN PROKARYOTES AND EUKARYOTES AND FUNCTION
COMMON ENZYMES
Dna A
Dna B
Dna C
SSB proteins
Topoisomerase-I
Topoisomerase-II/GYRASE
RNA primase
DNA ligase
FUNCTION
Opens duplex at specific site
Unwinds DNA
Helps Dna B
Binds ss-DNA
Relax supercoils by cutting strand
Relieves torsional strain by dna uncoiling
Synthesis of RNA primer
For sealing DNA/Joining nicks
SPECIFIC ENZYMES IN PROKARYOTES AND EUKARYOTES
PROKARYOTES
DNA Polymerase I
DNA Polymerase II
EUKARYOTES
α
€
β
γ
δ
FUNCTION
Gap filling
DNA proof
reading+repairing
DNA repairing
Mitochondrial DNA
synthesis
Synthesis of dna on
template strand
PROCESS:
A)
B)
C)
D)
FORMATION OF REPLICATION FORK:
INTIATION
ELONGATION
TERMINATION
A) FORMATION OF REPLICATION FORK :
1)ORIGIN OF REPLICATION AND UNWINDING OF DNA BEGINS FROM A=T
RICH REGION BECAUSE IT IS EASIER TO BREAK 2 HYDROGEN BONDS
BETWEEN THEM AS COMPARED TO 3 HYDROGEN BONDS BETWEEN GC
2)AFTER BREAKAGE STRANDS OPEN UP AND SSB PROTEINS BINDS THE
SITE TO PREVENT RE-COILING AND THUS REPLICATION FORK FORMS
B) INITIATION IN DNA SYNTHESIS :
1) INITIATION OF DNA SYNTHESIS REQUIRES PRIMING BY A SHORT
LENGTH OF RNA ABOUT 10-200 NUCLEOTIDES LONG
C)ELONGATION IN DNA SYNTHESIS:
1) ELONGATION OCCURS BY BOND FORMATION BETWEEN RNA
PRIMER AND NUCLEOTIDE BASES.
2) ONLY NUCLEOTIDES COMPLEMENTARY TO TEMPLATE
STRAND FORMS BOND
3) 1ST BOND FORMATION OCCURS DUE TO NUCLEOPHILIC
ATTACK BY –OH GRP OF RNA PRIMER
4) IN THIS WAY SUBSEQUENT NUCLEOTIDES ARE ADDED BY
THIS NUCELOPHILIC ATTACK.
WHEN 2 STRAND UNWIND AT REPLICATION FORK,THE LEADING STRAND
FACES DNA POLYMERASE CORRECT 5’ TO 3’ DIRECTION SO THAT
SYNTHESIS OF LONG CONTINOUS STRAND TAKES PLACE.
ON LAGGING STRAND THIS IS NOT POSSIBLE THEREFORE REPLICATION
PROCEEDS IN DISCONTINOUS WAY SYNTHESIZING SHORT SEGMENTS OF
DNA & THESE SEGMENTS ARE JOINED BY DNA LIGASE
PROOFREADING:
IT IS DONE BY RNA PRIMER WHEN A WRONG NUCLEOTIDE IS
INCORPORATED
D) TERMINATION : WHEN REPLICATION BUBBLES MMET EACH OTHER
REPLICATION COMES TO AN END i.e NO PERTICULAR TERMINATION
SITE.
REPLICATION IS BIDIRECTIONAL IN PROKARYOTES AS WELL AS
EUKARYOTES HOWEVER REPLICATION PROCEEDS FROM MULIPLE ORIGIN
IN EACH CHROMOSOME AS MANY AS 100 IN HUMANS SINCE EUKARYOTES
HAVE LARGE GENOMES
CHROMATIN RE-ORGANISATION:
AFTER REPLICATION DNA STARTS COILING AROUND HISTONE PROTEINS
TO RE-GAIN ITS COILED POSITION
REGULATION:
REPLICATION IS TIGHLY REGULATED SO THAT APPROPRIATE NUMBER OF
CELLS CONSTITUTING EACH TISSUE ARE PRODUCED DURING
DEVELOPMENT AND THROUGHOUT LIFE.
CONTROL OF INITIATION STAGE IS PRIMARY MECHANISM FOR
REGULATING CELLULAR DNA REPLICATION .
SIGNIFICANCE:
REPLICATION OF DNA IS IMPORTANT FOR SYNTHESIS OF
DNA,GROWTH,REGENRATION,REPAIRING,REPRODUCTION AND
ADAPTATION AND ALSO CAUSES GENETIC DISORDERS
CONCLUSION:
DNA REPLICATION IS NECESAARY FOR SUSTENCE AND FOR
ADVANCEMENT OF LIFE AS IT IS THE MOLECULE THAT UNDERGOES
MUTATION AND RECOMBINATION AND GETS PASS ON TO THE NEXT
GENERATION AND IS THE MAIN DRIVING FORCE MANIFESTING
EVOLUTION.