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 SEMI-CONSERVATICE 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 / FUNCTION
Dna A / Opens duplex at specific site
Dna B / Unwinds DNA
Dna C / Helps Dna B
SSB proteins / Binds ss-DNA
Topoisomerase-I / Relax supercoils by cutting strand
Topoisomerase-II/GYRASE / Relieves torsional strain by dna uncoiling
RNA primase / Synthesis of RNA primer
DNA ligase / For sealing DNA/Joining nicks

SPECIFIC ENZYMES IN PROKARYOTES AND EUKARYOTES

PROKARYOTES / EUKARYOTES / FUNCTION
DNA Polymerase I / α / Gap filling
DNA Polymerase II / € / DNA proof reading+repairing
β / DNA repairing
γ / Mitochondrial DNA synthesis
δ / Synthesis of dna on template strand

PROCESS:

A)  FORMATION OF REPLICATION FORK:

B)  INTIATION

C)  ELONGATION

D)  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.