WATSON-CRICK MODEL:-DNA
PRESENTED BY:- NIKHIL SAINI
DMLT (2016),6TH SEMESTER
VMMC & S.J.H

CONTENT
1. HISTORY
2. STRUCTURE OF DNA
3. IMPORTANT FEATURES OF WATSON&CRICK MODEL
4. BIOLOGICAL IMPORTANCE OF DOUBLE HELIX DNA

HISTORY
● DNA WAS FIRST ISOLATED BY THE SWISS PHYSICIAN FRIEDRICH
MIESCHER
IN 1869 AS NUCLEIN BECAUSE IT RESIDE IN NUCLEUS.
● IN 1951, J.D. WATSON (AN AMERICAN BIOLOGIST) AND F.H.C.
CRICK
( A BRITISH PHYSICIST) START WORKING TOGETHER AT THE
CAVENDISH LAB
WITHIN THE UNIVERSITY OF CAMBRIDGE.
● ,1953, WATSON AND CRICK SUGGESTED WHAT IS NOW ACCEPTED
AS THE
FIRST CORRECT DOUBLE-HELIX MODEL OF DNA STRUCTURE.

FIG:- JAMES WATSON{RIGHT SIDE) & F.CRICK ON{LEFT SIDE}

CONTINUE.....
● THE IMAGE IS TAKEN BY ROSALIND FRANKLIN AND M. WILKINS
● ON 28 FEBRUARY 1953 CRICK INTERRUPTED PATRONS' LUNCHTIME AT THE EAGLE PUB IN
CAMBRIDGE
TO ANNOUNCE THAT HE AND WATSON HAD "DISCOVERED THE SECRET OF LIFE".
● FOR THIS EPOCH-MAKING DISCOVERY, WATSON, CRICK AND WILKINS GOT NOBEL PRIZE IN
1962.

STRUCTURE OF D.N.A
DNA IS THE LONG POLYMER OF DEOXYRIBONUCLEOTIDES. IT ACTS
GENETIC
GENETIC MATERIAL IN HUMANS AND SOME OTHER ORGANISMS ALSO.
EACH NUCLEOTIDE CONSISTS OF THREE SUB-UNITS.
1. A PENTOSE SUGAR CALLED DEOXYRIBOSE.
2. 4 NITROGENOUS BASES ADENINE & GUANINE ARE PURINE BASES
AND
THYMINE & CYTOSINE ARE PYRIMIDINE BASES.
3. A PHOSPHATE GROUP[ PO4 ] POSITIONED ON SUGAR.

IMPORTANT FEATURES OF WATSON-CRICK
MODEL
● THE B-DNA IS MADE BY TWO POLYNUCLEOTIDE CHAINS, WHERE THE BACK HONE IS
CONSTITUTED BY
SUGAR- PHOSPHATE. AND THE BASES PROJECT INSIDE.
● THE TWO CHAINS HAVE ANTI-PARALLEL POLARITY.IT MEANS,IF ONE CHAIN HAS THE
POLARITY 5' TO 3' , THE OTHER HAS 3' TO 5'.
● THE TWO STRANDS ARE HELD TOGETHER BY HYDROGEN BONDS (H-BONDS) FORMING
BASE-PAIRS.
• ADENINE [A] ALWAYS PAIR WITH THYMINE [T] BY TWO HYDROGEN BONDS AND GUANINE
[G] ALWAYS PAIR WITH CYTOSINE [C] BY THREE HYDROGEN BONDS.AS A RESULT PURINE
ALWAYS COMES OPPOSITE TO A PYRIMIDINE. THIS GENERATE APPROX. A UNIFORM
DISTANCE B/W THE TWO STRANDS OF THE HELIX.

CONT......
+ THE BASE COMPOSITIONS OF DNA OBEY CHARGAFF'S RULES.
+ ACCORDING TO THEM THE FOUR NUCLEOTIDES ARE NOT PRESENT IN
EQUAL
AMOUNTS IN A DNA MOLECULE. BUT AMOUNT OF PURINES( A+G) AND
THAT OF
PYRIMIDINES(T+C) IS ALWAYS EQUAL. IN OTHERS WORD A=T AND G=C.
THIS IS
CALLED CHARGAFF'S RULE.

CONTINUE........
• THE DIAMETER OF DNA IS 20NM OR 20 A. ADJACENT BASES ARE SEPARATED
0.34 NM OR BY 3.4 A ALONG THE AXIS.
• THE LENGTH OF A COMPLETE TURN OF HELIX IS 3.4 NM OR 34 A I.E. THERE ARE
10BP PER TURN. (B- DNA-WATSON RICK DNA)
• THE DNA HELIX HAS A SHALLOW GROOVE CALLED MINOR GROOVE. A DEEP
GROOVE CALLED MAJOR GROOVE

BIOLOGICAL IMPORTANCE OF DNA

CONT.......

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