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GUIDED BY CLETUS

J.M DSAUZA

SIR

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WATSON & CRICKDISCOVERED THE DOUBLEHELICAL STRUCTURE OF DNA

IN 1953 BY X- RAY DIFFRACTION

DNA was first isolated by the Swissphysician Friedrich who, in 1869,discovered a microscopic substance inthe pus of discarded surgical bandages.

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THE STRUCTURE OF DNA

DNA consists of two long chain

of polymer i.e. polynucleotidecomposed of mono-meric unit callednucleotides.

Each nucleotide constitute of anucleoside and a phosphate group ,

while nucleotide consists of pentosesugar and nitrogenous base.

MINOR GROOVEExposes edges from which C1

atoms extends

MAJOR GROOVEExposes opposite edges of base

pairs

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DNA

GEOMETRY

PERIPHERY OF DNA SUGAR-PHOSPHATECHAINS

CORE OF DNA BASES WHICH ARESTACKED IN A PARALLEL

FASHION

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CHARGAFFS RULESA = TG = C

COMPLEMENTARY BASE-PAIRING

WHERE A & T ARE BOUNDBY TWO HYDROGENBONDS WHILE G & C AREBOUND BY THREE

HYDROGEN BONDS

HOW DOES THE BASE PAIRING OCCURES ?

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B- DNA

A- DNA

Z-DNA

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B DNA

B-DNA IS BIOLOGICALLY THE MOST COMMON

RIGHT-HANDED HELIX

COMPLEMENTARY BASE-PAIRING (WATSON-CRICK)

A-TG-C

EACH BASE PAIR HAS ~ THE SAME WIDTH10.85 A FROM C1 TO C1

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IDEAL B-DNA HAS 10 BASE PAIRS PERTURN

PITCH = 10 X 3.4 = 34 A PER COMPLETETURN

AXIS PASSES THROUGH MIDDLE OFEACH BP

MINOR GROOVE IS NARROW

MAJOR GROOVE IS WIDE

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RIGHT-HANDED HELIX

WIDER AND FLATTER THAN B-DNA11.6 BPPER TURN

PITCH OF 34 A

BASE PLANES ARE TILTED 20 DEGREES WITHRESPECT TO HELICAL AXISHELIX AXIS PASSES ABOVE MAJOR GROOVE

DEEP MAJOR GROOVE AND SHALLOWMINOR GROOVE

A-DNA

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A-DNA HAS BEEN OBSERVED IN 2CONTEXTS:

AT ACTIVE SITE OF DNAPOLYMERASE (~ 3 bp )

GRAM (+) BACTERIA

UNDERGOING SPORULATION

Segments of DNA where the bases have beenchemically modified by methylation may undergo alarger change in conformation and adopt the Z form

Z DNA

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A LEFT-HANDED HELIX

12 BASE PAIRS PER TURN

A PITCH OF 44 DEGREES

A DEEP

MINOR GROOVE

ITS SEEN IN CONDITIONS OF HIGH SALTCONCENTRATIONS

NO DISCERNIBLE MAJOR GROOVE

REVERSIBLE CHANGE FROM B-DNA TO Z-DNA INLOCALIZED REGIONS MAY ACT AS A SWITCH TOREGULATE GENE EXPRESSION

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Structural features of the three major forms of DNA

Geometry attribute A-DNA B-DNA Z-DNA Helix sense right-handed right-handed left-handed

Repeating unit 1 bp 1 bp 2 bp

Rotation/bp 33.6 35.9 60/2bp

Mean bp/turn 10.7 10.0 12

Inclination of bp to axis +19 -1.2 -9

Rise/bp along axis 2.3 3.32 3.8

Pitch/turn of helix 24.6 33.2 45.6

Mean propeller twist +18 +16 0

Glycosyl angle anti anti C: anti,G: syn

Sugar pucker C3'-endo C2'-endoC: C2'-endo,G: C2'-exo

Diameter 25.5 23.7 18.4

Structural features of the three major forms of DNA

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Unusual DNA structure

Palindrome vs. mirror repeat.

Triple helix.

Hoogsteen base pairing.

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The word palindrome isderived from the Greek

palndromos, meaningrunning back again (paln =AGAIN + drom,dramen =RUN). A palindrome is aword or phrase which readsthe same in both directions

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Hairpin DNA structure is formedbecause of the bending created bythe repeated sequences.

HAIRPIN AND CRUCIFORM STRUCTURES

OF DNA

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TRIPLE HELICS

Here the non-Watson-Crick base-pairing allows thethird strands to wind around the duplexes and formtriple-stranded helices

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THANK YOU