models of dna structures
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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
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