DNA structure
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Transcript of DNA structure
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1DNA structure
Prepared By ZAHIDA UMAR
Dept of Environmental Sciences
GC, University, Lahore
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What DNA is?
DNA structure
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What DNA is?
DNA structure
1. A polymer of deoxyribonucleotides i.e. DNA
2. Double-stranded
3. A “double helical” structure
• common axis for both helices
• antiparallel relationship between 2 DNA strands
4. Structural purposes, or are involved in regulating the use
of this genetic information.
5. Individual deoxynucleoside triphosphates are coupled by
phosphodiester bonds
• Esterification
• terminal ends : 5’ and 3’
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Discovering the structure of DNA
but 1st described by James Watson and Francis Crick in 1953.
DNA Discovery
DNA structure was discovered in 1869 by chemist Friedrich Miescher.
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DNA structure
DNA Organization
This image illustrates how DNA is organized in human body.
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Chemical structure of DNA
DNA Topology
•DNA forms a double helix in which the nucleotide bases are attached to deoxyribose units linked through phosphate groups. The bases in the center of the DNA helix always occur in complementary matched pairs, with cytosine linking to guanine and thymine linking to adenine through hydrogen bonding.
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Sugar Phosphate Backbone
Esterification
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•This backbone is constructed from alternating ribose sugar and phosphate molecules which are highly polar. Because the backbone is polar, it is hydrophilic which means that it likes to be immersed in water. The asymmetric ends of DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends, with the 5' end being that with a terminal phosphate group and the 3' end that with a terminal hydroxyl group.
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Nitrogenous Bases
DNA Bases
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•The interior portion of a DNA molecule is composed of a series of 4 nitrogenous bases: Adenine (A),Guanine (G), Thymine (T), Cytosine (C). These bases are classified into two types; Purines , Pyrimidines
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“Chargoff’s rule” A = T & C = G
Nitrogenous Bases
DNA Bases
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DNA Grooves
DNA structure
•There are two grooves twisting around the surface of the double helix:
1.Major groove 2. Minor groove
•The major groove is 22 Å wide and the other, the minor groove, is 12 Å wide. The narrowness of the minor groove means that the edges of the bases are more accessible in the major groove.
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Sense & AntisenseDNA strands
DNA STRANDS
•If DNA sequence is the same as that of a messenger RNA copy that is translated into protein is called SENSE & sequence on the opposite strand is called ANTISENSE
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DNA Supercoiling
Almost all DNA molecules in cells can be
considered as circular, and are on average
negatively supercoiled.
Counter helical turn
DNA Supercoiling
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Alternative Structure of DNA
DNA structure
A-Dna B-Dna Z-Dna
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DNA structure
•Bases are the anti conformation
•Right handed helix•planes of bases are tilted 20 ° relative the helix axis. •2.3 A rise between base pairs
•Sugars are in the 3’ endo conformation.
O
OH (OH)
O
BASEO
O
H (OH)
OBASE
2' endo (3' exo) B-DNA
1'
3' endo (A-DNA)
3'
1'3'
2'
5'
5'
2'
5.9 A
•11 bases per helix turn •Helical pitch = 25.3 A
A – DNA
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DNA structure
•Bases are the anti conformation
B – DNA
•Right handed helix•planes of bases are nearly perpendicular to the helix axis. •3.4 A rise between base pairs
•Sugars are in the 2’ endo conformation.
•10.4 bases per helix turn •Helical pitch = 34 A
O
OH (OH)
O
BASEO
O
H (OH)
OBASE
2' endo (3' exo) B-DNA
1'
3' endo (A-DNA)
3'
1'3'
2'
5'
5'
2'
7.0A
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DNA structure
•Narrow and deep minor groove
Z – DNA
•Left handed helix
•planes of bases are tilted 9° relative the helix axis. •3.8A rise between base pairs
•Flat major grooves
•12bases per helix turn
•Helical pitch = 45.6 A
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DNA structure
Z – DNA 5’-GCGCGCGCGCGCG3’-CGCGCGCGCGCGC
C: sugar is 2’-endo, base is antiG: sugar is 3’-endo, base is syn
Sugar and base conformations in Z-DNA alternate
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Alternative DNA
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Prepared By ZAHIDA UMAR
B.Sc Hons, Dept of Environmental
Sciences