A molecule that carries most of the genetic instructions used in the development, functioning and reproduction of all known living organisms. DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic acids compose the three major macro-molecules essential for all known forms of life. Most DNA molecules consist of two bio-polymer strands coiled around each other to form a double helix.

1. DNA
-Vishakha

2. DNA is a polymer. The monomer units of DNA are nucleotides, and the polymer is
known as a "polynucleotide."
First discovered by James D. Watson and Francis Crick, the structure of DNA of
all species comprises two helical chains each coiled round the same axis, and each
with a pitch of 34 Angstroms' (3.4 nanometers) and a radius of
10 Angstroms (1.0 nanometers).
Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing
base attached to the sugar, and a phosphate group.
A nucleobase linked to a sugar is called a nucleoside
(A nucleoside is one of the four DNA bases covalently attached to the C1'
position of a sugar)
and a base linked to a sugar and one or more phosphate groups is called a
nucleotide (A nucleotide is a nucleoside with one or more phosphate groups
covalently attached to the 3'- and/or 5'-hydroxyl group(s).).
There are four different types of nucleotides found in DNA, differing only in the
nitrogenous base.

3. Purine Bases
Adenine and guanine are purines. Purines are the larger of the two types of bases
found in DNA.
The 9 atoms that make up the fused rings (5 carbon, 4 nitrogen) are
numbered 1-9. All ring atoms lie in the same plane.

4. Pyrimidine Bases
Cytosine and thymine are pyrimidines.
The 6 stoms (4 carbon, 2 nitrogen) are numbered 1-6. Like purines, all pyrimidine
ring atoms lie in the same plane.

5. Deoxyribose Sugar
The deoxyribose sugar of the DNA backbone has 5 carbons and 3 oxygens.
The carbon atoms are numbered 1', 2', 3', 4', and 5' to distinguish from the
numbering of the atoms of the purine and pyrmidine rings. The hydroxyl groups
on the 5'- and 3'- carbons link to the phosphate groups to form the DNA
backbone.
Deoxyribose lacks an hydroxyl group at the 2'-position when compared to
ribose, the sugar component of RNA.

6. DNA Backbone
The DNA backbone is a polymer with an
alternating sugar-phosphate sequence.
The deoxyribose sugars are joined at both
the 3'-hydroxyl and 5'-hydroxyl groups to
phosphate groups in ester links, also
known as "phosphodiester" bonds.

7. DNA Double Helix
DNA is a normally double stranded macromolecule. Two polynucleotide chains, held
together by weak thermodynamic forces, form a DNA molecule.
Features of the DNA Double Helix
Two DNA strands form a helical spiral,
winding around a helix axis in a right-
handed spiral
The two polynucleotide chains run in
opposite directions
The sugar-phosphate backbones of the
two DNA strands wind around the helix axis
like the railing of a sprial staircase
The bases of the individual nucleotides
are on the inside of the helix, stacked on
top of each other like the steps of a spiral
staircase.

8. Base Pairs
Within the DNA double helix, A forms 2 hydrogen bonds with T on the opposite
strand, and G forms 3 hyrdorgen bonds with C on the opposite strand.
DNA with high GC-content is
more stable due to intra-
strand base stacking
interactions

9. In the canonical Watson-Crick DNA base pairing, adenine (A) forms a base pair
with thymine (T) and guanine (G) forms a base pair with cytosine (C).
In RNA, thymine is replaced by uracil (U).
Alternate hydrogen bonding patterns, such as the wobble base pair and
Hoogsteen base pair, also occur—in particular, in RNA—giving rise to complex
and functional tertiary structures.
A Hoogsteen base pair is a variation of base-pairing in nucleic acids such as the
A•T pair.
In this manner, two nucleobases on each strand can be held together by hydrogen
bonds in the major groove.
A Hoogsteen base pair applies the N7 position of the purine base (as a hydrogen
bond acceptor) and C6 amino group (as a donor), which bind the Watson-Crick
(N3–N4) face of the pyrimidine base.
The angle between the two glycosylic bonds (ca. 80° in the A• T pair) is larger and
the C1′–C1′ distance (ca. 860 pm or 8.6 Å) is smaller than in the regular
geometry.
In some cases, called reversed Hoogsteen base pairs, one base is rotated 180°
with respect to the other.

10. The majority of nucleotide bases in DNA
link together with Watson-Crick pairing.
However, there are a few that link together
with Hoogsteen pairing. It has been
thought for quite some time that Hoogsteen
pairing only occurs in DNA when it is either
damaged or bound to some other molecule
(like a protein or a drug).

11. •dA-dT and dG-dC base pairs are the
same length, and occupy the same space
within a DNA double helix. Therefore the
DNA molecule has a uniform diameter.
•dA-dT and dG-dC base pairs can occur in
any order within DNA molecules

17. Geometry attribute: A-form B-form Z-form
Helix sense right-handed right-handed left-handed
Repeating unit 1 bp 1 bp 2 bp
Rotation/bp 33.6° 35.9° 60°/2
Mean bp/turn 11 10.5 12
Inclination of bp to axis +19° −1.2° −9°
Rise/bp along axis 2.4 Å (0.24 nm) 3.4 Å (0.34 nm) 3.7 Å (0.37 nm)
Rise/turn of helix 24.6 Å (2.46 nm) 33.2 Å (3.32 nm) 45.6 Å (4.56 nm)
Mean propeller twist +18° +16° 0°
Glycosyl angle anti
anti pyrimidine: anti,
purine: syn
Sugar pucker C3'-endo C2'-endo
C: C2'-endo,
G: C2'-exo
Diameter 23 Å (2.3 nm) 20 Å (2.0 nm) 18 Å (1.8 nm)