Download The components of DNA

The components of DNA
• The individual nucleobases are:
NH 2
O
N
N
N
H
Adenine
H2N
O
N
HN
N
NH 2
N
N
Guanine
N
H
N
H
NH
O
Cytosine
N
H
O
Thymine
• The nucleoside is formed by a bond with
the ribose sugar at the N9 position of A and
G and the N6 position of C and T.
The stability of double-helical
DNA
NH 2
The double helical form of DNA
N
is formed by phosphodiester
linkages between the 5’ -OH
O
N
end of one ribose and the
HO
3’-OH of the next.
O
The double helix is
H
H
stabilized by hydrogen
H
H
OH
H
bonding in canonical base pairs
and by stacking interactions between the bases.
DNA conformations
A form
B form
Z form
Conformations of the ribose
• B is 2’-endo anti
• A is 3’-endo anti
• Z is 2’-endo syn
The first X-ray crystal structure
of d(GCGCGC) was in the Zform
• Watson-Crick DNA
was determined based
on fiber diffraction. This was B form.
• The expectation that crystalline DNA
would also be in the B-form was shattered
by the structure of the hexamer
d(GCGCGC)
• Raman and infrared spectroscopy have
played a key role in comparisons of the
various forms of DNA.
The Dickerson dodecamer
The first x-ray crystal structure of
DNA in the B-form was obtained by
Dickerson using the dodecamer with
the sequence [d(CGCGAATTCGCG)]2.
The structure shown has the stacking
pattern of B-form DNA found in most
DNA in solution. DNA can, however,
be bent or coiled. Coiling is facilitated
by regions of adenines known as phased
A tracts. Such coiling permits the
packing of DNA in structures that
for chromosomes.