Chapter 28: Nucleosides, Nucleotides and Nucleic Acids
Deoxyribose Nucleic Acid : DNA
There 2 important biological processes involving DNA:
replication to produce a "copy" of the DNA
protein biosynthesis via RNA
DNA is based on the carbohydrate deoxyribose and the pyrimidine
bases cytosine and thymine and the purine bases adenine and guanine.
The structure is the famous double helix discovered by Watson and Crick
(shared Nobel Prize1962).
There are 3 forms of DNA : A-DNA (compact), B-DNA (common)
and Z-DNA (left hand double helix)
B-DNA involves two strands of nucleic acids twisted around each
other with a right hand twist in an antiparallel fashion.
Antiparallel means the sense of the individual strands is in opposite directions.
The double helix is stabilised by H-bonding between the nitrogen
bases in the interior of the helix.
Note how the bases in the helix are p-stacked
parallel to each other 3.4 Angstroms apart.
Careful inspection of the DNA helix shows a wide gap = the major groove,
and a narrow gap = the minor groove between the strands.
Show hydrogen bonds
Show backbone
Show bases
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The double helix of a partial strand of
DNA
An important observation was mad e by Chargaff : the amounts of adenine
(A) and thymine (T) were about equal, around 19.5% and so were the amounts
of guanine (G) = cytosine (C) at 30.3%.
This led to the realisation that the hydrogen bonding interactions
between
specific base pairs was important : A=T and GºC
These interactions are shown below:
G-C base pair
A-T base pair
This interaction is so specific that it means that these are the only
pairings that are observed : A=T and GºC
only
