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The Structure of DNA
The molecular structure of DNA allows it to carry and transfer information that controls the
physicaland behavioural characteristics of an organism.
Nucleotides are nitrogen containing organic molecules. Each nucleotide contains:
A pentose (5C sugar)
One or more phosphate group
A nitrogen containing base which could be one of the following
o Adenine (A) a double-ring purine
o Guanine (G) a double-ring purine
o Cytosine (C) a single-ring pyramidine
o Thymine (T) a single-ring pyramidine
Nucleotides combine by condensation reactionsto form long chains called nucleic acidsor
The nucleotides are linked by covalent phosphodiester bonds (between carbon 3 and
the hydroxyl group of the other) which can be broken by hydrolysis.
The order in which the nucleotides are combined is variableand this is what enables
DNA to store genetic information for all the organisms on the planet.
Double helix structure
Each polynucleotide chain has a sugar-phosphate
backbone with the bases projected inwards (this can
act to protect the genetic information).
Polynucleotide chains are antiparallel (run in opposite directions).
There is a 5'end (with the carbon 5 of the deoxyribose sugar
closest) and a 3'end.
The two polynucleotide chains are joined via the organic bases
with hydrogen bonds . This allows the strands to separate during
translationand transcription . Although individually these bonds
are very weak,
in combination they are very strong .
Complementary base pairing
Adenine (A) always binds with thymine (T) (2 hydrogen bonds)
Guanine (G) always binds with cytosine(C) (3 hydrogen bonds)
The structure of the nucleotides mean that only these can bind (purine with a pyramidine).
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Each base pair is 0.34 nmapart and there are 10 base pairsper turn of the helix . The
complementary base pairing can only occur if the two strands run antiparallel to each other.
Therefore the sequence of one strand determines the sequence of the other .…read more