In the DNA molecule:
The total amount of purine nucleotides and pyrimidine nucleotides is not always equal
There are two strands, which run parallel in the 5' → 3’ direction
The proportion of adenine in relation to thymine varies with the organism
There are two strands, which run antiparallel - one in 5' → 3’ direction and the other in 3’→ 5’
the salient features of the Double-helix structure of DNA include that it is made of two polynucleotide chains with anti-parallel polarity. This means if one chain has the polarity 5' → 3', the other has 3' → 5' . Furthermore, Chargaff's rules state that for double-stranded DNA, the ratio between Adenine and Thymine is constant and equals one, and the total amount of purines equals pyrimidines .
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