Index >> Cellular Components >> Deoxyribonucleic Acid DNA

Thus, in all, DNA samples examined by them the amount of A +C as equal to the amount of G+ T thus the moles per cent (G+C) varied with DNA from different species and organisms . This is now generally referred to as base ratio (A+T / G+C) and the per cent GC content of DNA is used as a measure of relatedness in organisms.

Although this is true for organisms containing Double stranded DNA, we now know that some bacteriophages contain DNA that is single stranded and in such DNA A+C will not be equal to G+ T.
Although from early chemical studies it was concluded that DNA is a long chain of nucleotides, it did not tell us the shape nor the length of the chains. In 1953, James Watson and Francis Crick correctly predicted the molecular structure of DNA based on the information that Chargaff had col1ected on the GC content and from the X-ray diffraction analysis of DNA preparations by Wilkins and his colleagues.

The diameter of the helix is 20 Å. For reasons of space and orientation, adenine on one chain pairs with only thymine on the adjacent chain and similarly guanine forms hydrogen bonds only with cytosine. It has now been proved be doubt that DNA in all procaryotic and eucaryotic organisms consists of two anti parallel chains of polynucleotides, arranged in a structure identical to that proposed by Watson and Crick. This structure is referred to as the right handed B helix The validity of Watson Crick model of DNA is not questioned any more. However, minor modifications have been recognized and these are; (I) not all DNA molecules exist in the form of double helix. The DNA of some bacteriophages is single stranded; (II) The number of hydrogen bonds between guanine and cytosine is three and not two as proposed earlier, (III) DNA from a variety of species occasionally contain a number of other modified bases addition to A, T. G and C and (iv) in addition to the B helical structure, DNA can also exist in other helical structures such as A,C and Z (left handed), each having slightly different dimensions and possible different roles