... put into two groups Pyrimidines are bases of a single ring structure with three pyrimidine bases (cytosine, thymine and uracil) Purines are bases of a double ring structure with only two purine basis (adenine and guanine) The joining up of one nucleotide to another, between the sugar and phosphate groups forms a polynucleotide chain. In DNA molecules, the nucleotides contain one of the four bases - adenine, guanine, cytosine or thymine. In RNA molecules, the nucleotide contain one of the bases - adenine, guanine cytosine or uracil, which shows that RNA replaces thymine with uracil and is single stranded unlike the double stranded DNA. In 1951, American chemist Erwin Chargaff analysed DNA by using chromotography to separate the four bases in DNA. The amounts of each base were measured quantitatively and the results showed that adenine and thymine were similar and the amounts of cytosine and guanine were similar. These examples are shown in ...

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