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DNA is the single most important molecule found within cells It is a stable polynucleotide which contains coded information for inherited characteristics It is contained in chromosomes in the nucleus of an eukaryot cell The essential features of the Watson-Crick model are summarised below 1 The two helical polynucleotide chains are coiled around a common axis The two chains have opposite polarity ie they are antiparrallel 2 The regular repeating sugar phosphate backbone of each strand lies on the outside of the helix The purine and pyrimidine bases project inwards at 900 to the axis of the helix 3The two strands are held together by hydrogen bonding between pairs of bases such that guanine always pairs with cytosine and adenine always pairs with thymine this is called complementary base pairing 3 The diameter of the helix is 20 nm and adjacent bases are separated by 034 nm and inclined at 360 relative to each other This means that each complete turn of the double helix contains about 10 base pairs 4 The amount of guanine is usually equal to that of cytosine The monomers of RNA and DNA are called nucleotides Each nucleotide has three parts A Five Carbon or Pentose Sugar The sugar will be one of two very similar pentose rings Ribonucleic acids contain the sugar ribose Deoxyribonucleic acids contain the sugar deoxyribose The only difference between these two sugars is that deoxyribose contains one oxygen atom less than ribose Pentose sugars are essential because they are involved in linking different nucleotides together by condensation reactions The Nitrogen-Containing Bases There are two types of bases found in nucleic acids The purine bases have two nitrogen containing rings while the pyrimidines have only one In DNA the purines are adenine A and guanine G and the pyrimidines are cytosine C and thymine T In RNA the purine bases are the same as in DNA but the pyrimidines are cytosine and uracil U These rings
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