Mononucleotides are made up of 3 parts. A 5-Carbon (or Pentose sugar), a nitrogen-containing base and phosphoric acid. The pentose sugar in RNA is ribose, and in DNA, deoxyribose. Deoxyribose has one less oxygen atom then ribose. There are two types of nitrogen-containing bases in mononucleotides. There is the purine base, which have two nitrogen-containing rings, and there is the pyrimidine base, which only has one. The rings have the chemical property of being bases because of the nitrogen atoms they contain. DNA contains combinations of four different bases with equal numbers of pyrimidines and purines. The purines are adenine (A) and guanine (G). The pyrimidines are cytosine (C) and thymine (T). In RNA, the thymine is replaced with uracil (U). A phosphate group (PO43−) is the third component of a nucleotide. Inorganic phosphate ions are present in the cytoplasm of every cell. This phosphate group makes the mononucleotides. The sugar, base and phosphate group are joined together by condesation reactions to form the nucleotide. A hydrogen bond is when there is a dipole-dipole force between an electronegative atom, and a hydrogen atom, attached to oxygen, nitrogen or fluorine. Complementary base pairing is when two nucleotides on opposite DNA strands are connected with hydrogen bonds. Adenine joins with thymine, and guanine joins with cytosine. A DNA molecule is made up of two polynucleotide strands twisted around each other. The sugars and phosphates form the backbone of the molecule. Pointing inwards from this 'spine' are the bases, which pair up in specific ways. A purine always pairs with a pyrimidine, which makes a DNA double helix, a massive molecule that resembles a spiral staircase. The two strands are held by hydrogen bonds betwwen the complementary base pairs. There are ten of these pairs for each complete twist of the helix. The two strands are known as the 5' (5 prime) and 3' (3 prime) strand, named according to the numer of the carbon atom in the pentose sugar.
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