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Complementary Nucleotide Bases
DNA* is the information molecule of the cell. DNA’s capacity to store and transmit heritable information depends on interactions between nucleotide bases and on the fact that some combinations of bases form stable links, while other combinations do not. Base pairs that form stable connections are called complementary bases.
Consistent pairings of complementary bases allow cells to make double-stranded DNA from a single strand template, create messenger RNA from DNA and synthesize proteins from individual amino acids by matching nucleotides bases on messenger RNA with their complementary bases on transfer RNA.
The polynucleotides chains that make up DNA and RNA form via covalent bond*s between sugar and phosphate subunits of neighboring nucleotides along a chain. In addition to the strong covalent bonds that hold polynucleotide chains together, bases along a polynucleotide chain can form hydrogen bonds with bases on other chains (or with bases elsewhere on the same chain, as with secondary structure in RNA).
The formation of stable hydrogen bonds depends on the distance between two strands, the size of the bases and geometry of each base. Stable pairings occur between guanine and cytosine and between adenine and thymine (or adenine and uracil in RNA). Three hydrogen bonds form between guanine and cytosine. Two hydrogen bonds form between adenine and thymine or adenine and uracil.
Complementary pairs always involve one purine and one pyrimidine base*. Pyrimidine-pyrimidine pairings do not occur because these relatively small molecules do not get close enough to form hydrogen bonds. Purine-purine links do not form because these bases are too large to fit in the space between the polynucleotide strands. Asymmetry in the structure of non-complimentary purine - pyrimidine pairs cause some crowding and prevent stable bonds from forming.
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