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9.1 The Structure of DNA The DNA molecule is a polymer of nucleotides. Each nucleotide is composed of a nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate group. There are four nitrogenous bases in DNA, two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). A DNA molecule is composed of two strands. The genetic code & codon table (article) | Khan Academy Most codons specify an amino acid. Three "stop" codons mark the end of a protein. One "start" codon, AUG, marks the beginning of a protein and also encodes the amino acid methionine. Codons in an mRNA are read during translation, beginning with a start codon and continuing until a stop codon is reached. mRNA codons are read from 5' to 3' , and ... Nucleic acids (article) | Khan Academy Each nucleotide is made up of three parts: a nitrogen-containing ring structure called a nitrogenous base, a five-carbon sugar, and at least one phosphate group. The sug...