Genetic Code

Genetic information specifies the correct sequence of the amino acids in a polypeptide chain. Each of the twenty different types of amino acids is represented in a DNA molecule by a triplet code, consisting of sequences of three nucleotides. That is, the sequence C, G, T in a DNA strand represents one kind of amino acid; the sequence

Each nucleotide of a nucleic acid consists of a 5-carbon sugar (S); a phosphate group (P); and an organic, nitrogenous base (B).

Figure

Each nucleotide of a nucleic acid consists of a 5-carbon sugar (S); a phosphate group (P); and an organic, nitrogenous base (B).

Figure

A single strand of DNA consists of a chain of nucleotides connected by a sugar-phosphate backbone.

Figure 4.18

Figure 4.18

DNA consists of two polynucleotide chains. The nitrogenous bases of one strand are held to the nitrogenous bases of the second strand by hydrogen bonds (dotted lines). Note that the sugars point in opposite directions—that is, the strands are antiparallel.

G, C, A represents another kind; and T, T, A still another kind. Other sequences encode instructions for beginning or ending the synthesis of a protein molecule.

The sequence of nucleotides in a DNA molecule dictates the sequence of amino acids of a particular protein molecule and indicates how to start or stop the protein's synthesis. This method of storing information for protein synthesis is the genetic code. However, because DNA molecules are located in the nucleus and protein synthesis occurs in the cytoplasm, and because the cell must keep a permanent copy of the genetic instructions, the genetic information must somehow get from the nucleus into the cytoplasm for the cell to use it. RNA molecules accomplish this transfer of information.

The genetic code is said to be universal because all species on earth use the same DNA base triplets to specify the same amino acids. Researchers deciphered the code in the 1960s. When the media mentions an individual's genetic code or that scientists are currently breaking the code, what they really are referring to is the sequence of DNA bases comprising a certain gene or genome — not the genetic code (the correspondence between DNA triplet and amino acid).

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