When one or two nucleotides were inserted, protein synthesis was completely abolished. This was later confirmed experimentally Francis Crick and Sydney Brenner used the chemical mutagen proflavin to insert one, two, or three nucleotides into the gene of a virus. In other words, a given amino acid could be encoded by more than one nucleotide triplet. Scientists theorized that amino acids were encoded by nucleotide triplets and that the genetic code was degenerate. In contrast, there are 64 possible nucleotide triplets (4 3), which is far more than the number of amino acids. Nucleotide doublets would not be sufficient to specify every amino acid because there are only 16 possible two-nucleotide combinations (4 2). Given the different numbers of “letters” in the mRNA and protein “alphabets,” scientists theorized that combinations of nucleotides corresponded to single amino acids. The Genetic Code Is Degenerate and Universal Ribosomes are able to read the genetic information inscribed on a strand of messenger RNA and use this information to string amino acids together into a protein. However, the translation to protein is still systematic and colinear, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on.įigure 15.3 Instructions on DNA are transcribed onto messenger RNA. The translation to protein is a bit more complex because three mRNA nucleotides correspond to one amino acid in the polypeptide sequence. The copying of DNA to RNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every nucleotide read in the DNA strand. Because the information stored in DNA is so central to cellular function, it makes intuitive sense that the cell would make mRNA copies of this information for protein synthesis, while keeping the DNA itself intact and protected. The decoding of one molecule to another is performed by specific proteins and RNAs. The flow of genetic information in cells from DNA to mRNA to protein is described by the Central Dogma ( Figure 15.3), which states that genes specify the sequence of mRNAs, which in turn specify the sequence of proteins. ![]() The Central Dogma: DNA Encodes RNA RNA Encodes Protein It is the variety of amino acid side chains that gives rise to the incredible variation of protein structure and function. The side chain may be nonpolar, polar, or charged, as well as large or small. Each amino acid is composed of an amino group ( N H 3 + N H 3 +), a carboxyl group (COO -), and a side chain (blue). Figure 15.2 Structures of the 20 amino acids found in proteins are shown.
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