Proteins

"DNA makes RNA, RNA makes protein, and proteins make us." Francis Crick Although DNA is the carrier of genetic information in a cell, proteins do the bulk of the work. Proteins are long chains containing as many as 20 different kinds of amino acids. Each cell contains thousands of different proteins: enzymes that make new molecules and catalyze nearly all chemical processes in cells; structural components that give cells their shape and help them move; hormones that transmit signals throughout the body; antibodies that recognize foreign molecules; and transport molecules that carry oxygen. The genetic code carried by DNA is what specifies the order and number of amino acids and, therefore, the shape and function of the protein.

The "Central Dogma"—a fundamental principle of molecular biology—states that genetic information flows from DNA to RNA to protein. Ultimately, however, the genetic code resides in DNA because only DNA is passed from generation to generation. Yet, in the process of making a protein, the encoded information must be faithfully transmitted first to RNA then to protein. Transferring the code from DNA to RNA is a fairly straightforward process called transcription. Deciphering the code in the resulting mRNA is a little more complex. It first requires that the mRNA leave the nucleus and associate with a large complex of specialized RNAs and proteins that, collectively, are called the ribosome. Here the mRNA is translated into protein by decoding the mRNA sequence in blocks of three RNA bases, called codons, where each codon specifies a particular amino acid. In this way, the ribosomal complex builds a protein one amino acid at a time, with the order of amino acids determined precisely by the order of the codons in the mRNA.

In 1961, Marshall Nirenberg and Heinrich Matthaei correlated the first codon (UUU) with the amino acid phenylalanine. After that, it was not long before the genetic code for all 20 amino acids was deciphered.

A given amino acid can have more than one codon. These redundant codons usually differ at the third position. For example, the amino acid serine is encoded by UCU, UCC, UCA, and/or UCG. This redundancy is key to accommodating mutations that occur naturally as DNA is replicated and new cells are produced. By allowing some of the random changes in DNA to have no effect on the ultimate protein sequence, a sort of genetic safety net is created. Some codons do not code for an amino acid at all but instruct the ribosome when to stop adding new amino acids.

Table 1. RNA triplet codons and their corresponding amino acids.   U C A G U UUU Phenylalanine UUC Phenylalanine UUA Leucine UUG Leucine UCU Serine UCC Serine UCA Serine UCG Serine UAU Tyrosine UAC Tyrosine UAA Stop UAG Stop UGU Cysteine UGC Cysteine UGA Stop UGG Tryptophan C CUU Leucine CUC Leucine CUA Leucine CUG Leucine CCU Proline CCC Proline CCA Proline CCG Proline CAU Histidine CAC Histidine CAA Glutamine CAG Glutamine CGU Arginine CGC Arginine CGA Arginine CGG Arginine A AUU Isoleucine AUC Isoleucine AUA Isoleucine AUG Methionine ACU Threonine ACC Threonine ACA Threonine ACG Threonine AAU Asparagine AAC Asparagine AAA Lysine AAG Lysine AGU Serine AGC Serine AGA Arginine AGG Arginine G GUU Valine GUC Valine GUA Valine GUG Valine GCU Alanine GCC Alanine GCA Alanine GCG Alanine GAU Aspartate GAC Aspartate GAA Glutamate GAG Glutamate GGU Glycine GGC Glycine GGA Glycine GGG Glycine

A translation chart of the 64 RNA codons.