How can DNAAbbreviation for deoxyribonucleic acid; the polymer of nucleotides that constitutes the genetic material of chromosomes. and RNAAbbreviation for ribonucleic acid; a biological polymer of nucleotides that is involved in protein synthesis. molecules act as blueprints for the manufacture of proteins? The exact details were unraveled in the early 1960s mainly by Marshall Nirenberg (born 1927) at the National Institutes of Health and H. G. Khorana (born 1922) at the University of Wisconsin, workA mechanical process in which energy is transferred to or from an object, changing the state of motion of the object. which earned them the Nobel prize in 1968. They showed that each amino acidA carboxylic acid containing an amino group (-NH2). In an alpha amino acid, the amino group is attached to the carbon atom adjacent to the carboxyl group. in a protein is determined by a specific codonA three-base sequence in a molecule of messenger RNA; a codon contains genetic information that specifies which amino acid will be incorporated into a protein at a specific point. of three nitrogenous bases in the DNA or RNA chain. The details of this genetic codeThe rules that govern how a sequence of three-base codons in a DNA molecule can be translated into a sequence of amino acids in a protein. are given in the table below. As an example of how this code works, let us take the section of RNA shown in Fig.3 on Nucleic Acid Structure. This has the sequence UCAUGG. This is part of the instructions for building a polypeptideA polymer of many amino acids joined by amide linkages or "peptide bonds." chain containing the amino acid serine (UCA) followed by the amino acid tryptophan (UGG).
The Genetic Code for RNA
Note: (a) A termination codon is indicated by TERM. (b) AUG, the codon for methionine is also the initiation codon. All protein synthesisFormation of substances with more complicated sturctures than do their precursors. begins at this codon, though this initial methionine is often removed during post-transcriptional processing.
Since each codon corresponds to three places in the nucleic acid chain and since there are four kinds of nitrogenous bases to fill each place, there are a total of 43 = 64 different possible codons. Since there are only 20 amino acids, the genetic code is degenerate—several different codons correspond to the same amino acid. This degeneracy acts as a safeguard against errors in reading the code. Thus UCU, UCC, UCA, and UCG all correspond to serine. If a mistake is made in reading the third base in this triplet, no harm is done since serine is still produced. On the molecular level transfer RNAs (tRNAAbbreviation for transfer RNA; the RNA that brings the correct amino acid to a ribosome where it is attached to a growing protein.), the molecules reading the codons and providing the correct amino acid, can pair with multiple codons. This only occurs in terms of the third base in the codon. For instance, G pairs with C, but is also capable of pairing with U. Some tRNAs even employ a fifth nitrogenous base, inosinate(I) which is capable of pairing with A, U or C. This use of multiple pairing with the third codon by tRNA is called the wobble hypothesis, and was first proposed by Francis Crick. Notice that while a tRNA can pair with multiple codon in the wobble hypothesis, it can only pair with codons for the same amino acid, and each codon is still specific to only one amino acid.
There are three additional features of the genetic code. First, AUG, the codon for Methionine also serves as an initiation codon, and, with help from other signals, is where protein systhesis begins. A second feature is that reading RNA for protein synthesis goes from the 5' carbon end of the nucleic acid to the 3' carbon end. A final important feature of the genetic code is the existence of three termination codons. These correspond to an instruction for ending a polypeptide chain. How these features work is best illustrated by an example.
EXAMPLE 1 Decode the RNA fragment
5' A C C U U A U G A C G C C U G U C C A U U A A C G A U 3'
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. First, we must decide which direction to read the RNA code. Synthesis goes from the 5' end to the 3' end, so this segment is read left to right. Had it been displayed 3' to 5', we would have needed to read it from right to left.
Second, we need to look for an initiation codon, AUG. This codon appears starting at the sixth letter in. Thus, we can divide the sequence up like this, with the start codon bold:
Third, let us see if there is a stop codon in this sequence. Sure enough, the fifth codon after the start codon, UAA is a stop codon. Thus, the entire sequence to be translated, in bold:
which translates to the amino acid sequence:
Notice in the example, that if we had not started with the initiation codon, an entirely different protein would have been formed. Look at what would have happened if we had simply started at the beginning of the sequence:
a stop codon appears in a new place, and the translated protien is:
This highlights the importance of the reading frame, the place where codons start being read. Notice that, since codons are 3 bases long, any sequence has three different reading frames. Without the initiation codon, there would be no way to identify the correct reading frame. In addition to the AUG initiation codon, other elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. regulate initiation. In bacteria, a sequence of bases before the initiation codon, called the Shine-Dalgarno sequence precedes the AUG codon, specifying where to begin translation. A different set up occurs in eukaryotes. An initiation complexA central metal and the ligands surrounding it; also called coordination complex. forms, but instead of having a specific sequence connected to the initiation codon, the complex slides along the mRNAAbbreviation for messenger RNA; the RNA that conveys information from the genetic code on DNA to a ribosome where transfer RNA attaches the correct amino acid to a growing protein. strand, until it finds the AUG initiation codon.