Understanding Inheritance

1“1=1~ GENETIC CODE I

(a) RNA Codons for the Twenty Amino Acids Secor base — What triplet of ribcmucleotides directs the Amino-acid u c A G abbreviations addition of, say, the amino acid alanine to a protein that is being synthesized? Of ly- Phe Ser Tyr Cys u Ala = Alanine sine? C)fany one of the twenty amino acids Phe Ser Tyr Cys c Arg = Arginine u Leu Ser STOP STOP A Asp = Aspartic acid found in proteins? That was the problem to Asn = Asparagine Leu Ser STOP Trp G be faced after advancement of the ideas Cys = Cysteine that a gene is a string of deoxyribonucle- Leu Pro His Arg u Glu = Glutamic acid otide triplets, that the string of deoxyribo- Leu Pro His Arg c Gln = Glutamine c Leu Pro Gln Arg A + Gly = Glycine nucleotidetriplets is transcribed into a string g ~ His = Histidine Leu Pro Arg G of ribonucleotide triplets, and that the string ~ Gln ~ Ile = Isoleucine of ribor ucleotide triplets is translated into a ~ lie Thr Asn Ser u % Leu = Leucine ii o Lys = Lysine string of amino acids–a protein. The results Ile Thr Asn Ser c /4 Met = Methionine Ile Thr Lys Arg A of research on the problem is condensed in Phe = Phenylalanine the genetic code, a listing of the sixty-four Met (start) Thr Lys Arg G Pro = Proline possible ribonucleotidetriplets and the amino Val Ala Asp Gly u Ser = Serine Thr = Threonine acid (or translation command) correspond- Val Ala Asp Gly c G Trp = Tryptophan Val Ala Glu Gly A ing to each. Fortunately for those who worked Tyr = Tyrosine Val Ala Glu Gly G on the Iproblem, the genetic code is organ- — Val = Valine ism-independent. That is, the same genetic code is used by virtually all organisms. Shown in (a) is the usual representation of codons instead of RNA codons, Each single- the genetic code. The letters U, C, A, and G stranded deoxyribonucleotide triplet listed Researchers began to crack the genetic are symbols for the ribonucleotides contain- in (b) isthesequence of the so-called sense code in the early 1960s, Marshall Nirenberg ing the bases uracil, cytosine, adenine, and strand of a DNA codon—the strand that and his collaborators added a synthetic guanine, respectively. The symbols in the does not serve as a template for synthesis RNA, consisting entirely of repetitions of a body of the table are three-letter abbrevia- of FINA. Note that most of the amino acids single ribonucleotide, say U, to a bacterial tions for the amino acids. To find the amino are specified by at least two codons, For extract that contained everything neces- acid specified by a particular codon (say the example, phenylalanine is specified by two sary for protein synthesis except RNA. The codon CAG), locate the first nucleotide (C) codons: TTT and TTC. Arginine is specified result was astringof the amino acid phenyl- along the left side of the table and the by a total of six codons: CGT, CGC, CGA, alanine, They concluded that the ribonucleo- second nucleotide (A) along the top of the CGG, AGA, and AGG. In general, the more tide triplet UUU codes for phenylalanine. table. Their intersection pinpoints one of an amino acid is used in protein synthesis Other ribonucleotide triplets were decoded four amino acids. Of those four the one the likelier it is to be specified by more than by performing similar experiments with syn- aligned with the third nucleotide (G) is the one codon. Note also the start codon (ATG) thetic RNAs containing only A’s, C’s, or G’s amino acid in question. Thus the amino acid and the three stop codons (TAA, TGA, and or various combinations of ribonucleotides. glutamine (Gin) is specified by the three- TAG) that are used to signal the beginning By 1966 research teams led by Har Gobind nucleotide sequence CAG. and end of protein synthesis, The substan- Khorarla and Marshall Nirenberg had tive difference between the two versions of crackecj the entire genetic code, Shown in (b) is another version of the ge- the genetic code is that in (b) the deoxyribo- netic code, one expressed in terms of DNA nucleotide T replaces the ribonucleotide U,

(b) DNA Codons for the Twenty Amino Acids

~la Arg Asp Asn Cys Glu Gln Gly His Ileu Leu Lys Met Phe Pro Ser Thr Trp Tyr Val STOP — (START) G3J. AGA GAT AAT TGT GAA CAA GGA CAT ATA TTA AAA ATG TTT CCA AGT ACA TGG TAT GTA TM!. CXG AGG GAC AAC TGC GAG CAG GGG CAC ATT TTG AAG TTC CCG AGC ACG TrlC GTG TAG G12T CGA GGT ATC CTA, CCT TCA ACT GTT TGA GCC CGG GGC CTG CCC TCG ACC GTC cGT CTT TCT CGC C1’c TCC

48 Los Alamos ,ScZence Number 20 1Q9?