04 Molecular Basis of Inheritance - part 06 - Genetic Code

04 Molecular Basis of Inheritance - part 06 - Genetic Code


Genetic Code: 
  • DNA is a master molecule of a cell that initiates, guides, regulates and controls the process of protein synthesis. 
  • To perform this complicated function, it must carry the requisite information for the synthesis of proteins. 
  • Obviously this information has to be verily located in the DNA itself. 
  • The site for storing this information lies in the sequence of nucleotides (i.e. nitrogen bases),as evidenced by Yanofski and Sarabhai (1964). 
  • About, 20 different types of amino acids are involved in the process of synthesis of proteins. 
  • DNA molecule has 4 types of nitrogen bases to identify these 20 different types of amino acids. 
  • According to F.H.C. Crick, this information is stored in the form of coded language (cryptogram) called genetic code, that contains code words (codons) each one specifying (representing) specific amino acid. 
  • Genetic code, therefore, is a collection of base sequences that correspond to each amino acid. 
  • A single nitrogen base in a codon (singlet codon) will encode for only four different types of amino acids. 
  • A combination of two nitrogen bases (doublet codon) will specify only 16 different types of amino acids. 
  • A combination of three nitrogen bases (triplet codon) will specify 64 different types of amino acids. 
  • Hence G. Gamov (1954) suggested that in a codon, there must be combination of three consecutive nitrogen bases that will be sufficient to specify 20 different types of amino acids. 
  • Thus, there would be 64 different codons (code words) in the dictionary of genetic code and that each code word has to be a triplet codon. 
  • Every three consecutive nucleotides in DNA will constitute a triplet codon. Genetic code is a triplet code, was evidenced first by Crick (1961) using "frame- shift mutation". 
  • However, M. Nirenberg and Matthaei were able to synthesize artificial m-RNA which contained only one type nitrogenous base i.e. Uracil (Homopolymer). This synthetic poly-U sequence was transferred to protein synthesizing enzymes. 
  • A small polypeptide molecule was produced/ formed by the linking of phenylalaninemolecules. This explains that UUU codes for phenyl alanine. 
  • Later different homopolymer codons were deciphered. Codons formed by two or more bases were also tried. 
Dr. Har Gobind Khorana : 

  • He devised a technique for artificially synthesizing m- RNA with repeated sequences of known nucleotides. 
  • By using synthetic DNA, Dr. Khorana prepared chains of polyribonucleotides with known repeated sequences of two or three nucleotides. eg. CUC UCU CUC UCU. 
  • This resulted in formation of polypeptide chain having two different amino acids placed alternately (Leucine and Serine). 
  • Similarly polynucleotide chain with three- nitrogen base repeats gave polypeptide chain with only one amino acids. Eg. CUA CUA CUA CUA (leucine). 
  • Later, Severo Ochoa established that the enzyme (polynucleotide phosphorylase) was also helpful in polymerising RNA with defined sequences in a template- independent manner (i.e. enzymatic synthesis of RNA). 
  • Finally Nirenberg, Matthaei and Ochoadeciphered all the 64 codons in the dictionary of genetic code.
  • During replication and transcription, a nucleic acid is copied to form another nucleic acid. These two processes are based on complementarity principle. 
  • During translation, genetic information is transferred from a polymer of nucleotides to a polymer of amino acids. Here, complementarity principle does not exist. 
  • It is evident that change in nucleic acid (genetic material) results in the change in amino acids of proteins. This clearly explains that genetic code directs the sequence of amino acids during synthesis of proteins.

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