Genetic Code

Genetic code is the coded information present in the sequence of nitrogen bases of DNA, and it determines the sequence of amino acids in proteins. It is the relationship between the sequence of nucleotides in DNA/mRNA and the sequence of amino acids in a protein.

• DNA acts as the master molecule of the cell because it stores the information needed for protein synthesis, and this information lies in the sequence of nucleotides.
• The order of bases in DNA controls the kind and order of amino acids in proteins, so the genetic code acts as a blueprint for protein formation.

• According to Francis H. C. Crick, the genetic code is stored in the form of code words called codons, and each codon specifies a particular amino acid.
• Since there are only 4 nitrogen bases but about 20 amino acids, a single base cannot code for all amino acids.
• A singlet codon would code for only 4 amino acids, and a doublet codon would code for only 16 amino acids.
• Therefore, George Gamow (1954) proposed that the code must be a triplet code, in which three consecutive nitrogen bases form one codon.
• A triplet code gives 64 codons (4³ = 64), which is enough to code for 20 amino acids.
• Thus, each codon is made of three consecutive nucleotides.

• Out of the 64 codons, 61 codons code for amino acids, while 3 codons do not code for any amino acid and therefore act as stop codons.
• The three stop codons are UAA, UAG, and UGA.
• AUG serves as the start codon and codes for methionine.

• The triplet nature of the code was first proved by Crick (1961) through the frame-shift mutation experiment.
• Marshall Nirenberg and Matthaei synthesized artificial poly-U mRNA containing only uracil and added it to a protein-synthesizing system.
• This produced a polypeptide made only of phenylalanine, showing that UUU codes for phenylalanine.
• Later, other homopolymer codons and codons made of two or more bases were also deciphered.
• Har Gobind Khorana developed a method for synthesizing RNA molecules with repeated known nucleotide sequences, which helped in decoding more codons.
• For example, repeated sequences like CUC UCU CUC UCU produced a polypeptide with alternating amino acids, while CUA CUA CUA produced leucine.
• Severo Ochoa showed that the enzyme polynucleotide phosphorylase could polymerise RNA with defined sequences in a template-independent manner, which also helped in deciphering the code.
• Finally, Nirenberg, Matthaei, and Ochoa deciphered all 64 codons of the genetic code dictionary.

• The genetic code is triplet, because each codon has three bases.
• It is degenerate, meaning that more than one codon can specify the same amino acid.
• Many synonymous codons differ in the third base, such as GUU, GUC, GUA, and GUG, all of which code for valine.
• This flexibility at the third base is called the wobble effect.
• Most of the genetic code is universal, meaning it is nearly the same in prokaryotes and eukaryotes.

• During replication and transcription, one nucleic acid forms another nucleic acid based on complementarity, but during translation, genetic information passes from nucleotides to amino acids, where direct complementarity does not exist.
• A change in nucleic acid sequence causes a change in the amino acid sequence of proteins.
• Therefore, the genetic code directly determines the structure of proteins and controls protein synthesis.

• The genetic code is the coded information in the base sequence of DNA/mRNA that determines the amino acid sequence in a protein.
• It is a triplet code - three consecutive bases form one codon, proposed by George Gamow (1954).
• There are 64 codons in total: 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA).
• AUG is the start codon and codes for methionine.
• It was deciphered mainly by Nirenberg, Khorana, and Ochoa (poly-U mRNA showed UUU = phenylalanine).
• It is degenerate (one amino acid can have several codons, usually differing in the third base - the wobble effect) and nearly universal.
• A change in the base sequence alters the amino acid sequence, so the code directly controls protein synthesis.