GENETIC CODE AND ITS PROPERTIES - Genetic code - triplet codons, start/stop signals and key properties

Oct 12, 2026

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Genetic code refers to the nucleotide sequence on messenger RNA (mRNA) that is complementary to DNA, and it governs how amino acids are assembled into a polypeptide chain during protein synthesis. With only four bases (A, U, G, C) present on mRNA, the code resolves how 20 amino acids are specified. The concept of the triplet (three bases per amino acid) was supported by work including Gamow’s proposal and experimental cracking by Marshall Nirenberg and Har Gobind Khorana. The document also outlines properties such as triplet structure, comma-less formatting, non-overlapping organization, universality, degeneracy, non-ambiguity, collinearity, and the roles of initiation and termination codons, plus questions for further study like wobble hypothesis.

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Genetic Code and Its Properties

Basic definition and role in protein synthesis

Triplet codons, sense codons, and stop codons

Initiation codon (AUG) and coding of methionine

Key properties: triplet, comma-less, non-overlapping, universality, degeneracy, ambiguity, collinearity

UR/S4M/2020 GENETIC CODE AND ITS PROPERTIES Genetic code are the sequence of nucleotides or nitrogenous bases present on messenger RNA (m RNA) . As they are originating from DNA their bases are complementary to the segment of DNA, from where they are originated. Genetic code plays the key role in arranging the amino acid sequences in a polypeptide chain during protein synthesis. In our body there are 20 different types of amino acids but the number of nitrogenous bases on mRNA is just four (A,U,G and C) . So, there was confusion earlier that- how these four base code for 20 amino acids during protein synthesis? This confusion was overcome by the research contribution of different geneticist. Gamow (1951) first raised a concept that genetic code may be triplet ie. three bases combinely code for one amino acid. This was established by Marshall Nirenberg through artificial synthesis of genetic code in the laboratory. He synthesize a polyuridylic molecule (composed of three uracil bases-UUU), which was found to code for amino acid phenylalanine when introduced to a cell-free medium containing all the enzymes required for protein synthesis (extracted from bacteria E. Coli) and all the 20 amino acids. This artificial synthesis of genetic code is described as the cracking of genetic code. Har Govind Khorana also synthesize artificial gene. Both the workers shared Nobel Prize in 1968. Tab-Showing the triplet codons of m RNAand the amino acids coded by them (in blue colour), three Termination codons (in red colour) and initiation codon, code for methionon(green colour) Among the 64 triplet codons 61 is sense codon, ie. they code for amino acids. There are three codons –UAA, UAG and UGA they do not code for any amino acid, known astremination codon.These codons performa a very important function to terminate the polypeptide chein synthesis , at the time of protein synthesis in the cytosol. They are also known as stop codon. Genetic code AUG is considered as the initiation codon being it is always present at the initiation point. AUG code for amino acid methionine. Properties of genetic code- • Triplet-Three nitrogenous bases comprises one genetic code arranged in a specific sequence. (About this property we have discussed already in the above paragraph) . • Commaless- there is no comma or any punctuation mark between the adjacent genetic codes. • Non-overlapping-On the mRNA, one codon do not overlap with the next codon. EgAUG GCA ACG GGA etc. • Universality- Same genetic code is present in all organism including virus, bacteria and all living organism. • Degeneracy of genetic code-as the number of genetic code is more than the number of amino acids, it is implide that one amino acid may be coded by more than one codon. This is known as degeneracy. To understand see the table above . • Ambiguity-A genetic code is always nonambiguous, ie. One codon never code for two different amino acids. • Collinearity-The codons on mRNA and the corresponding amino acid residues in the polypeptide chain have a linear arrangement. NB:- These are few lines only to guide you. You please go through the chapter and try to solve the following- 1. Where from the genetic codes are originated? Write about their functional role? 2. The initiation codon present on the 3′ end or 5′ end? 3. AUG is initiation codon. but, can it be present on the other parts of mRNA? 4. Is there any genetic code which can also be an initiation codon apart from AUG ? 5. Is there any genetic code, which does not show degeneracy? 6. Why termination codons do not code for any amino acid? 7. What is Wobble hypothesis?

FAQs

What is genetic code and how does it relate to DNA and mRNA?

Genetic code is the nucleotide sequence on mRNA. Because mRNA originates from DNA, its bases are complementary to the DNA segment they come from.

How does the triplet codon concept explain coding of 20 amino acids with four bases?

The genetic code is based on triplets, meaning three nitrogenous bases form a code for one amino acid. Although mRNA contains only four bases, the combinations of triplets allow specification of many amino acids.

What are initiation and termination codons, and what do they do?

AUG is the initiation codon and codes for methionine at the start of translation. Termination codons (UAA, UAG, UGA) do not code for amino acids and instead stop polypeptide chain synthesis.

Oct 12, 2026

Upload by ahduag