Chapters
In this article, we will discuss protein synthesis and the universal genetic code. We will specifically discuss that polypeptide is encoded by a gene and that a gene is a sequence of nucleotides that creates components of a DNA molecule. We will also describe the principle of the universal genetic code in which various triplets of DNA bases either encode for specific amino acids or correspond to start and stop codons. So, let us get started.
Protein Synthesis - From Gene to Polypeptide
- A gene refers to the sequence of nucleotides that create a component of a DNA molecule. A single DNA molecule has several genes.
- This sequence of genes, i.e. nucleotides bases codes for the generation of a particular polypeptide, i.e. protein
- The molecules of proteins are composed of a series of amino acids that are bonded together. The shape and behaviour of these molecules are determined by the exact sequence of these amino acids. Remember that the initial sequence of amino acids is referred to as a primary structure of the protein molecule
- Hence, the genes of DNA molecules control the structure of the protein and protein function. This is because the exact sequence of the amino acids is determined by them.
In the next section of the article, we will discuss what is a universal genetic code.
The Universal Genetic Code
We have already discussed above that a gene is a sequence of nucleotide bases in a DNA molecule that codes to produce a particular sequence of amino acids, that in turn create a specific polypeptide (protein).
The DNA nucleotide base code present within a gene is a triplet or a three-letter code. Each triplet of bases, i.e. sequence of three bases ciphers for a single amino acid. These triplets of bases are referred to as codons. Each codon codes for a different amino acid. Remember that 20 distinct amino acids are used by the cells to create different proteins. For instance:
- The amino acid valine is coded as CAG
- The amino acid lysine is coded as TTC
- The amino acid leucine is coded as GAC
- The amino acid glycine is coded as CCG
Some of the sequences of three bases code for start and stop signals. These signals inform the cells where the particular genes start and stop. This helps the cells to read the DNA correctly. Remember that the code never overlaps. It also generates the correct sequences of amino acids that it needs to function properly.
Number of Bases
Since there are four bases, hence 64 distinct triplets are possible (
). Although, there exist only 20 different amino acids in biological proteins. As a result, multiple codons code for the same amino acids which results in the degeneration of the code (the effect of mutations are limited by it).
The genetic code is universal which implies that almost every living organism employs the same code. Now, what does it mean? Well, it simply means that in all the living organisms, the same codons code for the same amino acids, hence, genetic information is passed between the species.
In the next section of the article, we will discuss the features of the universal genetic code.
Characteristics of Genetic Code
The genetic code contains 64 triplets of nucleotides which are known as codons. Except for three (UAA, UAG, UGA), each of the 64 codons encodes for one of the 20 amino acids used in the protein synthesis. It results in some redundant codes because the majority of the amino acids are encoded by more than one codon.
The genetic code refers to the set of rules by which living organisms convert information encoded in genetic material like DNA or RNA sequences into proteins. Now, let us discuss some of the important characteristics of the genetic code.
Features of the Genetic Code
Some of the important characteristics of the genetic code are discussed below:
Triplet nature
A genetic code for 64 distinct combinations (4 x 4 x 4) is possible with the triplet code. It provides a lot of information in the DNA molecule to identify where all the 20 amino acids are placed. When experiments were conducted to decode the genetic code, it was the code that was a triplet. The three-letter codes of nucleotides are known as codons. Examples include AUG and AAA, etc.
Degeneracy
The genetic code is degenerate which implies that the same amino acid is coded by multiple base triplets. For instance, the three amino acids alanine, leucine, and arginine have six similar codons.
Non-overlapping
The genetic code is non-overlapping which implies that the adjoining codons do not overlap. The non-overlapping code indicates that the same letter is not employed for two different codons. In other words, no single base can participate in the creation of more than one codon.
Comma less
No signal is present to indicate the end of one and the start of the next codon. It means that the genetic code is comma less (free of the comma).
Non-ambiguous
A specific codon always codes for the same amino acid. More than one codon can code the same amino acid, however, the same codon should not code for two or more different amino acids. It means that the code is non-ambiguous.
Universality
The genetic code is based on the experiment conducted on the bacterium Escherichia coli, however, it can be applied to other organisms too. One of the key features of the genetic code is its universality. It implies that the same sequence of three bases encodes the same amino acids in all living organisms ranging from simple microorganisms to complex, multicellular organisms like human beings.
Polarity
The genetic code is polar which implies that the code is always read in the fixed direction, i.e. in the 5′ → 3′ direction.
Chain beginning codons
The triplets GUG and AUG play dual roles in the bacterium E.coli. When they are present in between the two ends of a cistron (middle position), they encode for the amino acids methionine and valine respectively in a middle position in the protein molecule.
Chain termination codons
The three out of 64 triplets UAA, UAG, and UGA do not encode for any amino acid. In the beginning, they were described as non-sense codons, as opposed to the rest of the 61 codons, which are referred to as sense codons.
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