Genetic code – a divine writing

Ever wondered why your nose resembles your father’s nose? This is due to the inheritance of traits from generation to generation. Our modern understanding of how traits may be passed through generations comes from the principles proposed by Gregor Mendel in 1865. DNA is the genetic material that is transferred from the parent generation to offspring. DNA can be very well related to the genetic code. Genetic code refers to the specialized sequence of nucleotides on RNA (ribonucleic acid) that eventually determines the sequence of amino acids in a protein. Every individual that carries the genetic code for certain characteristics will show evidence of that characteristic. A lot of scientists also refer to the genetic code to be a blueprint. The genetic code is not only a simple collection of nucleotides but rather more complex. 

Discovery

Some of the early works suggest that Francis crick made certain remarks on the genetic code along with the discovery of the double helix nature of DNA with James Watson. However the origin and evolution of the genetic code remain complex to understand till now, there were various attempts to understand but none were able to completely crack the science behind it. Many other theories like the ‘stereochemical hypothesis’ came up for the same. Some of the studies also suggest that Francis Crick gave the ‘frozen accident hypothesis’ for genetic code. Few other contributions were made, for instance, Sydney Brenner. Marshall Nirenberg also gave his remarks on the same at the same time. [Source- The Genetic Code: Francis Crick’s Legacy and Beyond – PMC (nih.gov)]

George Gamow, who was a physicist proposed that the genetic code should be a combination of bases since there are only 4 fundamental nitrogenous bases that have to code for 20 amino acids. As per his suggestion, if 20 amino acids have to be coded, the genetic code should be a combination of 3 nucleotides. If we calculate-4³ (4 × 4 × 4), it would be 64 codons (at least sufficient for 20 amino acids). After that came the need to prove the triplet genetic code. Har Gobind Khorana developed a chemical method for synthesizing RNA molecules in a specifically defined combination of nitrogenous bases. Marshall Nirenberg worked on a cell-free system in order to perform protein synthesis. Another use was of Severo Ochoa enzyme called polynucleotide phosphorylase that played a role in polymerizing RNA with particular sequences in a specialized template-independent manner.

The codon

“Codon” is a beautiful biological term used to denote the specialized sequence of nucleotides (three in number) that together form the genetic code in an RNA/DNA molecule.

What is the genetic code used for?

Genetic code as a term is used to explain how the combination of nitrogenous bases of RNA [adenosine (A), cytosine (C), guanine (G), and uracil (U)] are read by the tRNA while translation to convert the genetic information of mRNA to proteins. The genetic code is seen to be present in the form of a row of three nucleotides or the codon which codes for a single amino acid.

How is the genetic code used?

Genetic code is the set of codes used by cells to translate information encoded within genetic material (DNA or mRNA sequence) into proteins. Thus it is the key basis of translation as well as genetic mechanisms.

Also read- Prescription Writing – My Biology Dictionary

Image Source: NCERT

Salient features of genetic code

Linear:

It always has a linear form where it is represented by sequences of ribonucleotide bases. The sequence of amino acids seen in a protein tends to correspond to the sequence of nucleotide bases in the gene (DNA) and subsequently the mRNA. It must be noted that it is the beauty of genetic code that even a minor alteration in a particular codon can result in a change of amino acid in the corresponding position in the respective protein. Thus this arrangement of code and protein is said to be co-linear.

Triplet:

As per the discovery, the genetic codon was found to be a triplet. It has the role to specify which amino acid will be added next during protein synthesis (translation process). It is known that 61 codons out of a total of 64 codons have the role of code for amino acids while 3 codons play the role of stop codons. 

Universal:

The word universal highlights that the same code is applicable throughout the living organisms in nature i.e. universal. It serves as a potential common language to translate nucleotide sequences of DNA/RNA to amino acid sequences of proteins.  For instance, GGG codes for Glycine in all organisms and UUU particularly code for phenylalanine in every organism. Certain exceptions do exist. Eg. UGA is a stop codon as per normal code however, it codes for tryptophan in mitochondria. Some exceptions are also seen in some protozoans. 

Degenerate:

It is a special feature which implies that a given amino acid can be specified by more than one triplet codon. For instance, codons GAA and GAG both code for Glutamic acid. These codons coding the same amino acid tend to differ in any of their three positions. 

Image Source: www.researchgate.net

Nonoverlapping and commaless:

Three nucleotides of a codon in mRNA tend to specify one amino acid in a protein. There is no concept of overlapping. This implies that successive triplets have to be read in order (contiguous fashion). The code is hence continuous and commas less with no letter being wasted. 

Start and stop codon:

AUG, which codes for methionine is an initiator codon. There are three stop codons- UAG is Amber, UGA is Opal, and UAA is Ochre. They are also referred to as termination codons. Another name given is ‘nonsense’ codons. 

Unambiguous:

There is no ambiguity in the genetic code. A codon will always code for a particular amino acid despite its position. Each codon will specify only one amino acid. For instance, UAU codes only Tyrosine. 

Keep up the work geneticists. Thank you for reading at MBD.

Team MBD

Watch more here-Genetic Code 3-D – YouTube