The genetic code is the set of complete information of the protein manufactured from RNA.
Inheritance of traits is possible due to the transfer of genetic information from a parent to the offspring. The process of transfer of genetic information starts with the replication and then transcription and translation of DNA. In the transcription process, the genetic information which is stored in the DNA gets copied into a form of RNA. The process of translation is completed by the ribosome which pairs amino acids as specified by mRNA (messenger RNA), with the help of tRNA (transfer RNA) molecules to carry amino acids and read the mRNA three nucleotides at one given time. The amino acid codon table has 64 entries and is very similar in all organisms; i.e., the genetic code is the same for the tiniest organism to the largest one. The genetic code table is a summarisation of an organism’s genetic code, a set of relationships between codons and amino acids.
The genetic code is the set of complete information of the protein manufactured from RNA. Therefore, even a minor change in the sequence can lead to the alteration in the formation of amino acids. It was Dr George Gamow who observed that there were 43 therefore 64 probable permutations of the 4 DNA bases, if we consider 3 at a time, then it would be lowered down to 20 distinct combinations provided that the order was irrelevant. He also proposed that these 20 combinations may code for the 20 amino acids which may be the only constituents of every protein.
The genetic code is stored on any one of the two strands of DNA molecule as a straight, well-spread and non-overlapping sequence of the nitrogenous bases namely, Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These abbreviation letters are used for the code words. The genetic code words consist of a three-lettered word called codons. These letters are written in a sequence with the length of the DNA strand. Each codon is a unique combination of these letters. Once they form a polypeptide chain, they will be interpreted as a single amino acid. In total, they can form up to 64 codons with the unique combinations of four words. Cells read their nucleotides in codons and decode mRNAs. Most codons specify amino acid, three stop codons together signify the end of a protein, one AUG codon which is a start codon signifies the beginning of a protein along with encoding the amino acid methionine. Cells read codons in a messenger RNA (mRNA) during translation starting with a start codon and until reaching the stop codon. The cells read the mRNA from 5’ to 3’. The mRNA specifies the order of the amino acids in a protein from N-terminus to C-terminus. Each codon is known to code only one and specific type of amino acid. Some codons together code one amino acid.
A table of the various combinations of the 3 nucleotides present that come together to form different types of amino acids is given below. [Image will be Uploaded Soon]
In the codon table, we see the different types of amino acids produced based on the codons translated by the cell. So the table shows 20 types of amino acids produced overall (see footer of the image) The abbreviations and names of all 20 amino acids are listed below. Ala= Alanine Arg= Arginine Asn= Asparagine Asp= Aspartic acid Cys= Cysteine Glu= Glutamic acid Gln= Glutamine Gly= Glycine His= Histidine Ile= Isoleucine Leu= Leucine Lys= Lysine Met= Methionine Phe= Phenylalanine Pro= Proline Ser= Serine Thr= Threonine Trp= Tryptophan Tyr= Tyrosine Val= Valine So, the below codons specify the amino acid ARG or Arginine are CGU, CGC, CGA, CGG, AGA, and AGG (see red markings in below figure) [Image will be Uploaded Soon]
Q1. What does the Codon Sequence Look Like? Ans: The sequence has a lot of information like the N-terminus and C-terminus. The mRNAs are decoded by cells by interpreting data in codons, i.e. the nucleotides in groups of three. The four nucleotides in RNA can inhabit any of the 3 probable combinations. Out of the 64 codon groupings, 61 specify the amino acids, and the remaining three (UAA, UAG, or UGA) serve as the stop signal. In the above diagram, UAG is the stop signal. These groupings depict the end of protein synthesis. There is one more codon – AUG, with an interesting characteristic. It serves as a start signal for beginning the translation. [Image will be Uploaded Soon] Q2. How to Read the Codon Table? Ans: In the codon table we see a lot of different abbreviations in the main body of the table. These look like ‘Phe’ and ‘Gly’. They are nothing but the different types of amino acids produced If you look at the first row, fourth column, third item, it says stop. This is because the 3 nucleotides / codon groupings referred to as UAA broken down as below –
Now, we already know that UAA is the stop signal to signify the ‘stop’ codon, i.e. indicating the cells to stop decoding mRNAs of the translation process of making a protein. That is why the table reads as ‘stop’ in this position.