Information in the DNA is encoded in the form of a genetic code. The genetic code includes nitrogen base sequences in an mRNA chain.
Before understanding the genetic code, we need to understand the term ‘gene’.
Genetic code is a set of instructions by which information present in molecules of heredity such as DNA or RNA sequences is translated into proteins by the ribosomes of cells of living organisms.
The part of the genome that codes for the amino acids in proteins or RNA (Ribonucleic acid) is called a gene.
Source: Genetic code
Decoding the information present in the nucleic acids, DNA or RNA is the basic function of genetic coding and is called gene expression.
The genetic codon is a set of three-letter combinations of nucleotides specific for a particular amino acid or a stop signal. Francis Crick proposed the concept of codons in 1961.
A total of 64 possible combinations of the three-letter codons can be built from the four nucleotides.
From the 64 possible combinations, there are 61 codes for amino acids, and three of them act as stop signals.
One codon is specific for one amino acid, and since there are twenty amino acids, one amino acid may be coded for by more than one codon. This is called degeneracy or redundancy of the genetic code.
1. Sense codons
These are the codons that code for amino acids. There are 61 sense codons that code for 20 amino acids
2. Signal codons
These codons code for signals during the synthesis of proteins. AUG, UAA, UAG, and UGA are the four codons that code for signals. These are of two types:
Start codons: They begin the process of translation, hence also known as initiation codons, as they start the synthesis of polypeptide chains. GUG codes for the amino acid Valine which is an example of a start codon. AUG is another example that codes for amino acid Methionine.
Stop codons: They give the signal for termination of polypeptide chains synthesis. Also known as termination codons, the code for the termination and release of the synthesized polypeptide chains. They do code for any particulate amino acid. Hence they are also called Non-sense Codons. Examples are UGA, UAG, and UAA.
Among most genes, whether in animals, plants, or microorganisms, the same codons are assigned to the same amino acids and the same start and stop signal with a few exceptions.
Three RNA bases code for one amino acid in a triplet code. 64 codons code for 20 amino acids and the starting and stopping signals in the synthesis of polypeptide chains.
Three bases or nucleotides code for one amino acid. In a non-overlapping code, a single letter is read just once. Therefore in a non-overlapping code, six bases will code for two amino acids.
There are no demarcation commas or lines between the codons, so they are continuous. If a single base is deleted in a comma less code, it changes the entire sequence of the following amino acids after the point of deletion.
Of the 64 codons present in the genetic code, 61 codons code for 20 amino acids, but none codons code for more than one amino acid. Therefore it is ambiguous.
In many cases, several codons code for the same amino acid. Only two amino acids, namely, methionine and tryptophan are coded by a single codon each. Isoleucine is coded by three codons, five amino acids by four codons each and three amino acids by six codons each.
The other nine amino acids are coded by two codons each. This redundant code system protects an organism against any dangerous mutation in case any single base gets mutated as other codons will code for the same amino acid, and there will be no changes in the polypeptide chain.
This is the way of reading a message from a codon. The genetic code has a particular direction of reading that message. Reading from the opposite direction will code for some other amino acid. Since the reading is done from the five ends to the three end direction, the polarity of the genetic code is five ends to three ends.
1. What is a genetic code and its type?
The genetic code is a combination of rules which define how the four-letter code of DNA is translated into amino acids. The two types can be expressed as either RNA codons or DNA codons.
2. Why is genetic code important?
It provides evidence for the common origin of life on earth since it is similar in most living organisms.
3. What are the three principles of the genetic code?
The genetic code is always read in the 5’ to 3’ direction Codons are read as non-overlapping parts during translation There are start codons and stop codons in the synthesis of amino acids.
4. How does the genetic code work?
It is the way the four bases of DNA, A, C, G, and T, are attached that the ribosome of the cell can translate the code and synthesize proteins.
5. What are the four main features of the genetic code?
The genetic code is a universal, non-overlapping, triplet, and redundant.
6. What are the four properties of the genetic code?
The genetic code is non-ambiguous, has a polarity, comma less, and is a triplet codon.
7. What is genetic code explained in detail?
8. How many genes do humans have?
Humans have between 20,000 and 25,000 genes.
9. What does DNA stand for?
It stands for Deoxyribonucleic acid.
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