Genetic Code

Introduction

Genes contain information about different characters and characteristics of living beings. Genetic information is passed down through the generations in the form of code. It determines an organism’s characteristics, for instance, whether it is a bacterium or a human. The genetic code one possesses controls the entire life process.

We shall learn about genetic code definition and its properties in this section.

Genetic Code

We can define genetic code as a collection of rules for living cells to translate information encoded in genetic material (DNA or mRNA sequences). Ribosomes are in charge of completing the translation process. They use tRNA (transfer RNA) molecules to carry amino acids and read the mRNA, three nucleotides at a time, to link the amino acids in an mRNA-specified (messenger RNA) order.

Characteristics of Genetic Code

It is interesting to watch the complicated process of decoding genes and then to record them in the progeny to create a new set of genes. Millions of chemical reactions take place in a cell and requires well-controlled environment to make up the overall biochemical process.

Let us now proceed to better understand the key characteristics of genetic code.

  • Each codon has three bases, resulting in a total of 64 codons. Only 61 of the 64 codons can produce amino acids. The remaining three are used as stop codons during the translation process
  • One codon is in charge of directing the reactions that result in the production of amino acids. As a result, the procedure is quite particular
  • The synthesis of some amino acids may need numerous codons. This is referred to as genetic code degeneracy. For example, valine (Val) synthesis requires four separate consecutive codons – GUA, GUC, GUU, and GUG
  • Codons have a role in mRNA production; on the other hand, mRNA is in charge of generating proteins
  • The genetic code is also unique in that it is universal. It signifies that a single codon results in the production of single amino acid. Phenylalanine (Phe), for example, has the genetic code UUU. It is shared by all living creatures. This indicates that a bacterium’s Phe will be comparable to that of a human
  • Codons can have many functions at times. AUG, for example, is the genetic code for Methionine (Met). It also serves as a start codon or initiator
  • The basic genetic code is largely universal throughout all living things

Properties of Genetic Code

  • Triplet code : A codon is represented by 3 successive nucleotides on mRNA in 5’→3’
  • Unambiguous and Universal: All organisms have the same set of 64 codons
  • Degenerate code: One amino acid may be coded by more than one codon
  • Non-overlapping code; Same nucleotide cannot act as component of the next codon in succession
  • Commaless: Genetic code is read in continuation and every nucleotide takes part in the formation of genetic code
  • Start and stop codons: AUG is the start codon for each protein synthesis. It codes for methionine. # codons, UAA, UAG and UGA are termination codon and do not code for any amino acid
  • Polarity: Each triplet is read from 5′ to 3′, with the first base being 5

These characteristics of genetic coding are discussed further below.

Triplet Code

A codon, often known as a code word, is a sequence of nucleotides that specifies an amino acid. There is substantial evidence that three-nucleotide sequence codes for an amino acid in a protein, indicating that the code is a triplet.

Three-base codons are made up of the four nucleotide bases (A, G, C, and U). Sense codons are included in the 64 codons (that specify amino acids). As a result, there are 64 codons for 20 amino acids. This means that for one amino acid  more than one codon exists.

Commaless Code

There is no space between the codons for punctuation, indicating that each codon is directly adjacent to the one before it with no nucleotides between them.

Non-overlapping Code

The code is read in groups of three nucleotides, and a nucleotide that is part of one triplet is never part of the next.

As an example,

Serine has the 5′-UCU-3′ code.

The code 5′-AUG-3 represents methionine’.

Polarity

Each triplet is read from 5′ to 3′, with the first base being 5′, the middle base being 3′ and the last base being 3′. This means that codons have a fixed polarity, meaning that if the codon is read backward, the base sequence will reverse, resulting in two different proteins being specified.

Postulates of the Genetic Code

The following are the genetic code postulates:

  • AUG – is a codon that begins a strand of DNA
  • In most situations, one amino acid correlates to many mRNA codons in the genetic code. The first two nucleotides of a codon for one amino acid are identical, whereas the third differs
  • The genetic code is made up of three triplets. Codon refers to the triplet of mRNA
  • Stop codons are UAG, UAA and UGA
  • The nucleotide sequence can only be recognised one way, triplet by a triplet
  • All organisms have the same genetic code

Mutations of Genetic Codes

Not every person is the same. In fact, it has been discovered that a specific physiological feature vanishes. The genetic codes are altered and erased during transcription and replication, resulting in this condition. Distinct portions of DNA are altered and removed during the process, resulting in mutations.

In the process, genes are gained and lost, leading to new physical features in an organism. Consider the following scenario. Sickle cell anaemia is a blood condition that occurs when the amino acid valine (Val) is replaced by the amino acid glutamine (Gln) in a specific gene sequence.

Conclusion 

To summarise, genetic code is a collection of rules for living cells to translate information encoded in their genetic material. These set of rules help in understanding what is the information and how it is to be read and understood.

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