Genetic code

Introduction

The genetic material found in the organisms is arranged in the form of  sequential codes in a permutation and combination styles called codons. Codons are arranged logically so that each codon has its own significance . A codon contains  bases  AUGTC (Adenine, Uracil, Guanine, Thiouracil and cytosine) are arranged   in some order so that they form a sequence that help in controlling the specific trait in an individual.

 

Codons are linked with the synthesis of amino acids therefore, a variety of combinations of AUTGC give rise to a specific genetic code that synthesizes the correct sequence of amino acids. The amino acids formed by the sequential organization of the bases help in the protein synthesis. For the first time, Har Gobind Khorana has developed a formula to explain genetic coding. By taking an example of the RNA molecule, he proved that bases are arranged in a specific manner and the combinations of bases form the homopolymers and copolymers in the RNA. At the end, Marshall Nirenberg’s has proposed a model called cell-free system for protein synthesis. He was able to decode the sequence of codons resulting in a table of 64 unique codon combinations forming a variety of amino acids as shown in the table below.

 

The Genetic Code Chart of 64 combinations

The genetic material in DNA is arranged into 64 codon combination helping in the synthesis of various amino acids. AUTGC undergoes series of permutations and combinations resulting in these combinations. Some of the combinations can be AAA, AUG, GCA, UUU, TTT, CCC, UTC and the list goes on. As a result, different amino acids are produced. Amino acids assist in protein synthesis - a basis for cell division and growth.

 

 

Features of Codons

A. A genetic codon is a Triplet hence the codons for a given amino acid must have 3 letters such as AAA, AUG, ACG, etc.

B. In a codon,  a degenerate may be present. A degenerate is the one in which the same codon can be repeated to form the same amino acid, for example, if we look at the table above, GAT and GAC both can form the same amino acid Aspartic acid.

C. Codon is characterized by a non-overlapping tendency ( with a few exceptions) where the same letter –base is not used in the formation of more than one codon. 

D. The code is commaless entity which means, the codon is a continuous series with no comma in the middle. For example, -C-A-G   -A-U-G............

E. The Code is Unambiguous as a particular codon can only code for a specific amino acid without being duplicated.  However, rarely a codon can form a degenerate.

F. The Code is Universal and common in all organisms irrespective of whether they are prokaryotes or eukaryotes.

 

The genetic code and the mutations

Mutation is an inborn error in chromosomes characterized by the manipulation of the number and sequence of chromosomes resulting in malfunction of some of the genes. A healthy mutation is necessary for the variations to occur through generations. On the other hand, an abnormal mutation can cause wrong expression of alleles resulting in genetic illness in the resulting progeny. The underlying causes of mutation can be the alteration of single base units in DNA, deletion, insertion, or rearrangement of larger sections of genes in a given  chromosomes. There are a number of ways a DNA can be altered, as a result, a variety of mutations such as addition, deletion, substitution and frameshifting can occur. Mutations are caused by the change in lifestyle, pollution, carcinogens, stress and familial factors.

 

Mutations 

1. A missense mutation is a point mutation in which a  change in the nucleotide sequence results in a codon that codes for a different amino acid. It is a type of non-synonymous substitution resulting in the abnormal protein synthesis.

2. The nonsense mutation can result in the improperly functioning protein or no protein synthesis at all. It occurs when the altered DNA sequence prematurely signals the cell to stop building a protein.

3. Insertion mutation results in a change in the number of DNA bases of a gene. An insertion is characterized by the addition of  extra piece of DNA into the existing series of codons that results faulty protein synthesis.

4. Deletion occurs when one or more DNA bases are deleted. Although very few deletions are allowed in the few base pairs; but if it happens on a larger scale, it hampers the protein structure.

5. Duplication occurs when a piece of DNA is copied twice or more times in an abnormal way.

6. Frameshift is a mutation as a result of the addition or deletion of a base pair or base pairs in the DNA of a gene resulting in the translation of the genetic code into an unnatural reading frame from the position of the mutation to the end of the gene.

7. Repeat expansion occurs when the short sequences of nucleotide repeats in a row. For instance,  a trinucleotide made up of 3-base-pair sequences can be repeated 3 times consecutively.

 

Illustrative examples of gene mutation

Take the sentence written below as an example, each word corresponds to a codon-nucleotide sequence and the entire sentence forms a genetic matter.

1. Normal codon- RABBIT EATS GREEN GRASS

2. Insertion error occurs when an additional B comes in the word RABBIT, SO the expression is - RABBBIT EATS GREEN GRASS. Similarly, if we now insert two letters in the same word –say BE now the expression is RABBBIET EATS GREEN GRASS.

3. Deletion error is when a letter is deleted, say, for example, RA0BIT EATS GREEN GRASS. Here, B is absent indicating ZERO in its place.

 

t-RNA as an adapter molecule

A transfer RNA (tRNA) is the most important component of genetic coding. It serves as a bridge between amino acids and mRNA as it helps to an mRNA codon to find the right amino acid. Each tRNA molecule has a unique folded structure with 3 hairpin shaped loops as shown in the image below. One among these loops contains an anticodon. The anticodon sequence has the ability to recognize and decode an mRNA codon so that the right mRNA fits with appropriate amino acid. An individual tRNA carries its corresponding amino acid attached to the tail. When a tRNA recognizes and binds to its corresponding codon in the ribosome, the tRNA will then transfer the appropriate amino acid to the end of the growing amino acid chain. Ultimately, the tRNAs and ribosomes keep decoding the mRNA molecule until the entire sequence is translated into a useful form.

 

t-RNA adapter molecule.

t-RNA Adapter molecule

 

 

Read more

1.       

Introduction to genetics and heredity

2.

Gene mutation

3.

Human genome project

4.

Mendelian inheritance

 

Questions

  1. What is a genetic code?

  2. Briefly explain the features of codons.

  3. What is the relationship between protein synthesis and genetic codes?

  4. Mention the purines and pyrimidines.

  5. Explain the t-RNA adapter molecule.

 



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