Genetic code

Introduction:

The genetic material found in the organisms is arranged through codes in a sequential manner. Codons are arranged on some criteria instead of being scattered ; this shows the fact that genetic material is scientifically organised . The bases  AUGTC (Adenine, Uracil, Guanine, Thiouracil and cytosine) are so paired that they form a sequence help in controlling the particular trait. Various combibation of AUTGC give rise to a specifi gentetic code forming amino acid sequences. The amino acids formed by the sequential organisation of the bases help in protein synthesis ensuring the survival of cells. Har Gobind Khorana has developed a formulae to explain genetic coding. With an example of RNA molecule, he proved that there are specific combinations of bases (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 define how to decode/decipher the codons resulting in a table of 64 unique codon combinations for DNA as shown below .

 

The Genetic Code Chart of 64 combinations

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

 

Features of Codons

A. A Code is a Triplet: The codons for a given amino acid must have 3 letters (AAA, AUG, ACG, etc).

B. In a codon, there can be a degenerate. A degenerate is formed if there is more than one codon for a single amino 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 comma less which means the codon is a continuous one instead of a comma (or punctuation) is present in between two codons. For example, -C-A-G   -A-U-G............

E. The Code is Unambiguous becasue any given codon can only code for a particular amino acid without being duplicated. Hence there is no question of misrepresentation of codons resulting in the formation of inappropriate amino acid.

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 or acquired change of the genetic structure, numbers or 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 genetic illnesses in the resulting progeny. The underlying causes of mutation can be the alteration of single base units in DNA, the deletion, insertion, or rearrangement of larger sections of genes or 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 are classified as

1. A missense mutation is the one in which there is a  change in the DNA base pair forming a wrong substitution of an amino acid that generates an abnormal protein.

2. The nonsense mutation results in the formation of a protein that functions improperly or there is 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 in a gene. In an insertion, there is an extra piece of DNA being inserted into the codons to generate a faulty protein synthesis.

4. Deletion occurs when one or more DNA bases get deleted. Although very few deletions are allowed in the few base pairs; however, if it happens on a larger scale may cause harm to the individual. 

5. Duplication is where a piece of DNA is copied twice or more times in an abnormal way resulting in altered function of the protein.

6. Frameshift mutation shifts the grouping base pairs that change the code for amino acids, consequently, there will be a protein that is usually nonfunctional.

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 this sentence as an example given below, each word corresponds to a codon-nucleotide sequence, 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 part of genic coding. It acts as a bridge between amino acids and mRNA as it helps to an mRNA codon to find the right amino acid. Structurally, each tRNA molecule has a unique folded structure with 3 hairpin shaped loops. One among these loops has a sequence called the anticodon. The anticodon sequence has the ability to recognize and decode an mRNA codon so that the right mRNA fits with appropriate amino acid. Each individual tRNA carries its corresponding amino acid attached to its 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

 

 

Features of Codons

1. A Code is a Triplet: Any codons for a given amino acid must have 3 letters (AAA, AUG, ACG, etc).In a codon, there can be a degenerate. A degenerate is when there is an occurrence of more than one codon for a single amino acid. There are 61 functional codons as shown above.

2. Codon is characterized by a non-overlapping tendency where the same letter –base is not used in the formation of more than one codon. But there may be overlapping sometimes.

3. The code is comma less which means the codon is a continuous one instead of a comma (or punctuation) is present in between two codons. For example, -C-A-G   -A-U-G

4. The Code is Unambiguous because a given codon always codes for a particular amino acid, wherever it is present instead of coding with inappropriate amino acid.

5. The Code is Universal as it is present in all organisms irrespective of they are prokaryotes or eukaryotes.



img-1

img-1

img-1