Genetic Code And Mutation - An Overview

Genetic code and mutations are integral aspects of biology that profoundly influence the diversity and functioning of living organisms. The genetic code is the universal language by which genetic information encoded in DNA or RNA is translated into proteins. It contains codons of specific nucleotides sequences, which determine the amino acids incorporated into proteins during protein synthesis. Understanding the genetic code has paved the way for deciphering the intricacies of gene expression and protein function.

• Mutations are changes that occur in the DNA sequence.
• It can alter the genetic information carried by an organism. 
• Various factors, such as errors during DNA replication, exposure to mutagens, or natural genetic variations, can cause mutations. 
• Mutations can have diverse effects, ranging from negligible to detrimental or beneficial.
• They are the ultimate source of genetic variation and play a vital role in evolution.

Also Read: Difference Between DNA and RNa

Key terms: Genetic code, Mutations, DNA sequence, Amino acids, Protein synthesis, Gene.

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What is Genetic code?

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One of the remarkable properties of the genetic code is its universality. This means that the code is nearly identical in all organisms, from simple bacteria to complex mammals. The universality of the genetic code allows scientists to study and compare genes across different species. 

For example, a specific codon in one organism will typically encode the same amino acid in another organism, facilitating our understanding of gene function and protein structure across various organisms.

• The discovery and deciphering of the genetic code were significant milestones in molecular biology. 
• In the early 1960s, scientists conducted groundbreaking experiments to decipher the relationship between codons and amino acids. 
• Through their work, they established the correspondence between specific codons and the amino acids they encoded.

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Properties of Genetic Code

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Genetic codes in each organism have several properties.

Redundancy

Multiple codons can encode the same amino acid because the genetic code is redundant or defective. Due to this redundancy, a mutation that modifies a single nucleotide in the codon may not necessarily affect the encoded amino acid. Thus, providing some error tolerance.

Non-Overlapping 

The codons can be readed in a non-overlapping manner during translation. Each codon is recognized as a separate unit, and there is no overlap or sharing of nucleotides between adjacent codons.

Start and Stop Codons

The genetic code includes specific codons that serve as start and stop signals. The start codon, AUG (adenine-uracil-guanine) in RNA or ATG (adenine-thymine-guanine) in DNA, begins protein synthesis. The stop codons (UAA, UAG, and UGA) signal the termination of protein synthesis, and these codons encode no amino acid.

Also Read: Klinefelter syndrome

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What is Mutation?

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Mutations are alterations that take place in an organism's DNA sequence.

• In organisms, one or more mutations can occur at the same time or throughout life.
• A number of things can contribute to the development of mutations. 
• For example, mistakes in DNA replication, exposure to mutagens (such as certain chemicals or radiation), or even innate genetic variants that develop over time. 
• Both germ and somatic cells can experience mutations.
• Mutations can have different effects on an organism. 
• Some mutations are silent, meaning they do not alter the resulting protein's amino acid sequence.
• Such mutation happens due to the genetic code's redundancy. 
• Other mutations can be neutral, meaning they result in a slight change in the protein but have no significant impact on the organism's phenotype. 
• However, some mutations can be detrimental, leading to genetic disorders or decreased fitness. 
• For example, mutation causes genetic diseases like cancer, diabetes, klinefelter syndrome etc.
• Mutations can be beneficial on rare occasions.
• It provides an evolutionary advantage in specific environments.

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Types of Genetic Code

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Genetic code is nearly identical in all organisms and consists of specific codons that correspond to amino acids or serve as start or stop signals during protein synthesis. However, mutations can occur within the genetic code, leading to changes in the DNA sequence and subsequent alterations in the proteins produced. 

Standard Genetic Code

The standard genetic code is most common and widely observed across organisms. It consists of 64 codons, including 61 that specify amino acids and three stop codons. Mutations can affect the standard genetic code by altering the sequence of nucleotides that changes the codons and, consequently, the amino acid sequence of the resulting protein.

Non-Standard Genetic Code

Some organisms, such as certain bacteria and organelle like mitochondria have variations of the standard genetic code. 

• These non-standard genetic codes may have differences in codon assignments. 
• It means that particular codons encode different amino acids than in the standard code. 
• Mutations can occur in these organisms that further modify the non-standard genetic code. 
• This results in potentially impacting protein structure and function.

Expansion of the Genetic Code

In addition to the standard and non-standard genetic codes, some organisms have expanded their genetic codes. 

• This expansion involves the incorporation of new codons and amino acids beyond the standard set. 
• Certain organisms, such as some archaea and bacteria, have evolved additional transfer RNA (tRNA) molecules and enzymes. 
• Such molecules recognize and incorporate these novel codons during protein synthesis. 
• Mutations in these organisms may contribute to further expansions or modifications of the genetic code.

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Types of Mutation

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Mutations can be categorized into different types based on the specific changes in the DNA sequence. Following are some different types of mutations.

1. Point Mutations

Point mutations involve the substitution of a single nucleotide with another nucleotide. There are three types of point mutations.

Silent Mutations

These mutations occur when a nucleotide substitution does not change the protein's amino acid sequence. The genetic code is redundant, with multiple codons often encoding the same amino acid. As a result, the protein's function remains unaffected.

Missense Mutations

In missense mutations, a nucleotide substitution leads to incorporating a different amino acid in the protein. This can alter the structure or function of the protein, depending on the specific amino acid change and its location within the protein.

Nonsense Mutations

Nonsense mutations occur when a nucleotide substitution creates a premature stop codon in the DNA sequence. This causes the protein synthesis to terminate prematurely, resulting in a shorter and often non-functional protein.

2. Frameshift Mutations

Frameshift mutations involve the insertion or deletion of nucleotides in a DNA sequence, causing a shift in the reading frame of codons during protein synthesis. This alteration can completely change the amino acid sequence downstream of the mutation site. It produces a non-functional protein.

3. Insertions and Deletions (Indels)

Indels are mutations that occur when nucleotides are added to or subtracted from the DNA sequence. These mutations can cause frameshifts and reading frame disruptions, which can significantly alter the amino acid sequence of the protein produced.

4. Chromosomal Rearrangements

Chromosomal rearrangements involve large-scale changes in the structure or arrangement of chromosomes. Examples of chromosomal rearrangements are as follows.

Deletion 

When deletion takes place. a segment of the chromosome is lost during DNA replication or recombination, resulting in the loss of genetic material.

Duplication

In this condition, a segment of the chromosome is duplicated. It leads to an increase in the copy number of specific genes. One copy (the original one) keeps the original function while the second copy evolves new ones. This can create chances for evolutionary innovation.

Inversion

Inversion mutation occurs when a segment of the chromosome is reversed in orientation. This can disrupt gene expression patterns or lead to altered protein interactions.

Translocation

In translocation, genetic material is exchanged between non-homologous chromosomes. Translocations can significantly affect gene regulation and may contribute to the development of certain genetic disorders.

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Mechanisms of Mutations

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Mutation arises through certain mechanisms and each type of mutation has a specific mechanism.

Substitution

A single nucleotide is substituted with another nucleotide during DNA replication. This can lead to a change in the corresponding codon, potentially altering the protein's amino acid sequence.

Insertion

The DNA sequence contains one additional nucleotide, which causes a frameshift mutation. As a result, codons' reading frames change. Thus, frequently results in an entirely new amino acid sequence downstream of the mutation site.

Deletion

A nucleotide is deleted from the DNA sequence, causing a frameshift mutation similar to an insertion. This alters the reading frame of codons and can significantly affect the resulting protein.

Duplication

A segment of the DNA sequence is duplicated, leading to multiple copies of a particular region. Mutations can occur within the duplicated segment, potentially causing changes in the genetic code or subsequent alterations in protein structure and function.

Chromosomal Rearrangements

Large-scale rearrangements of chromosomes, such as deletions, inversions, or translocations, can disrupt the standard reading frame and arrangement of genes. This can affect the genetic code and lead to mutations by altering gene expression or interfering with average protein production.

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Things to Remember

• Genetic codes are specific sequences of three nucleotides that code for amino acids.
• Genetic codes are codons that carry information for protein synthesis.
• The Start and Stop codon signals for initiation and termination of protein synthesis, respectively.
• Mutation is a change in genetic codes, gene or DNA sequences.
• Mutation can be caused by mutagens, replication errors or by natural variations.
• Types of mutation involve frameshift, mis-sense, non-sense, duplication and indels.
• The mechanisms that cause mutation are inversion, insertion, deletion, translocation and chromosomal rearrangements.
• Mutation can be silent, neutral or lethal.
• The lethal or harmful mutations cause disease like cancer, diabetes, klinefelter syndrome etc.
• Mutation can be beneficial too as it gives diversity and variations.