Mutation-causes and its effects |Definegenetics

What is mutation?

The mutation is a change in DNA. It happens where one or more nucleotides is deleted, inserted or substituted for a different nucleotide. Mutations results in a change in the sequence of the nucleotides basis. If this change occurs in a gene it can alter the order that amino acids are coded for which in turn will change the shape of the protein.

You've probably watched a movie or read a comic book about a person who got superhero powers because of a mutation. Unfortunately this is science fiction. In real life a mutation won't give you superhuman strength or the ability to read people's minds, but they can cause a change or be it not so exciting one.

Introduction

 A change in a protein shape can result in the protein acting differently or not working at all. For example if there is a mutation in a gene that codes for an enzyme the resulting protein chain that is produced might not fold into the correct shape. So the active site of the enzyme no longer fits the substrate. The majority of our DNA does not code for a protein this is called non-coding DNA. In fact only 1% of the 3 million nucleotides that make up human DNA are actually protein coding genes. That's a tiny amount, some of the non-coding DNA regions do however have other roles such as controlling if and how genes are expressed while mutations can occur to a gene itself. They might also occur in an area that controls if a gene is expressed and therefore whether a particular protein is made. Thus mutations in these non-coding expression sections can also affect phenotype by altering gene expression more often than not the mutation occurs in a region of the DNA that does not code for a protein or control expression. This means that most mutations have no effect on phenotype.

Examples:

Group of mutated mice with different colors 

Look at everyone around you maybe they have different hair or eyes or skin-color. There is a lot of genetic variation in all populations humans, animals,  plants everything. This is because genetic variation in a population arises from mutations. For example in a population of black mice the mutation might arise which causes white fur. This mutation would not be an advantage to mice living in the wild because the white mouse would easily be spotted by predators and eaten. It is not likely to survive to adulthood and breed to pass on this white hare gene to its offspring. However if a mutation causes a new phenotype that is useful such as better hearing, then the new gene will get passed on to offspring and the number of mice in the population with this new phenotype will increase. This gradual change in a population is known as natural selection and is the mechanism by which evolution occurs. In populations of bacteria mutations often occur which by chance gives an individual resistance to antibiotics. Because this is an advantage and bacteria reproduces so quickly this mutation can spread very quickly throughout a population. This is how antibiotic resistant bacteria evolve and is a problem for humans.

Typical bacteria mutated to antibiotics

Types of mutation

As living organisms reproduce and grow, they are constantly replicating DNA with a high degree of precision. However there are about one in every billion nucleotides that are replicated and error occurs in the replication. There are two main classes of mutations. One is known as base substitution mutation and the other

is known as frameshift mutation.

Base substitution mutations occurs when a single nucleotide replaces another in the DNA sequence. Base substitution mutations can be further divided into:

1. Silent mutations 
2. Missense mutations and
3. Nonsense mutations

1.Silent mutation:-

 Now let's dig deep into each of this type of the mutations. The base substitution produces no change in the amino acid sequence of the resulting protein, it is a silent mutation. Due to the overlapping nature of the genetic code and one third of its all substitutions lead to the silent mutations. Since there is no change in the amino acid sequence of the protein, silent mutations affect the genotype of the organism but not the phenotype effect. 

2.Missense mutation:-

Next is the missense mutation. If a base substitution mutation caused a single amino acid to change to the protein the mutation is missense mutation. Depending on the nature of the amino acid substitution, the missense mutation can be harmful, neutral or even beneficial in very rare cases.

3.Nonsense mutation:-

 The third type is a nonsense mutation. If the base substitution causes a codon to change from one coding amino acid to the stop codon, it is a nonsense mutation. In general nonsense mutations are harmful since they lead to the premature termination of the protein synthesis.

Frameshift mutations:-

Now let's look the second type that is a frameshift mutation. If nucleotides are inserted into or removed from the DNA sequence the resulting mutation is known as a frameshift mutation. Because the ribosome translates messenger RNA into the protein by reading the mRNA in 3 nucleotide codons. If one or two nucleotides are removed or inserted the result is a shift in the reading frame of the codons. This leads to the change in every amino acid that follows the site for frameshift mutation. Frameshift mutation usually leads to the non-functional protein production. Now let us look at the type of frame shift.

*Frame shift insertion:  If one or more bases are added to the DNA sequence causing a shift in the reading frame of the resulting codons, the mutation is a frameshift insertion.

*Frame shift deletion:  If one or more bases are removed from the DNA sequence causing a shift in the leading frame and the reading frame of the resulting codons, the mutation is a frameshift deletion. 

So what causes mutations?

Well this is where science fiction meets science facts. In the backstory of many superheroes there will be a meeting with a radioactive substance, be it cosmic rays or radioactive waste in real life. Exposure to ionizing radiation such as gamma rays, x-rays and ultraviolet rays and some chemical mutagens like chemicals in tobacco can increase the incidence of mutations. If these mutations happen to a region of DNA that controls cell division, cancer can occur. This is because cancer is uncontrollable cell division. Thus exposure to ionizing radiation increases the risk of developing cancer. For example UV light is a risk factor for skin cancer and smoking is a risk factor for lung cancer.

1. The ionizing radiation.

Sign of radioactive zone with high radiation

 Ionizing radiation such as x-rays and gamma-rays can cause some of the molecules within the cell to lose electrons, becoming highly reactive ions and free radicals. Some of these radical ions are free radicals can combat with the bases of the DNA resulting in error in the DNA replication and therefore cause mutations. Even more seriously three groups can react to the sugar phosphate backbone of the DNA and cause the break in the chromosome.

2. Non ionizing radiation.

 Non ionizing radiation in the form of UV light is also mutagenic because it causes adjacent thymine bases to covalently bind to one another producing thymine dimers. Such dimers can cause serious harm or death to the cell if they are not repaired. Since these dimers prevent the cells from properly transcribing and replicating such DNA. 

3. Nucleotide analogs.

Nucleotide analogs are compounds that are structurally similar to normal nitrogenous bases but with different base pairing properties. This compounds can become incorporated into growing DNA during replication replacing their related base. Once incorporated, the nucleotide analog can inhibit further replication or cause miss matching of the figure between the replication. For example five bromo uracil is a nucleotide analogue of thymine but it often pairs with guanine rather than adenine. Incorporation of five bromouracil can therefore lead to a base substitution mutation of a guanine for an adenine.

4. some chemical mutagens.

Bio hazardous chemicals

  These directly alter the structure of the nitrogenous bases of the DNA for example nitrous acid can chemically alter the adenine base so that the base pair with cytosine rather than thymine. During replication this change causes a base substitution mutation in the daughter DNA. Some chemical mutagens cause small insertions or deletions of nucleotide base pairs which can lead to the frameshift mutations. Example of such frameshift mutations include acridine which is a dye commonly used for a mutagen in genetic research. Benzopyrene which is found in smoke and ethidium bromide which is useful for the laboratories for staining the DNA. These chemicals are the right size and have the right chemical properties to slip between the stacked base pairs of the DNA producing a bulge in the molecule. During DNA replication this bulge can lead to the insertion or deletion of one or more bases in the newly synthesized DNA.