Mutations and Genetic Code: The translation is the process of the conversion of nucleic acid information into amino acids. This genetic information is encrypted in the form of a code called genetic code or codon. The genetic code is a set of information encoded in the sequence of nucleic acids that does the coding for proteins to be synthesized. Any change in genetic codes might lead to mutation.

Mutations and Genetic Code

Mutation

The phenomenon of change occurring within the DNA sequence is termed because of the mutation. This is mainly caused either by the interior factors or through external factors including smoking, UV rays, etc. Apart from these factors, there are other causes for the DNA variation in an organism, and it’s the recombination. Mutation leads to the alteration in the expression of the genotype and phenotype. Eventually, this might affect cells or maybe damage the organism.

A variation which is caused by the change in the building block and base pair of DNA is termed as a point mutation. An example of point mutation is Sickle cell anaemia.

The sickle cell anaemia is a disease condition caused due to the change in a single base pair of the gene. In this condition, the codon for the amino acid glutamate is replaced by that for valine.

Consider a statement that is made up of the following words each having three letters like genetic code.

RAM HAS RED CAP

If we insert a letter B in between HAS and RED and rearrange thes tatement, it would read as follows:

RAM HAS BRE DCA P

Similarly, if we now insert two letters at the same place, say BI’. Now it would read,

RAM HAS BIR EDC AP

Now we insert three letters together, say BIG, the statement would read

RAM HAS BIG RED CAP

The same exercise can be repeated, by deleting the letters R, E and D, one by one and rearranging the statement to make a triplet word.

RAM HAS EDC AP
RAM HAS DCA P
RAM HAS CAP

The conclusion from the above exercise is very obvious. Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion. However, such mutations are referred to as frameshift insertion or deletion mutations. Insertion or deletion of three or its multiple bases insert or delete in one or multiple codon hence one or multiple amino acids, and reading frame remains unaltered from that point onwards.

Genetic Code

• It is the sequence of bases in mRNA, that codes for a particular amino acid in the protein synthesis.
• Each code is made up of three nucleotides called a triplet. codons are nearly universal, except for some protozoans and mitochondrial codons.
• More than one triplet codon code for same amino acid, so the code is degenerate.
• There are a total of 64 codons, of which 61 code for amino acids.
• 3 codons do not code for any amino acids, they are called stop codons- UAA, UAG, UGA.
• AUG is the start codon as well as codes for the amino acid methionine.

DNA is the Genetic Material

• Alfred Hershey and Martha Chase (1952) gave unequivocal proof that DNA is the genetic material.
• In their experiments, bacteriophages (viruses that infect bacteria) were used.
• They grew some viruses on a medium that contained radioactive phosphorus and some others on sulphur containing radioactive medium.
• Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not. In the same way, viruses grown on radioactive sulphur contained radioactive protein, but not radioactive DNA because DNA does not contain sulphur.
• Radioactive phages were allowed to attach to E. coli bacteria. As the infection proceeded, viral coats were removed from the bacteria by agitating them in a blender. The virus particles were separated from the bacteria by spinning them in a centrifuge.
• Bacteria which were infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria.
• Bacteria that were infected with viruses that had radioactive proteins were not radioactive. This indicated that the proteins did not enter the bacteria from viruses. It proved that DNA is a genetic material that is passed from virus to bacteria.

Properties of Genetic Material

• It became established that DNA is the genetic material from the Hershey-Chase experiment.
• In some viruses, RNA was also reported as genetic material, e.g. Tobacco mosaic viruses, QB bacteriophage, etc.
• According to the above mentioned rules, both the nucleic acids (DNA and RNA) have the ability to direct duplications.
• Stability can be explained in DNA as the two strands being complementary if separated by heating come together in appropriate conditions.
• The 2′ — OH group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable, hence it is reactive.
• DNA is chemically less reactive and structurally more stable when compared to RNA. Thymine also confers additional stability to DNA. So, among the two nucleic acids, the DNA is a predominant genetic material.
• Both RNA and DNA are able to mutate. Viruses having RNA genome and having shorter life span mutate and evolve faster.
• DNA is dependent on RNA for protein synthesis, while RNA can directly code for it. The protein synthesising machinery has evolved around RNA. This concluded that the DNA being more stable is suitable for storage of genetic information, while for the transmission of genetic information, RNA is suitable.

Characteristics of a Genetic Material

• It should be able to replicate.
• It should be chemically and structurally stable.
• It should provide scope for slow changes (mutation) that are required for evolution.
• It should be able to express itself in the form of ‘Mendelian characters’.

 

 

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By Team Learning Mantras