Question

We also discussed many errors that could occur in the genetic code (point mutations). Please briefly describe each of the four types of point mutations and how these relate to the four aspects of the genetic code described above: specificity, the genetic code is non-overlapping, continuous (no punctuation) and degenerate.

Answer 1

Answer- Point mutations are of following types-

• Insertion and deletion mutations.
• Non sense mutation.
• Mis sense mutation.
• Silent mutation.

Non sense, Mis sense, and Silent mutations may be placed together as substitution mutations because these three mutations are caused by substitution of one base.

• Insertion and deletion mutations - An insertion mutation occurs when an extra base pair is added to a sequence of bases. A deletion mutation is the opposite; it occurs when a base pair is deleted from a sequence. These two types of point mutations are grouped together because both of them can drastically affect the sequence of amino acids produced.
• ​​​​​Non sense mutation - A nonsense mutation occurs when one nucleotide is substituted and this leads to the formation of a stop codon instead of a codon that codes for an amino acid. A stop codon a certain sequence of bases (TAG, TAA, or TGA in DNA, and UAG, UAA, or UGA in RNA) that stops the production of the amino acid chain. It is always found at the end of the mRNA sequence when a protein is being produced, but if a substitution causes it to appear in another place, it will prematurely terminate the amino acid sequence and prevent the correct protein from being produced.
• Mis sense mutation - A missense mutation also occurs when one nucleotide is substituted and a different codon is formed; but this time, the codon that forms is not a stop codon. Instead, the codon produces a different amino acid in the sequence of amino acids. For example, if a missense substitution changes a codon from AAG to AGG, the amino acid arginine will be produced instead of lysine. A missense mutation is considered conservative if the amino acid formed via the mutation has similar properties to the one that was supposed to be formed instead. It is called non-conservative if the amino acid has different properties that structure and function of a protein.
• Silent mutations- In case of a silent mutation, a nucleotide is substituted but the same amino acid is produced anyway. This can occur because multiple codons can code for the same amino acid. For example, AAG and AAA both code for lysine, so if the G is changed to an A, the same amino acid will form and the protein will not be affected.

Properties of genetic code -

• Specificity - The genetic code is specific in the sense that same codon cannot specify different amino acids.
• Non-overlapping - In translating mRNA molecules the codons do not overlap but are “read” sequentially. Thus, a non-overlapping code means that a base in a mRNA is not used for different codons.
• Continuous - The genetic code is continuous in the sense that there is no gap between codons and codons are read sequentially one after another.
• Degenerate - More than one codon may specify the same amino acid; this is called degeneracy of the code.

The code degeneracy is basically of two types: partial and complete, which are explained as follows-

• Partial degeneracy occurs when first two nucleotides are identical but the third (i.e., 3′ base) nucleotide of the degenerate codons differs, e.g., CUU and CUC code for leucine.
• Complete degeneracy occurs when any of the four bases can take third position and still code for the same amino acid (e.g., UCU, UCC, UCA and UCG code for serine).