Genetic code

The genetic code is the set of instructions in a gene that tells the cell how to make a specific protein. A codon is a trinucleotide sequence of DNA or RNA that corresponds to a specific amino acid.

There are 64 different codons:

• 61 codons specify amino acids.
• The remaining 3 codons act as stop signals.

The start codon is the first codon of an mRNA transcript translated by a ribosome. It is AUG, which codes for the amino acid methionine.

Stop codons halt translation and are UAA, UGA, and UAG.

Features of the genetic code

Nonoverlapping and contiguous: codons are continuous.

Unambiguous: each codon specifies only one amino acid.

Degenerate: an amino acid may be coded by more than one codon. The wobble position of a codon refers to the 3rd nucleotide in a codon. Changes are often seen at the wobble position in codons that code for a single amino acid.

Universal: the same code is used everywhere, including plants, humans, and microbes.

Mutations: A mutation is a change in the base sequence of DNA.

1. Point mutations are changes in a single nucleotide base. Point mutations can be of the following types:

   i) Silent: The new mutation still codes for the same amino acid as before, so there is practically no change.

   ii) Missense: The new mutation changes the codon so that it now specifies a different amino acid. For example, in sickle cell anemia due to a missense mutation (GAG to GTG), the amino acid valine is formed instead of glutamic acid.

   iii) Nonsense: The new mutation produces a stop codon, which prematurely stops translation and leads to the formation of a truncated protein product.

2. A frameshift mutation is a type of mutation involving the insertion or deletion of a nucleotide in which the number of deleted/inserted base pairs is not divisible by three. “Divisible by three” matters because the cell reads a gene in groups of three bases (codons). A frameshift mutation disrupts the reading frame, so the entire DNA sequence following the mutation will be read incorrectly.

3. Trinucleotide repeats are caused by errors in DNA replication leading to an abnormal number of repeated nucleotide triplets. Seen in Huntington’s disease, Friedrich’s ataxia. Fragile X syndrome and myotonic dystrophy.
4. Translocation is a type of chromosomal abnormality in which a chromosome breaks and a portion of it reattaches to a different chromosome. It is seen in cancers like Burkitt’s lymphoma, Philadelphia chromosome in CML, etc.
5. Aneuploidy is a change in chromosome number due to loss or gain of an entire chromosome. It is seen in Down’s syndrome (trisomy 21). Most cases result from nondisjunction, when homologous chromosomes or chromatids fail to separate in meiosis. This causes some cells to have extra chromosomes while others will have fewer chromosomes.
6. Microdeletions involve deletion of a small portion of a chromosome and are seen in Angelman and Prader-Willi syndromes.