Flow of Genetic Information in Biology

Study Notes

The central dogma of molecular biology describes the two-step process of transcription and translation, by which the information in genes flows into proteins: DNA → RNA → protein.

The pathway of protein synthesis is called translation because the language of nucleotide sequence on mRNA is translated into the language of an amino acid sequence.
The process of translation requires a genetic code, through which the information contained in nucleic acid sequence is expressed to produce a specific sequence of amino acids.

A, G, T, and C correspond to the nucleotides found in DNA.
These nucleotides are organized into codons.
The collection of codons is called genetic code.
There should be 20 codons for 20 amino acids.
Each codon should have 3 nucleotides to impart specificity to each of the amino acids for a specific codon.
1 nucleotide = 4 combinations
2 nucleotides = 16 combinations
3 nucleotides = 64 combinations (Most suited for 20 amino acids)

Genetic code is a dictionary that corresponds with the sequence of nucleotides and the sequence of amino acids.
Words in the dictionary are in the form of codons.
Each codon is a triplet of nucleotides.
There are 64 codons in total, and 3 out of these are non-sense codons.
There are 61 codons for 20 amino acids.

Specificity: Genetic code is specific (unambiguous).
A specific codon always codes for the same amino acid, e.g., UUU codes for phenylalanine, and it cannot code for any other amino acid.

Universal: Genetic code is the same in all living organisms.
The exception to universality is found in mitochondrial codons where AUA codes for methionine and UGA for tryptophan, instead of isoleucine and termination codon respectively of cytoplasmic protein synthesizing machinery.
AGA and AGG code for Arginine in the cytoplasm but in mitochondria, they are termination codons.

### Genetic Code - Redundant

Redundant: Genetic code is redundant, also called degenerate.
Each codon corresponds to a single amino acid, but a single amino acid can have multiple codons.
Except for tryptophan and methionine, each amino acid has multiple codons.

All codons are independent sets of 3 bases.
There is no overlapping.
Codon is read from a fixed starting point as a continuous sequence of bases, taken three at a time.
The starting point is extremely important and is called reading frame.

There are 3 codons out of 64 In the genetic code that do not encode for any amino acid.
They are called termination codons, stop codons, or nonsense codons.
The stop codons are UAA, UAG, and UGA.
They encode no amino acid, the ribosome pauses and falls off the mRNA.

The rules of base pairing are relaxed at the third position so that a base can pair with more than one complementary base.
Some tRNA anticodons have inosine at the third position.
Inosine can pair with U, C, or A.
This means that we don't need 61 different tRNA molecules, only half as many are required.

The first two bases in the codon in mRNA (5'-3') base pair traditionally with the 2nd and 3rd base of the anticodon in tRNA (5'-3').
Non-traditional base pairing is observed between the third base of the codon and the 1st base of the anticodon.
The reduced specificity between the third base of the codon and the complementary nucleotide in the anticodon is responsible for wobbling.