Genetic code

 

The genetic code is used by living cells to translate information encoded within genetic material, DNA or mRNA, sequences of nucleotide triplets, or codons into proteins. 

 

 

Translation of  genetic information is accomplished by the ribosome.

 

 

The ribosome links proteinogenic amino acids in an order specified by messenger RNA (mRNA), using transfer RNA (tRNA) molecules to carry amino acids and to read the mRNA three nucleotides at a time. 

 

 

There is a series of codons in part of a messenger RNA (mRNA) molecule. 

 

 

Each codon consists of three nucleotides, that usually corresponds to a single amino acid. 

 

 

The nucleotides are abbreviated with the letters A, U, G and C. 

 

 

mRNA, which uses U (uracil). 

 

 

DNA uses T (thymine).

 

 

mRNA molecules will instruct a ribosome to synthesize a protein according to this codon code.

 

 

The genetic codon code defines how codons specify which amino acid will be added next during protein synthesis. 

 

 

The three-nucleotide codon in a nucleic acid sequence specifies a single amino acid, and  the vast majority of genes are encoded with a single scheme: referred to as the genetic code.

 

 

The genetic code what determines a protein’s amino acid sequence.

 

 

It was postulated that sets of three bases encode the 20 standard amino acids used by living cells to build proteins.

 

 

Ribosomes, are the components of cells that translate RNA into protein.

 

 

As many as 40 non-natural amino acids have been added into protein by creating a unique codon and a corresponding transfer-RNA synthetase pair to encode it with diverse physicochemical and biological properties to be used as a tool to exploring protein structure and function or to create novel or enhanced proteins.

 

 

Translation starts with a chain-initiation codon or start codon. 

 

 

The most common start codon is AUG, which is read as methionine.

 

 

Stop codons are also called “termination” or “nonsense” codons:

 

They signal release of the nascent polypeptide from the ribosome.

 

 

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. 

 

 

Such errors, mutations, can affect an organism’s phenotype.

 

 

Mutations affect phenotype, especially if they occur within the protein coding sequence of a gene. 

 

 

Mutation error rates are typically 1 error in every 10–100 million bases.

 

 

Missense mutations can cause genetic diseases such as sickle-cell disease.

 

 

Nonsense mutations can cause genetic thalassemia.

 

 

Missense mutations generally change the properties of the coded amino acid residue among basic, acidic, polar or non-polar states.

 

 

Nonsense mutations result in a stop codon.

 

 

Frameshift mutations  disrupt the reading frame sequence by insertions or deletions of a non-multiple of 3 nucleotide bases.

 

 

These mutations likely cause a stop codon to be read, and may impair the protein’s function.

 

 

Frameshift mutations are rare because the protein being translated is essential for growth.

 

 

The absence of a functional protein may cause death before the organism becomes viable.

 

 

Tay–Sachs disease is due to a frameshift mutation.

 

 

Most mutations that change protein sequences are harmful or neutral.

 

 

Some mutations provide benefits. which 

 

allow the mutant to withstand particular environmental stresses, or reproduce more quickly. 

 

 

When mutations are beneficial these mutations  to become more common in a population through natural selection.

 

 

RNA virus genetic material have rapid mutation rates.

 

 

RNA viruses can evolve rapidly, and thus evade the immune system defensive responses.

 

 

The genetic code has redundancy but no ambiguity: codons GAA and GAG both specify glutamic acid, redundancy, neither specifies another amino acid, no ambiguity.

 

 

Redundancy is that errors in the third position of the triplet codon cause only a silent mutation or an error that would not affect the protein because the hydrophilicity or hydrophobicity is maintained by equivalent substitution of amino acids.

 

 

Amino acids are not affected at all by mutations at the third position of the codon, whereas a mutation at the second position is likely to cause a radical change in the physicochemical properties of the encoded amino acid. 

 

 

Changes in the first position of the codons are more important than changes in the second position.

 

 

Stereochemical genetic code is a result of a high affinity between each amino acid and its codon or anti-codon.

 

 

Stop codons are translational stops are most likely to terminate translation early in the case of a frame shift error.