Biology TRANSLATION

KEY TOPICS

star Charging of tRNA
star Ribosomes
star Translation Unit
star Process of Translation

CHARGING OF tRNA

● Translation refers to the process of polymerisation of amino acids to form a polypeptide.

● The order and sequence of amino acids are defined by the sequence of bases in the mRNA.

● The amino acids are joined by a bond which is known as a peptide bond.

● Formation of a peptide bond requires energy.

● Therefore, in the first phase itself amino acids are activated in the presence of ATP and linked to their cognate tRNA– a process commonly called as charging of tRNA or aminoacylation of tRNA to be more specific.

● If two such charged tRNAs are brought close enough, the formation of peptide bond between them would be favoured energetically.

● The presence of a catalyst would enhance the rate of peptide bond formation.

RIBOSOMES

● The color{Violet}"cellular factory" responsible for synthesising proteins is the color{Violet}"ribosome".

● The ribosome consists of color{Violet}"structural RNAs" and about 80 color{Violet}"different proteins".

● In its color{Violet}"inactive state", it exists as two subunits; a color{Violet}"large subunit" and a color{Violet}"small subunit".

● When the small subunit encounters an mRNA, the process of color{Violet}"translation" of the color{Violet}"mRNA to protein" begins.

● There are color{Violet}"two sites" in the color{Violet}"large subunit", for subsequent color{Violet}"amino acids to bind" to and thus, be close enough to each other for the color{Violet}"formation of a peptide bond".

● The ribosome also acts as a color{Violet}"catalyst" (color{Violet}"23S rRNA" in bacteria is the enzyme- color{Violet}"ribozyme") for the formation of color{Violet}"peptide bond".

TRANSLATIONAL UNIT

● A color{Violet}"translational unit" in mRNA is the sequence of RNA that is flanked by the color{Violet}"start codon (AUG)" and the color{Violet}"stop codon" and codes for a color{Violet}"polypeptide".

● An mRNA also has some color{Violet}"additional sequences" that are not translated and are referred as color{Violet}"untranslated regions (UTR)".

● The UTRs are present at both color{Violet}"5' -end" (color{Violet}"before start codon") and at color{Violet}"3' -end" (color{Violet}"after stop codon").

● They are required for color{Violet}"efficient translation" process.

PROCESS OF TRANSLATION

● For color{Violet}"initiation", the ribosome color{Violet}"binds" to the mRNA at the color{Violet}"start codon" (AUG) that is recognised only by the color{Violet}"initiator tRNA".

● The ribosome proceeds to the color{Violet}"elongation phase" of protein synthesis.

● During this stage, complexes composed of an color{Violet}"amino acid linked to tRNA", sequentially bind to the color{Violet}"appropriate codon" in mRNA by forming color{Violet}"complementary base pairs" with the tRNA anticodon.

● The ribosome moves from color{Violet}"codon to codon" along the mRNA.

● Amino acids are added one by one, translated into color{Violet}"Polypeptide sequences" dictated by DNA and represented by mRNA.

● At the end, a color{Violet}"release factor" binds to the color{Violet}"stop codon", terminating translation and releasing the color{Violet}"complete polypeptide" from the ribosome.