The Synthesis of polypeptide chain - Translation

 

• With the reference of central dogma,  When DNA is transcribed into a messenger RNA  during transcription , the mRNA is  translated into a polypeptide chain or  protein by the process of  translation .

• In translation, mRNA along with  tRNA and ribosomes work together to produce proteins. This is called Protein synthesis and takes place in cytoplasm. 

• During transcription, after the splicing, messenger RNA has been modified  from hnRNA and involved in  translation. It binds to a small subunit of the ribosome.

Read more about the Transcription

• Ribosomes  have two subunits,  a large subunit and a small subunit. Both subunits are united with each other by the Magnesium ions. 

• They contain a binding site for mRNA  at a smaller subunit and two binding sites for tRNA located in the large ribosomal subunit.

• The process of protein synthesis includes three major steps Initiation, Elongation and termination.

• The first step of protein synthesis is attachment of mRNA with the smaller unit of Ribosome.

• tRNA molecule is charged with the enzyme of aminoacyl synthese and ATP. This step is also known as aminoacylation of tRNA or charging of tRNA. Charged tRNA is capable to pick up amino acid by its accepter ends.

• As mRNA molecules are encountered with a smaller sub unit  of the ribosomes than a tRNA  molecule is also attached with the larger Sub unit of ribosomes.

• tRNA molecules make interaction with the mRNA molecule according to the base pair or genetic code   present on mRNA.

• As tRNA molecules make interaction with the initiation codon or  AUG  then  the process of translation begins. This step is called initiation.

Read more about the Genetic code

• As as the charged tRNA molecule pick up amino acid by  its amino acid acceptor end than renders the amino acids  at larger sub unit of ribosomes according to base pairs  present on  mRNA.

• Amino acids attached with tRNA are entered through the A Site and exited from P Site. After the initiation as first amino acid is rendered  by tRNA at a large subunit of the ribosomes.

• The ribosome flanks along the mRNA molecule  and  next tRNA molecule picks up next amino acid and also left it  at larger Subunit of ribosomes.

• Amino acids are  joined at the larger subunit of ribosome by the peptide bond. The  formation of peptide  bond  is catalysed by the enzyme peptidyl transferase which is also present at the largest subunit of the ribosomes.

• The process of joining of amino acid is catalysed by peptidyl transferase enzyme and synthesis of polypeptide chain  takes place. This step is called elongation.

• tRNA molecules join amino acids in a continuous manner  according to the codon  present on mRNA.

• As  the termination codon is approached then the process of protein synthesis is terminated. And a newly synthesised protein chain is released from cytoplasm. 

• There are so many genetic codes that are not translated during the protein synthesis.

• These are known as untranslated regions or UTR. This untranslated region is present on both 3 Prime and 5 prime ends.

• Finally , release factor bind with codon to terminate the protein synthesis and release the protein chain into cytoplasm.

Structure of tRNA

• Francis Crick postulated the role of tRNA in protein synthesis. 

• He used term adapter molecule for tRNA because it reads genetic code of mRNA by its one end and adapt amino acid by its another end.

Remember 👌👌 tRNA is called as sRNA or Soluble RNA.

• tRNA is specific for amino acid. For initiation of protein synthesis, there is specific tRNA therefore called as initiater tRNA. There are on tRNA for stop codon.

Clover leaf structure of tRNA

• It is secondary structure of tRNA. It has various arms that gives Clover leaf apperance to RNA. 

• Anticodon loop has three set of nucleotide that are complementary to codon present on mRNA. These set of nucleotides read genetic code during protein synthesis and add amino acid according to codon on mRNA.

• Amino acid acceptor end is present at 5' end with phosphate group. It accept amino acids according to codon on mRNA.

• DHU loop is called as D arm and include 3 to 4 base pairs. It terminated at loop structure called D loop. It has dihydrouridine as modified nucleotide.

• TΨC  has 4 to 5 base pair with pseudouridine as modified nucleotide.

• A variable loop is present between TΨC and anticodon loop with 3 to 21 bases.

• Tertiary structure of tRNA is inverted L shaped and gives 3 dimensional structure.