Question

. Explain the process of translation. Be sure to include (these are in no particular order): mRNA, ribosomes, A site, P site, E site, where peptide bonds occur, codon, anti-codon, stop codon, start codon, aminoacyl tRNA synthetase, tRNA, genetic code, initiation, elongation, termination, where the process takes place, and what is formed

Answer 1

Terms used in the process of translation :-

Codon- trinucleotide unit of gene that cides for a single amino acid.

Anticodon-a sequence of three nucleotides forming a unit of genetic code in a transfer RNA molecule, corresponding to a complementary codon in messenger RNA.

Genetic code- it is set of rules by which information encoded in the gene is translated into protein.

Start codon-AUG

Stop codon-UAA, AUG, UGA

TRANSLATION:-synthesis of protein from mRNA IS CALLED TRANSLATION.it occurs on ribosomes in the cytoplasm of cells.

mRNA is translated from 5' end tk 3' end priducing a protein synthesised from its amino terminal to carboxy terminal end.

ACTIVATION OF AMINO ACID:-

Also called charging of amino acid.

It takes place kn the chtosol of cell. In this process each of the 20 amino acids covalently bind to their respective tRNA at the expense of ATP as two high energy phosphate bonds therefore it is called charging of tRNA.

Aminoacyl tRNA synthase is a family of enzymesthat is required for attachment of amino acid to their corresponding tRNA. They are specificfor a particular amino acid and also for tRNA.

The extreme specificity of synthetase in recognizing both amino acids and tRNAs is responsible for high fidelity if translationof genetic message.

It implements genetic code by acting as molecular dictionaries. It is required for covalent attachment of carboxyl group specific aminoacid to 3' end of corresponding t-rna (isoaccepting t-rna).

2 ATPare required for this process.

Their are atleast 20 different tRNA and 20 different amino acyl synthetase in a protein synthesising protein. In addition to all this it has a proof reading and editing activity that can remove mischarge aminoacid from enzyme or tRNA molecule

STEPS OF TRANSLATION IN EUKARYOTES:-

1. INITIATION:-

FOUR STEPS:-

A. RIBOSOMAL DISSOCIATION:-two initiatio factors e1f³ and e1f-1A bind to 40s subunit of eukaryotic ribosome(80s). And this result kn dissociation of 80s particle into 40s subunit and 60s subunit

Ribosome has two binding sites for tRNA, A site(aminoacyl) and P (peptidyl) site.initially tRNA binds to P site.

B.FORMATION OF 43S PREINITIATION COMPLEX:-

first it involve binding of GTP with initiation factor e1f². This complex then bind to Met-tRNA,a tRNA which js specifically involved in the initiation codon AUG on mRNA. AUG serves as the START CODON in protein synthesis and codes for methionine.

This tertiary complex(GTP-e1f²-tRNA) binds to 40s RIBOSOMAL subunit to form 43 Spre initiation complex.which is stabilized by association with e1f³ and e1f-1A.

In eukaryotes e1f²is the controlling factor for initiation.

C. FORMATION OF 48 S INITIATION COMPLEX:-

binding of mRNA to 43s preinitiation comllex forms 48s initiation comllex. Binding of mRNA TO 43S pre initiation comolex is facillitated by 7-methylguanylate cap at 5' end of mRNA. It requires e1f⁴ also called cap binding protein and ATP.

This is scanning process,i.e 43s preinitiation complex binds close to cap structure at the 5' end of mRNA and moves down the mRNA untill it encounters the jnitiator codon AUG.

D. FORMATION OF 80S INITIATION COMPLEX:-.

combination of 48s initiation complex with 60s ribosomal subunit forms 80s initiation complex. This requires hydrolysis of e1f²bound GTP by e1f⅝ with release of all initiation factors

These initiation factors are recycled. At this stage the met-tRNA is on the Psite of the ribosome so that its anticodon pairs with initiating AUG codon on the mRNA and is now ready for elongation process.

2. ELONGATION:-

Elongation occurs in following steps:-

A. Binding of next aminoacyl tRNA specified by next codon in the mRNA to tha A sute of ribosome:-

This aminoacyl tRNA first bind to elongation factor eEF-1alpha and GTP to form 'aminoacyl tRNA-eEF-1alpha-GTP' Complex which then enters tha A site of ribosome with the release of eEF-1 alpha-GDP and phosphate.

Thus binding of new amknoacyl tRNA requires 1 GTP.

B. Formation of peptide bond:-

The methionine of tRNA of P site is transferred to new amino acid on tRNA of A site to form peptide bond between these two amino aicds.

This step is catalyzed by PEPTIDYL TRANSFERASE (a ribozyme-rnaacts as an enzyme).

As a result tRNA at A site contains two amino acids and P site becomes empty

C. Translocation:-

The ribosomes moves along the mRNA towrds its 3' end ,by a distance of one codon.

The movement of ribosome shifts the dipeptidyl tRNA from A site to P site, which causes release kf preceeding empty tRNA from Psiteto back in the cytosol.

Nkw third codon of mRNA is on the Asite and second codon on the P site.

Translocation requires elongation factors eEF² and hydrolysis of GTP.

THE ribosome with its attached dipeptidyl tRNA and mRNA is now ready for another elongation cylce to attach the third amino acid residue,which proceeds in precisely the same way as the addition of second . Thus the process of peptide synthesis occurs untill a termination codon is reached.

3. TERMINATION:-.

The elongation steps are repeated untill one of three (UAA, UAG ,UGA) termination or stop codon or nonsense codon of mRNA appears in the A site.

Once the ribosome reaches a termination codon releasing factors are capable of recognizing the termination signal present in the A site.

Eukaryotes have knly single releasing factor eRF.

Prokaryotes have three releasing factors RF-1,RF-2, RF-3.

Releasing factors in conjunction with GTP and peptidyl transferase hydrolyze the peptidyl tRNA bond when a nknsense codon occupies the A site.

This hydrolysis releases the polypeptide and tRNA from Psite.

Upon hydrolysis and release of polypeptide the mRNA is then released from the ribosome .

Ribosome then dissociates into 60S and 40S subunits which are then recycled.

POST TRANSLATIONAL MODIFICSTION OF POLYPEPTIDE CHAINS:-.

1. TRIMMING:-.

large precursor molecules are converted into smaller active proteins

2.COVALENT MODIFICATION

Phosphorylation done by kinases and reversed by phosphatases

Glycosylation N-glycosylation occurs in endoplasmic reticulum and O-glycosylation in golgi appratus

Hydroxylation-proline and lysine residues of the alpha chains of collagen are extensively hydroxylated by vitaminC dependent hydroxylases in endoplasmic reticulum

3. Protein folding

By proteins called chaperons

4. Protein degradation

Defective proteins are often marked for destruction by ubiquintination (attachment of small,highly conserved protein called ubiquitin).

Such proteins are rapidly degraded by proteosomes which is macromolecular ATP dependent proteolytic system located in cytosol.