TRANSLATION

• Translation involves “decoding” a messenger RNA (mRNA)

• The translation using its information to build a polypeptide or chain of amino acids.

• A polypeptide is basically a protein with the technical difference being that some large proteins are made up of several polypeptide chains.

The Genetic Code

• Triple : a sequence of three base (a codon ) is needed to specify one amino acid

• No overlapping code : no base are shared between consecutive codon

• Continuous code

• Degenerate : more than one codon can code for the same amino acid

• Universal : same in all organisms

• In an mRNA, the instructions for building a polypeptide come in groups of three nucleotides called codons.

• Have 64 codon

• There are 61 different codons for amino acids

• One codon, AUG, is a “start” signal to kick off translation it also specifies the amino acid methionine. (anticodon for AUG is UAC)

• Three codon ( UAA , UAG and UGA ) “stop” codons mark the polypeptide as finished. (don’t have anticodon).

Translation Requirement

• mRNA (codon) , G C C

• tRNA (anticodon), C G G

• rRNA (ribosome)

• various proteion factors

Step Of Translation The Genetic Message

Step 1 : amino acid activation

• In translation, the codons of an mRNA are read in order from the 5' end to the 3' end  by molecules called transfer RNAs (tRNA).

• tRNA has an anticodon, a set of three nucleotides that binds to a matching mRNA codon through base pairing.

• tRNAs bind to mRNAs inside of a protein-and-RNA structure (ribosome).

• As tRNAs enter slots in the ribosome and bind to codons, their amino acids are linked to the growing polypeptide chain (Met-Ile-Ser) in a chemical reaction.

• The end result is a polypeptide whose amino acid sequence mirrors the sequence of codons in the mRNA.

• Free energy of hydrolysis of ATP provides energy for bond formation

Step 2 :Chain Initiation

• In  all organisms , synthesis of polypeptide chain starts at the N- terminal end and grows to C- terminus.

• Initiation required: tRNAfmet  ,initiation codon (AUG) of mRNA ,30S ribosomal subunit,50 ribosomal subunit,initiation factors (Ifs) and GTP,Mg2+

• A ribosome (which comes in two pieces, large and small)

• An mRNA with instructions for the protein we'll build

• An "initiator" tRNA carrying the first amino acid in the protein, which is almost always methionine (Met)

• During initiation, these pieces must come together in just the right way .They form the initiation complex.

• Inside our cells and the cells of other eukaryotes, translation initiation , the tRNA carrying methionine attaches to the small ribosomal subunit.

• Together, they bind to the 5' end of the mRNA by recognizing the 5' GTP cap (added during processing in the nucleus).

• Then, they "walk" along the mRNA in the 3' direction, stopping when they reach the start codon (often, but not always, the first AUG).

• Example : tRNAfmet contain the triple 3’-UAG-5’and the triple base pairs with 5’-AUG-3’ in mRNA.

• 3’-UAC-5’ triplet on tRNAfmet recognizes the AUG triplet (the start signal) when it occurs at the beginning of the mRNA sequence that direct polypeptide synthesis (bind at P site).

Eukaryotic translation initiation

                                                       

Bacterial Translation Initiation

• In bacteria, the situation is a little different. Here, the small ribosomal subunit doesn't start at the 5' end of the mRNA and travel toward the 3' end. 

• Instead, it attaches directly to certain sequences in the mRNA. 

• These Shine-Dalgarno sequences come just before start codons and "point them out" to the ribosome.

Step 3: Chain Elongation

• Elongation is when the polypeptide chain gets longer.

• Uses 3 biding sites for tRNA  present on the 50S subunit of the 70S ribosome: P (peptidyl) site, A(aminoacyl) site ,E (exit) site.

• Requires: 70S ribosome, codons of mRNA , aminoacyl-tRNAs, elongation factors (EF),GTP and Mg2+

• First : a fresh codon is exposed in another slot, called the A site.  

                  : The A site will be the "landing site" for the next tRNA, one whose anticodon is a perfect

                     (complementary) match for the exposed codon.

                  :an aminoacyl-tRNA is bound to the A site 

                  : the P site is already occupied by tRNAfmet 

                  : 2nd amino acid bound to 70S initiation complex.Defined by the mRNA.

     Second : the peptide bond is formed ,the P site is uncharged.

     Third    : the uncharged tRNA is released (E site)

                  : the next peptidyl-tRNA is translocated to the P site

                  : the next aminoacyl-tRNA occupies the empty A site

• Translation read the mRNA from 5’ to 3’ direction

• Ribosom moves toward 3-end 

• Polypeptide sequence grows from N-end to C-end

Step 4 : Chain termination

• Termination happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site.Stop codons have no tRNA,(anticodon).

• Stop codons are recognized by proteins called release factors, which fit neatly into the P site (though they aren't tRNAs).

• Release factor (RFs) which either binds to UAA and UAG or UGA.

• GTP which is bound to RF

• The entire complex dissociates setting free the complete polypeptide ,the release factors,tRNA  mRNA ,and the 30S and 50S ribosomal subunits.