Translation

tRNAacceptor site of amino acid tyrosine

Anticodon arm

Anticodon: recognizes the codon of the mRNA. The recognition is facilitated by complementing base pairs.

• Every tRNA carries only one specific amino acid

• The third base in each codon may differ b/w two codons that code for the same a.a.– E.g, AAU and AAC both

code for tyrosine.

• If the tRNA’s anticodon is UUA , it can still bind to the codon AAC.

• This flexibility makes it possible for the correct a.a. to be added in the growing polypeptide chain, even when errors were made in the gene sequence of mRNA

• Aminoacyl-tRNA- is a tRNA molecule with its corresponding amino acid to its acceptor site at the 3’ end

• Aminoacyl-synthetase– is the enzyme that adds the appropriate

amino acid to each tRNA– there are at least 20 of them– each is specific for a particular amino acid

and tRNA

Ribosome

• rRNA is produced from a DNA template in the nucleolus of nucleus

• rRNA is packaged with a variety of proteins into ribosomal units, one larger than the other

• Subunits move separately through nuclear envelope pores into cytoplasm where they combine to form ribosomes

The ribosome brings the mRNA strand, the aminoacyl-tRNA, and the enzymes involved in building polypeptides together

• They facilitate complementary base pairing b/w tRNA anticodons and mRNA codon.

• Contains:• -one mRNA site • -two tRNA binding sites:

• P site-holds the tRNA attached to the growing polypeptide

• A site-holds the tRNA carrying the next amino acid to be added to polypeptide chain

The Steps in TranslationPolypeptide Chain Initiation

• mRNA attaches to the ribosome

• The start codon (AUG) is the first codon that is recognized by the ribosomes

• AUG ensures that the correct reading frame is used by the ribosomes

• AUG codes for methionine , therefore every protein initially starts with the amino acid methionine

• Met enters the P site of the ribosome which signals the process to begin

Polypeptide Chain Elongation

• The next tRNA with the appropriate anticodon attaches to the codon on the mRNA, (carrying the required amino acid) enters the A site.

• tRNA at the P site is released from its amino acid and the mRNA codon by enzyme action

• Triplet at A site moves to P site bringing next triplet to A site

• Amino acid at P site attaches to amino acid at A site (by a peptide bond)

• tRNA that have been released are recycled by aminoacyl-tRNA synthesize adding a correct amino acid to them

• Above process repeats itself until a STOP signal comes to the A site.

Polypeptide Chain Termination

• Stop signal does not bring any amino acid in as it moves to the P site and is released.

• Stop Codons: UAG, UGA, and UAA

• A protein which is known as a release factor recognizes that the ribosome has slowed down and aids in the release of the polypeptide chain from the ribosome

• The ribosome subunits fall off the mRNA

• Translation is over!

At this point, modifications to the polypeptide may include:

• sugars added to various amino acids through glycosylation

• Phosphates may be added to various amino acids through phosphorylation

• Enzymes may cleave the chain at various points