Translation

 Is the process in which mRNA provides a template for synthesis of polypeptide.

Over view of"central dogma" DNA → RNA → protein.

Components Required for Translation  A large number of components required for the synthesis of a polypeptide chain. These include  all the amino acids that are found in the finished product,  the m-RNA to be translated,  t-RNAs,  functional ribosomes,  energy sources,  enzymes,  and protein factors needed for the initiation, elongation, and termination of the polypeptide chain.  These gather in the cytosol prior to turning on the assembly line that creates a finished product.

1.Amino acids  All the amino acids that appear in the finished protein must be present at the time of protein synthesis. 2. Transfer RNA (t-RNA)  At least one specific type of t-RNA is required per amino acid.  In humans there are at least 50 species of t-RNA. Whereas bacteria contain 30 to 40 species.  Because there are only 20 different amino acids commonly carried by t-RNAs, some amino acids have more than one specific t-RNA molecule. This is particularly true of those amino acids that are coded for by several codons.

 Amino acid attachment site  Each tRNA molecule has an attachment site for a specific amino acid at its 3'-end.  When a tRNA has a covalently attached amino acid it is said to be charged,  when tRNA is not bound to an amino acid it is described as being uncharged.  The amino acid that is attached to the tRNA molecule is said to be activated.

 Anticodon  Each t-RNA molecule contains a three-base nucleotide sequence—the anticodon—that recognizes a specific codon on the mRNA.  This codon specifies the insertion into the growing peptide chain of the amino acid carried by that t-RNA.  Because of their ability to both carry a specific amino acid and to recognize the codon for that amino acid, t-RNA are known as adaptor molecules.

Typical a clover leaf diagram of t-RNA molecule

3. Messenger RNA (mRNA)  The specific mRNA required as a template for the synthesis of the desired polypeptide chain must be present. 4. Aminoacyl-t-RNA synthetases  This family of enzymes is required for attachment of amino acids to their corresponding t-RNAs. 5.Functionally component ribosomes  Ribosomes are large complexes of protein and r-RNA. They consist of two subunits—one large and one small.

 Ribosomal RNA (r-RNA)  Prokaryotic ribosomes contain 3 molecules of r-RNA, whereas eukaryotic ribosomes contain 4 molecules of r-RNA.  Ribosomal Protein  Are present in considerably greater numbers in eukaryotic ribosomes than in prokaryotic ribosomes. These proteins play a number of roles in the structure and function of the ribosome and its interactions with other components of the translation system.

 A and P sites on the ribosome  The ribosome has two binding sites for t-RNA molecules, the A and P sites, each of which extends over both subunits. Together, they cover two neighboring codons.  During translation, the A site binds an incoming aminoacyl-t-RNA as directed by the codon currently occupying this site. This codon specifies the next amino acid to be added to the growing peptide chain.  The P site codon is occupied by peptidyl-t-RNA. This t-RNA carries the chain of amino acids that has already been synthesized.

6.Protein factors  Initiation, elongation, and termination factors are required for peptide synthesis.  Some of these protein factors perform a catalytic function, whereas others appear to stabilize the synthetic machinery. 7. ATP and GTP are required as source of energy

Codon Recognition By tRNA  Recognition of particular codon in an mRNA sequence is accomplished by the anti codon sequence of the t-RNA. Some t-RNA recognize more than one codon for a given amino acid.  Antiparallel binding between codon and anti codon: Binding of the t-RNA anti codon to the mRNA codon follows the rules of complementary and anti parallel binding.  Wobble hypothesis: The mechanism by which t-RNA can recognize more than one codon for a specific amino acid is described by the “wobble” hypothesis in which the base at the 5'-end of the anti codon (the first base of the anti codon) is not a spatially defined as the other two bases. Movement of that first base allows nontraditional base pairing with the 3'-base of the codon (the last base of the codon). This movement is called “wobble” and allows a single t-RNA to recognize more than one codon.

Steps in Translation  The process of translation is divided into 3 separate steps: initiation, elongation, and termination. 1.Initiation  Initiation of protein synthesis involves the assembly of the components of the translation system. These components include the two ribosomal subunits, the mRNA to be translated, the aminoacyl-t-RNA specified by the first codon in the message, GTP, and the initiation factors, that facilitate the assembly of this initiation complex.  In prokaryotes, three initiation factors are known (IF- 1, IF-2, and IF-3), whereas in eukaryotes, there are at least nine.

2. Elongation  Elongation of the polypeptide chain involves the addition of amino acids to the carboxyl end of the growing polypeptide chain.  During elongation, the ribosomes moves from the 5'-end to the 3'-end of the mRNA that is being translated. Delivery of the aminoacyl-t-RNA whose codon appears next on the mRNA template is facilitated in prokaryote by elongation factors EF-Tu and EF-Ts and requires GTP. In eukaryotes, elongation factors are designated eEF.  The formation of the peptide bonds is catalyzed by peptidyl-transferase.  After the peptide bond has been formed the ribosome advances three nucleotides toward the 3'-end of the mRNA. This process is known as translocation.  This causes release of the uncharged t-RNA and movement of the peptidyl-t-RNA into the P site.

3. Termination  Termination occurs when one of the three termination codons moves into the A site.  These codons are recognized in by release factors:  RF-1, which recognizes the termination codons UAA and UAG.  RF-2, which recognizes UGA and UAA.  RF-3, which binds GTP and stimulates the activity RF-1 and RF-2.

Polysomes  Translation begins at the 5'-end of the mRNA, with the ribosome proceeding along the RNA molecule. Because of the length of most mRNA, more than one ribosome at a time can generally translate the message.  Such a complex of one mRNA and a number of ribosomes is called a polysome or polyribosome.