Translation

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 synthetasesThis family of enzymes is required for attachment

of amino acids to their corresponding t-RNAs.5.Functionally component ribosomesRibosomes 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 ProteinAre 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 ribosomeThe 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. TerminationTermination 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.