Protein Synthesis

Translation

Protein Synthesis

Translation

Initiation

Elongation

Termination

Translation

From DNA blueprint Protein

Once mRNA enters the cytoplasm, it is ready to be translated

Translation is made up three stages

Translation- Initiation

Initiation occurs when a ribosome recognizes a specific sequence, the 5' cap, on the mRNA and binds to that site

This process is the initiation of translation of nucleic acid into protein

Ribosomes

In eukaryotes, the ribosome consists of two subunits

A large subunit of 60SA small subunit of 40SThe S refers to the

rates at which various components sediment when centrifuged

The rates of a measure of size

Intact ribosome (80S)

Structure of RNA

E site (Exit site)

P site (Peptidyl tRNA binding site)

A site (Aminoacyl tRNA binding site)

mRNA binding site Small

subunit

Large subunit

Ribosomal subunits are a combination of ribosomal RNA (rRNA) and protein

Translation-Initiation

The ribosomes move along the mRNA in the 5'-3' direction, adding a new amino acid to the growing polypeptide chain

The two ribosomal subunits bind to the mRNA, clamping the mRNA between them

Ribosomes

Ribosomes synthesize different proteins by associating with different mRNAs and reading their coding sequence

Reading frame: a contiguous and non-overlapping set of three-nucleotide codons

There are 3 possible reading frames in an mRNA strand

Reading Frame

In theory any mRNA can be translated in any of three different "reading frames" depending on the nucleotide sequence at which translation starts.

Thus 3 different amino

acid sequences can be obtained from a single mRNA

The Role of Transfer RNA (tRNA)

The ribosome alone cannot synthesize the polypeptide chain

The correct amino acids must be delivered to the ribosome binding site

The molecule that delivers the amino acids is tRNA

tRNA

A small single-strandednucleic acid whose structure resembles acloverleaf

At one arm of tRNA isthe anticodon

Anticodon is a sequenceof three bases that recognizes the codon of the mRNA

The opposite arm carries the corresponding amino acid

mRNA

5' 3'

tRNA

tRNA and mRNA recognize each other by complementary base pairing

tRNA

Every tRNA carries only a specific amino acid, which means that at least 20 different tRNAs are required

Recall: there is 64 possible codons

In reality, anywhere between 20-64 types of tRNA molecules are available depending on the organism

Wobble Hypothesis

Proposed by Francis CrickThe theory proposes that the first two bases in

the codon and anticodon will form complementary pairs in the normal antiparallel fashion.

However, a degree of ‘wobble’ is allowed in the base-pairing at the third position.

This hypothesis accounts for the number of tRNAs that are observed

It also accounts for the degeneracy that is observed in the Genetic Code. The degenerate base is that in the wobble position.

Wobble Theory

Crick recognized that the following base-pair schemes were possible:

5' anticodon base 3' codon base

A U

C G

G C or U

U A or G

I A or C or U

Aminoacyl-tRNA

tRNA molecule with its corresponding amino acid attached to its acceptor site at the 3' end

Acceptor site of amino acid

Translation-Initiation

The mRNA joins to the small ribosomal unit at the 5' untranslated region. This binds to a special binding site on the small ribosomal subunit.

Translation-Initiation

The large ribosomal subunit attaches to the small subunit such that the first codon is aligned at the P binding site.

Translation-Initiation

A tRNA carrying the amino acid methionine attaches to the start codon (AUG) on the messenger RNA. This initiates elongation.

The AUG codon codes for methionine, i.e, every protein initially starts with this amino acid

The tRNA that carries methionine enters the P site

Translation-Initiation

Translation-Elongation

The next tRNA carrying the required amino acid enters the A site

The tRNA and its amino acid attaches to the A binding site.

Amino end of polypeptide

mRNA

Translation-Elongation

The methionine amino acid at the P site is bonded to the amino acid at the A site

There is a Peptide bond formation between the two amino acids Peptide

bond formation

Translation- Elongation

The tRNA at the P site is now free of amino acids

Translation-Elongation

Ribosome shifts over one codon (three bases)

Ribosome moves in the 3' direction down the messenger RNA

The A binding site is now open and a vacant tRNA is in the E binding site.Ribosome ready for next aminoacyl tRNA

Translation- Elongation

A new tRNA attached to an amino acid enters the vacant A site on the ribosome and the process repeats again

Amino end of polypeptide

mRNA

Translation-Termination

The ribosome will eventually reach a stop codon

The three stop codons are:

UGA UAG UAA

Since these codons do not code for an amino acid, there are no corresponding

tRNAs

Translation- Termination

Since, the stop codons do not code for an amino acid, there are no corresponding tRNAs

Translation- Termination

A protein known as a release factor recognizes that the ribosome has stopped translating

The release factor helps in the release of the polypeptide chain from the ribosome

The two subunits fall off the mRNA and translation officially stops