TRANSLATION (PROTEIN SYNTHESIS)

PREPARED BY: ZEESHAN AKRAMGROUP PARTICIPANTS: GROUP BPRESENTED TO: SIR ALI HAIDER SALEEM

Definition

Translation is the process in which cellular ribosomes create proteins From mRNA.

mRNA carries coded instructions for protein synthesis (translation) from the DNA in the nucleus.

The mRNA sequence is thus used as a template to assemble the chain of amino acids that form a protein.

SITE OF TRANSLATION

PROKARYOTES: In prokaryotes Transcription and translation occur together.

Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.

(a)

TRANSLATION

TRANSCRIPTION DNA

mRNARibosome

Polypeptide

SITE OF TRANSLATION

EUKARYOTES: In eukaryotes, transcription takes place in nucleus and translation occurs in cytoplasm. Transcription and Translation occur separately.

TRANSLATION INVOLVES:

mRNA Ribsomes….Ribosomal RNA Transfer RNA Amino Acids

mRNA:

Carries the information from nucleus. mRNA has specific codon information which

converts Amino acids into polypeptide chain. mRNA acts as a template.

tRNA:

Brings amino acids to the ribosome so it can build protein.

It has Anticodones (sequence of 3 nucleotides) Complementary to mRNA codons A correct match(bond or recognition) between

tRNA and an amino acid with the help of Aminoacyl-tRNA synthetase.

TYPES OF RIBOSOMES:

In Eukaryotes two types of ribosomes are found i-e 50s and 30s. Each subunit exists separately in the cytoplasm, but the two join together on the mRNA molecule at time of translation.

AMINO ACIDS:

Amino acids are the sequence of 3 nucleotides. Each amino acid has its own specific codon.

PHASES OF TRANSLATION:

There are 3 main phases.1. Initiation2. Elongation3. Termination

INITIATION:

mRNA attaches with small subunit of ribosome with the help of three initiation factor proteins(known as IF1, IF2, and IF3).

Ribosome has 3 parts.1. A Site (where tRNA enters with Amino acid)2. P Site (where peptide bond forms between A.A)3. E Site (where tRNA exits) Translation starts with start codon AUG

(Methionine) tRNA carries the AUG codon and attach at A site

of ribosome.

Largeribosomalsubunit

The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiationfactors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid.

2

Initiator tRNA

mRNA

mRNA binding site Smallribosomalsubunit

Translation initiation complex

P site

GDPGTP

Start codon

A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met).

1

Met MetU A C

A U G

E A

3

5

5

3

35 35

ELONGATION:

In the elongation stage, amino acids are added one by one to the preceding amino acid.

Peptide bond is formed between amino acids. It is facilitated by an enzyme called peptidyl

transferase. The process in which ribosomal subunits move

forward and shift or translocate the t-RNA from P site to E site is called translocation process.

Amino endof polypeptide

mRNA

Ribosome ready fornext aminoacyl tRNA

E

P A

E

P A

E

P A

E

P A

GDPGTP

GTP

GDP

2

2

site site5

3

TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysisof GTP increases the accuracy andefficiency of this step.

1

Peptide bond formation. An rRNA molecule of the large subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site.

2

Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs,bringing the next codon to be translated into the A site.

3

TERMINATION:

There are three termination codons that are employed at the end of a protein-coding sequence in mRNA: UAA, UAG, and UGA.

No t-RNAs recognize these codons. Thus, in the place of these t-RNAs, one of

several proteins, called release factors, binds and facilitates release of the mRNA from the ribosome and subsequent dissociation of the polypeptide chain.

Release factor

Freepolypeptide

Stop codon(UAG, UAA, or UGA)

5

3 3

5

35

When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA.

1The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome.

2 3 The two ribosomal subunits and the other components of the assembly dissociate.

MODIFICATION:

The protein which is formed during translation is inactive.

Transport to Endoplasmic reticulum Modification with lipids, carbohydrates etc if

needed. Then transport to target location.

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