termination of translation - protein synthesis

termination of translation

1. TERMINATION OF TRANSLATION DONE BY : STEFFY THOMAS I MSc BIOCHEMISTRY BHARHTHIAR UNIVERSITY

2. TRANSCRIPTION Transcription is the process by which the information in the strand of  DNA  is copied into a new molecule of messenger RNA   (mRNA). transcription occurs in three phases known as initiation, elongation and termination.

3. TRANSLATION Translation  is a process by which the genetic code contained within an mRNA molecule is decoded to produce the specific sequence of amino acids in a polypeptide chain. It occurs in the  cytoplasm  following transcription and, like transcription, has three stages: initiation, elongation and termination

4. TERMINATION OF TRANSLATION Elongation continues until the ribosome adds the last amino acid coded by mRNA. Termination of polypeptide synthesis requires a special signal. Termination ,the fourth stage of polypeptide synthesis is signaled by the presence of one of the three termination codon in the mRNA(UAA, UAG ,UGA),immediately following the final coded aminoacid

5. Translation is terminated by release factor when a stop codon is reached The final stage of translation , like initiation and elongation , requires highly specific molecular signals that decide the fate of mRNA -ribosome -peptidyl tRNA complex. Two types of specific protein release factors RFs have been discovered class I RFs and class II RFs. In prokaryotes there are two types of class I RFs : RF 1 And RF2 in eukaryotic cell class I RF (eRF1) Is the only kind. In prokaryotes and eukaryotes there is only one class II factor : RF3 And eRF2 respectively

6. In prokaryotes, there are three releasing factor totally (RF1, RF2 and RF3). Release factor 1 is responsible for the recognition of termination codon UAA and UAG and Release factor 2 is responsible for the recognition of UAA or UGA. The other task of the RF1 and RF2 is to trigger polypeptide hydrolysis from tRNA in P site. Class 1 RFs (RF1 and RF2) are similar in size and shape with tRNA and bind A site of the ribosome. Class 2 RFs – stimulate the dissociation of class 1 RFs from the ribosome after the release of polypeptide

7. once a termination codon occupies the ribosomal A site, three termination factors, or release factors-the proteins RF-l, RF-2, and RF3contribute to : (i) hydrolysis of the terminal peptidyl tRNA bond; (2) release of the free polypeptide and the last tRNA, now uncharged from the P site; and (3) dissociation of the 70S ribosome in to its 30S and 50S subunits, ready to start a new cycle of polypeptides synthesis.

8. These releasing factors RF1 and RF2, by binding at the termination codon induces peptidyl transferase to transfer the growing polypeptide to a water rather than to another aminoacid. The release factors have domains thought to mimic the structure of tRNA. In eukaryotes, a single release factor, eRF, recognizes all three termination codons.

9. Termination occurs in response to a termination codon in the A site. First, a release factor,R F( RF-1 or RF-2, depending on which termination codon is present)binds to the A site. This leads to hydrolysis of the ester linkage between the nascent polypeptide and the tRNA in the P site and release of the completed polypeptide . Finally the mRNA, Deacylated tRNA, and release factor leave the ribosome which dissociates into its 3 0S and 50S subunits aided by ribosome recycling factor (RRF), lF -3, and energy provided b y EF-G-mediated G TP hydrolysis . The 30S subunit complex with lF-3 is ready to begin Another cycle of translation

10. The termination of protein synthesis in eukaryotic cells is performed in a similar manner as of prokaryotes. However, 2 different release factors are located. The first one; eRF1; recognizes all three termination codons It can be said that it is the equivalent of RF1 and RF2 in prokaryotes. eRF3 binds with GTP and stimulates the separation of polypeptide from ribosome, with similar tasks with RF3 in prokaryotes. By providing termination, release factors enable protein synthesis cycle to end TERMINATION OF TRANSLATION IN EUKARYOTIC CELLS

11. RIBOSOME RECYCLING After release of polypeptide chain and RFs, ribosome (in its P and E site) is still connected to mRNA and uncharged tRNA. For ribosome enter a new cycle of polypeptide synthesis, it needs to release the mRNA and tRNA, to separate the large and small subunits of the ribosome. All of these events are called as the re-use of ribosome. In prokaryotes, the activating of ribosome recycling after the release of polypeptide for recycling ribosome involves EF-G and IF3.

12. RRF (ribosome recycling factor) binds to A site of the ribosome which is empty and imitates a tRNA; at the same time it allows the binding of EF-G to the ribosome; stimulates the release of uncharged tRNA in P and E sites. IF3 (being initiation factor) leads release of mRNA and separation of the two subunits of the ribosome. These subunits can participate in a new cycle .