Animation of Protein Synthesis (Translation) in Prokaryotes.
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Initiation of translation in prokaryotes:
The small ribosomal subunit is separated from the large subunit with the help of two initiation factors: IF1 and IF3. This complex then binds a to purine-rich region -- the Shine-Dalgarno sequence -- upstream of the AUG start codon on the mRNA. The Shine-Dalgarno sequence is base-paired to a complementary sequence on the 16S rRNA - a component of the small subunit. This alignment ensures that the start codon is in the right position within the ribosome. Another initiation factor - IF2 - brings in the initiator tRNA charged with the initiator amino acid N-formyl-methionine. The large ribosomal subunit joins the complex and all initiation factors are released. The ribosome has three sites: the A-site is the entry site for new tRNA charged with amino-acid or aminoacyl-tRNA; the P-site is occupied by peptidyl-tRNA - the tRNA that carries the growing polypeptide chain; the E-site is the exit site for the tRNA after it's done delivering the amino acid. The initiator tRNA is positioned in the P-site.
Elongation: A new tRNA carrying an amino acid enters the A-site of the ribosome. On the ribosome, the anticodon of the incoming tRNA is matched against the mRNA codon positioned in the A-site. During this proof-reading, tRNA with incorrect anticodons are rejected and replaced by new tRNA that are again checked. When the right aminoacyl-tRNA enters the A-site, a peptide bond is made between the two now-adjacent amino-acids. As the peptide bond is formed, the tRNA in the P-site releases the amino-acids onto the tRNA in the A-site and becomes empty. At the same time, the ribosome moves one triplet forward on the mRNA. As a result, the empty tRNA is now in the E-site and the peptidyl tRNA is in the P-site. The A-site is now unoccupied and is ready to accept a new tRNA. The cycle is repeated for each codon on the mRNA.
Termination: Termination happens when one of the three stop codons is positioned in the A-site. No tRNA can fit in the A-site at that point as there are no tRNA that match the sequence. Instead, these codons are recognized by a protein, a release factor. Binding of the release factor catalyzes the cleavage of the bond between the polypeptide and the tRNA. The polypeptide is released from the ribosome. The ribosome is disassociated into subunits and is ready for a new round of translation. The newly made polypeptide usually requires additional modifications and folding before it can become an active protein.
©Alila Medical Media. All rights reserved.
Support us on Patreon and get FREE downloads and other great rewards: patreon.com/AlilaMedicalMedia
Initiation of translation in prokaryotes:
The small ribosomal subunit is separated from the large subunit with the help of two initiation factors: IF1 and IF3. This complex then binds a to purine-rich region -- the Shine-Dalgarno sequence -- upstream of the AUG start codon on the mRNA. The Shine-Dalgarno sequence is base-paired to a complementary sequence on the 16S rRNA - a component of the small subunit. This alignment ensures that the start codon is in the right position within the ribosome. Another initiation factor - IF2 - brings in the initiator tRNA charged with the initiator amino acid N-formyl-methionine. The large ribosomal subunit joins the complex and all initiation factors are released. The ribosome has three sites: the A-site is the entry site for new tRNA charged with amino-acid or aminoacyl-tRNA; the P-site is occupied by peptidyl-tRNA - the tRNA that carries the growing polypeptide chain; the E-site is the exit site for the tRNA after it's done delivering the amino acid. The initiator tRNA is positioned in the P-site.
Elongation: A new tRNA carrying an amino acid enters the A-site of the ribosome. On the ribosome, the anticodon of the incoming tRNA is matched against the mRNA codon positioned in the A-site. During this proof-reading, tRNA with incorrect anticodons are rejected and replaced by new tRNA that are again checked. When the right aminoacyl-tRNA enters the A-site, a peptide bond is made between the two now-adjacent amino-acids. As the peptide bond is formed, the tRNA in the P-site releases the amino-acids onto the tRNA in the A-site and becomes empty. At the same time, the ribosome moves one triplet forward on the mRNA. As a result, the empty tRNA is now in the E-site and the peptidyl tRNA is in the P-site. The A-site is now unoccupied and is ready to accept a new tRNA. The cycle is repeated for each codon on the mRNA.
Termination: Termination happens when one of the three stop codons is positioned in the A-site. No tRNA can fit in the A-site at that point as there are no tRNA that match the sequence. Instead, these codons are recognized by a protein, a release factor. Binding of the release factor catalyzes the cleavage of the bond between the polypeptide and the tRNA. The polypeptide is released from the ribosome. The ribosome is disassociated into subunits and is ready for a new round of translation. The newly made polypeptide usually requires additional modifications and folding before it can become an active protein.
