MCAT Biochemistry
Protein synthesis, or translation, is the biological process where ribosomes utilize mRNA to make proteins. This occurs in the cytosol or rough ER depending on the type of proteins being made. tRNA is essential for adding amino acids to the growing polypeptide chain. Before translation begins, aminoacyl-tRNA synthetase uses ATP to charge the tRNA molecule. Translation consists of three stages: initiation, elongation, and termination.
During initiation, factors help bring the small ribosomal subunit, initiator tRNA, and mRNA together to form the initiation complex. The ribosome scans the mRNA until it locates the start codon, AUG, which codes for methionine. The large ribosomal subunit joins, and the initiation factors dissociate from the complex. In elongation, a new aminoacyl-tRNA enters the A site and ribozyme creates a peptide bond to join the amino acids. The ribosome then shifts to the next codon, allowing a new aminoacyl-tRNA to enter. Elongation continues until release factors recognize a stop codon, leading to termination, and the polypeptide is released.
Lesson Outline
<ul> <li>Protein synthesis (translation)</li> <ul> <li>Eukaryotic translation occurs in cytosol or rough ER depending on protein type</li> </ul> <li>Key components for protein synthesis</li> <ul> <li>mRNA as template</li> <li>tRNA for adding amino acids</li> <li>Charging tRNA with aminoacyl-tRNA synthetase and ATP</li> </ul> <li>tRNA and anticodons</li> <li>Aminoacyl-tRNA synthetase and charging process</li> <ul> <li>20 different versions, one for each amino acid</li> <li>Error checking mechanism</li> </ul> <li>tRNA binding sites on ribosome</li> <ul> <li>A site - incoming charged tRNA enters</li> <li>P site - holds growing polypeptide</li> <li>E site - where empty tRNA leaves</li> </ul> <li>Stages of translation</li> <ul> <li>Initiation</li> <ul> <li>Eukaryotic initiation factors</li> <li>Brings together small ribosomal subunit, initiator tRNA, and mRNA</li> <li>Finds start codon and recruits large ribosomal subunit</li> </ul> <li>Elongation</li> <ul> <li>Incoming tRNA enters A site</li> <li>Wobble effect and genetic code redundancy</li> <li>Formation of peptide bond with ribozyme</li> <li>Translocation to new codon with GTP</li> <li>Repeats until stop codon reached</li> </ul> <li>Termination</li> <ul> <li>Eukaryotic release factors recognize stop codon</li> <li>Polypeptide released from ribosome</li> <li>Requires GTP</li> </ul> </ul> </ul>
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FAQs
Ribosomes are essential organelles in cells that facilitate protein synthesis. They act as the site where mRNA and tRNA come together to facilitate translation, decoding the genetic information stored in the mRNA into amino acids that are then assembled into a polypeptide chain. The ribosome is composed of two subunits: the small subunit binds to the mRNA and the tRNA carrying the amino acids, while the large subunit catalyzes the formation of the peptide bond between the amino acids.
In eukaryotic translation, mRNA (messenger RNA) acts as a template for protein synthesis, carrying the genetic code in the form of codons (triplets of nucleotides). Each codon specifies a particular amino acid to be added to the growing polypeptide chain. tRNA (transfer RNA) molecules serve as adapters, binding amino acids in the cytoplasm and delivering them to the ribosome. The tRNA molecule recognizes the codon in the mRNA through its anticodon, ensuring that the correct amino acid is added to the polypeptide chain.
Translation in protein synthesis can be divided into three stages: initiation, elongation, and termination. During initiation, the ribosomal subunits assemble around the mRNA, and the first tRNA carrying the start amino acid (usually methionine) binds to the ribosome at the start codon. Elongation involves the decoding of mRNA codons and the formation of peptide bonds between amino acids, facilitating the growth of the polypeptide chain. Finally, termination occurs when a stop codon is reached in the mRNA; the ribosome recognizes the stop codon, the polypeptide chain is released, and the ribosome disassembles.
Elongation is the phase of protein synthesis where the polypeptide chain is extended by the addition of amino acids. This process is facilitated by the ribosome, which moves along the mRNA, decoding each codon and binding the correct tRNA. The tRNA brings the corresponding amino acid to the ribosome, where the large subunit catalyzes the formation of a peptide bond between the amino acid and the growing polypeptide chain. The ribosome then shifts by one codon, releasing the empty tRNA and allowing a new tRNA with its amino acid to enter. This process repeats until a stop codon is encountered.
Termination in translation occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) in the mRNA sequence. Stop codons do not code for any amino acids, and instead signal the end of the protein synthesis process. When the ribosome reaches a stop codon, a release factor protein binds to the site, causing the ribosome to release the newly synthesized polypeptide chain into the cytoplasm. After the release, the ribosome dissociates into its subunits, which can then initiate a new round of translation with another mRNA molecule.
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