Figure A below shows the stage in translation when an incoming aminoacyl-tRNA ha

Question

Figure A below shows the stage in translation when an incoming aminoacyl-tRNA has bound to the A-site on the ribosome (step 1). Using the components shown in Figure A as a guide, show on Figures B and C what happens in the next two steps. Step 2 represents the peptidyl transferase reaction inherent to the large ribosomal subunit (indicate where the peptide chain is attached and where tRNAs #3 and 4 are located). At step 3 the EF-G dependent translocation of the small subunit on the mRNA has taken place (indicate the positions of tRNAs #3, 4 and the next aminoacyl tRNA, #5).

For step 2 show the nucleophilic attack that forms the peptide bond and releases one amino acid from its tRNA. You will need to include a figure showing the amino group of the incoming acylated tRNA and the peptidyl tRNA.

Explanation / Answer

Translation involves “decoding” a messenger RNA (mRNA) and using its information to build a polypeptide, or chain of amino acids. For most purposes, a polypeptide is basically just a protein (with the technical difference being that some large proteins are made up of several polypeptide chains).

STEP 1:

n translation, the codons of an mRNA are read in order (from the 5' end to the 3' end) by molecules called transfer RNAs, or tRNAs.

Each tRNA has an anticodon, a set of three nucleotides that binds to a matching mRNA codon through base pairing. The other end of the tRNA carries the amino acid that's specified by the codon.tRNAs bind to mRNAs inside of a protein-and-RNA structure called the ribosome. As tRNAs enter slots in the ribosome and bind to codons, their amino acids are linked to the growing polypeptide chain in a chemical reaction. The end result is a polypeptide whose amino acid sequence mirrors the sequence of codons in the mRNA.

In order for translation to start, we need a few key ingredients. These include:

During initiation, these pieces must come together in just the right way. Together, they form the initiation complex, the molecular setup needed to start making a new protein.

STEP 2 Translation elongation is simply the ribosome travelling down the message, reading codons and bringing in the proper aminoacyl tRNA’s to translate the message out to protein. The incoming aminoacyl tRNA is brought into the ribosome A site, where it is matched with the codon being presented. Once it has been secured (by hydrolysis of GTP to “fix” it in place), the peptidyltransferase reaction occurs. This is where the bond between the peptide and the aminoacyl tRNA in the P site is broken, while a new bond is simultaneously formed between the (momentarily unattached) peptide and the new amino acid in the A site. The ribosome then moves over by 3 bases, the spent tRNA is ejected from the E site.

STEP 3

Polypeptides, like all good things, must eventually come to an end. Translation ends in a process called termination. Termination happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site.

Stop codons are recognized by proteins called release factors, which fit neatly into the P site (though they aren't tRNAs). Release factors mess with the enzyme that normally forms peptide bonds: they make it add a water molecule to the last amino acid of the chain. This reaction separates the chain from the tRNA, and the newly made protein is released.

What next? Luckily, translation "equipment" is very reusable. After the small and large ribosomal subunits separate from the mRNA and from each other, each element can (and usually quickly does) take part in another round of translation.

Related Questions

Navigate

• Browse (All)
• Browse F
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.