Question

Which statement about translation is NOT correct?

a. In order for translation to begin, the small ribosomal subunit must bind to the mRNA and the initiator tRNA, which is bound to methionine.

b. The initial amino acid in a polypeptide is located on the C-terminus.

c. The large ribosomal subunit is responsible for catalyzing peptide bonds between amino acids.

d. Translation is terminated when the mRNA stop codon moves into the ribosomal A site and a release factor binds in the place of tRNA.

e. At the end of termination, the small and large ribosomal subunits separate from one another, and the polypeptide is released

Answer 1

The mRNA carries genetic information. This genetic information is encoded as a DNA sequence from the chromosomes to the nucleolus. The ribonucleotides are read by translational machinery in a sequence of nucleotide triplets which are called codons. Each of those triplets codes for a specific amino acid.

The ribosomes translate this code to a specific sequence of amino acids. The ribosome is a multi-subunit structure that contains rRNA and proteins. It is the "factory" where amino acids are assembled into proteins. tRNAs are small noncoding RNA chains that transport amino acids to the ribosome. tRNAs have a site for amino acid attachment, and a site called an anticodon. The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo amino acid.

Aminoacyl tRNA synthetases catalyze the bonding which is between specific tRNAs and the amino acids that their anticodon sequences made for. As a product of this reaction an aminoacyl tRNA is created. In case of the prokaryotes, this aminoacyl-tRNA is carried to the ribosome by EF-Tu, where mRNA codons are matched through the complementary base pairing to specific tRNA anticodons. Aminoacyl-tRNA synthetases that mispair tRNAs with the wrong amino acids can produce mischarged aminoacyl-tRNAs, which can result in inappropriate amino acids at the respective position in protein. The "mistranslation" of the genetic code naturally occurs in most organisms at low levels, but certain cellular environments increases the permissive mRNA decoding, sometimes to the benefit of the cell.
The ribosome has three sites for tRNA to bind. They are the aminoacyl site, the peptidyl site and the exit site. With respect to the mRNA, these three sites are oriented 5’ to 3’ E-P-A, as the ribosome moves toward the  end of mRNA. Then the A-site binds the incoming tRNA with the complementary codon on the mRNA. The P-site holds the tRNA with the growing polypeptide chain. And the E-site holds the tRNA without its amino acid, and the tRNA is then released. When an aminoacyl-tRNA initially binds to its corresponding codon on the mRNA, it is in the A site. Then, a peptide bond forms between the amino acid of the tRNA in the A site and the amino acid of the charged tRNA in the P site. The growing polypeptide chain is transferred to the tRNA in the A site. Translocation occurs, moving the tRNA in the P site, now without an amino acid, to the E site; the tRNA that was in the A site, now charged with the polypeptide chain, is moved to the P site. The tRNA in the E site leaves and another enters the A site to repeat the process.

Therefore, c. The large ribosomal subunit is responsible for catalyzing peptide bonds between amino acids is not correct about translation.