The chemical mechanisms used to avoid errors in protein synthesis are different from those used during dna replication. dna polymerases use a 3' to 5' exonuclease proofreading activity to remove mispaired nucleotides incorrectly inserted into a growing dna strand. there is no analogous proofreading function on ribosomes. instead, proofreading for protein synthesis is carried out by the aminoacyl-trna synthetases. ldentify the mechanisms aminoacyl-trna synthetases use to ensure that the correct amino acid is attached to the appropriate trna. to distinguish between two amino acids with similar structures, some aminoacyl-trna synthetases use a proofreading site that recognizes and hydrolyzes the incorrect amino acid. the active site in each aminoacyl-trna synthetase has a high specificity for the correct amino acid substrate. after binding the correct amino acid, an aminoacyl-trna synthetase recruits the appropriate trna to the active site by recognizing a number of sequences and structural features in a specific trna. aminoacyl-trna synthetases have an additional active site that binds to non-cognate trnas. the trnas that bind to this second active are hydrolyzed and released from the enzyme. first, the aminoacyl-trna synthetase binds to its cognate trna by recognizing a number of specific sequences and structural features in the trna. next, the synthetase-trna complex recruits the correct amino acid to the enzyme active site. aminoacyl-trna synthetases can rapidly hydrolyze the ester linkage between incorrectly paired rnas and amino acids

transcription is a process in which information is rewritten. it is something our cells must do, in a more specialized and narrowly defined way. in biology, transcription is the process of copying out the dna sequence of a gene in the similar alphabet of rna. it needs to happen in the nucleus because that's where the dna is. dna is always inside the nucleus unless the cell is dividing.

transcription is the first step in gene expression, which involves copying a gene's dna sequence to make an rna molecule. transcription is performed by rna polymerases. it has three stages: initiation, elongation, and termination. rna molecules must be processed after transcription in eukaryotes.

explanation:

long version:

the protein synthesis consists two processes which are, transcription and translation. transcription takes place in the nucleus. it uses dna as a template to make an rna molecule. rna then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. transcription is the first part of the central dogma of molecular biology: dna → rna. it is the transfer of genetic instructions in dna to messenger rna (mrna). during transcription, a strand of mrna is made that is complementary to a strand of dna.  

these three stages are:

  initiation: in which   is the beginning of transcription.occuring when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

    elongation: in which is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

    termination: in which is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.

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