Chapter 22

An aminoacyl group must be attached at the _______ '-OH position to take part in protein synthesis.

3

An incoming aminoacyl-tRNA enters the ribosome at the ________ site.

A

The high fidelity of amino acid attachment to tRNA is maintained at the expense of _____ hydrolysis.

ATP

The random allocation of codons to amino acids argues against a nonrandom origin for the genetic code.

False. It is the nonrandom allocation of codons to amino acids that argues against a nonrandom origin for the genetic code.

Base modifications are essential for maintaining a tRNA's structural integrity.

False. They may help promote attachment of the correct amino acid, or strengthen codon-anticodon interactions.

The acceptor stem is not critical for an aminoacyl-tRNA synthetase-tRNA interaction.

False. The acceptor stem is often important in recognition.

The growing polypeptide is formed by the transfer of an aminoacyl group from the incoming aminoacyl-tRNA to the peptidyl-tRNA at the P site.

False. The peptidyl-tRNA at the P site transfers the peptidyl group to the aminoacyl-tRNA.

Translocation of the newly extended peptidyl-tRNA is carried out by the ribosome by reading the codons non-sequentially.

False. Translocation occurs so that the mRNA is read sequentially.

An mRNA directly recognizes amino acids.

False. tRNA is the link between the codons of mRNA and amino acids.

Match each of the following: Non Watson-Crick base pairs Acceptor stem Class I aminoacyl-tRNA synthetase Class II aminoacyl-tRNA synthetase 1. Can occur at the third position of the codon, as predicted by the "wobble" hypothesis 2. Contains a Rossmann fold, must recognize the anticodon, and aminoacylates the tRNA's 3'-terminal 2'-OH group. 3. Specific for smaller, hydrophilic amino acids, and aminoacylates the tRNA's 3'-OH group. 4. A 7 bp stem that includes the 5'-terminal nucleotide

Non Watson-Crick base pairs 1 Acceptor stem 4 Class I aminoacyl-tRNA synthetase 2 Class II aminoacyl-tRNA synthetase 3

Match each of the following: Nonsense codon tRNA Degenerate genetic code 1. Allows many amino acids to be specified by more than one codon. 2. Contains an anticodon, a trinucleotide sequence that is complementary to an mRNA codon. 3. Signals the ribosome to terminate polypeptide chain elongation.

Nonsense codon 3 tRNA 2 Degenerate genetic code 1

Match each of the following: Transpeptidation tRNAfmet Initiation factor Translocation IF-3 1. Required for initiation but is not permanently associated with the ribosome. 2. Shared control, tax advantages, increased skills and resources. 3. Easier to transfer ownership and raise funds, no personal liability. 4. Simple to set up and maintains control with founder. 5. The peptidyl-tRNA, together with its bound RNA moves entirely to the P site.

Transpeptidation 4 tRNAfmet 2 Initiation factor 1 Translocation 5 IF-3 3

Base modifications may help promote attachment of the proper amino acid to the acceptor stem.

True

The ribosome mediates the interactions of nonribosomal protein factors that promote polypeptide chain initiation, elongation and termination.

True

The nascent protein begins to fold after it emerges from the ribosome.

True. The exit tunnel is too small for folding to occur prior to exiting.

At the ribosome an mRNA codon must pair with the anticodon of the correct ______ .

aminoacyl-tRNA

Eukaryotic ribosomes are larger and more complex than ________ ribosomes.

prokaryotic

Ribosomes translate mRNA's with remarkable fidelity by a process known as ______ .

proofreading

The tertiary structure of tRNA is maintained by _________ interactions and base pairing within and between helical stems.

stacking