Chapter 11

Question 1

What is the one gene, one enzyme hypothesis?

Answer 1

the idea that each gene encodes a separate enzyme

Question 2

What are isoaccepting tRNAs?

Answer 2

different tRNAs that accept the same amino acid but have different anticodons

Question 3

What is the significance of the fact that many synonymous codons differ only in the third nucleotide position?

Answer 3

Synonymous codons code for the same amino acid, or, in other words, have the same meaning. A nucleotide at the third position of a codon pairs with a nucleotide in the first position of the anticodon. Unlike the other nucleotide positions involved in the codonanticodon pairing, this pairing is often weak, or “wobbles,” and nonstandard pairings can occur. Because the “wobble,” or nonstandard base-pairing with the anticodons, affects the third nucleotide position, the redundancy of codons ensures that the correct amino acid is inserted in the protein when nonstandard pairing occurs.

Question 4

Reading Frame

Answer 4

particular way in which a nucleotide sequence is read in groups of three nucleotides (codons) in translation; begins with a start codon and ends with a stop codon

Question 5

Overlapping Code

Answer 5

a code in which a single nucleotide might be included in more than one codon

Question 6

Nonoverlapping Code

Answer 6

each nucleotide is part of only one codon and encodes only one amino acid in a protein

Question 7

Initiation Codon

Answer 7

the first codon of the mRNA to specify an amino acid; most often AUG

Question 8

Termination Codon

Answer 8

codon in mRNA that signals the end of translation; the three most common stop codons are UAA, UAG, and UGA

Question 9

Sense Codon

Answer 9

codon that specifies and amino acid in a protein

Question 10

Universal Code

Answer 10

refers to the fact that particular codons specify the same amino acids in almost all organisms

Question 11

How is the reading frame of a nucleotide sequence set?

Answer 11

The initiation codon on the mRNA sets the reading frame.

Question 12

How are tRNAs linked to their corresponding amino acids?

Answer 12

For each of the 20 different amino acids commonly found in proteins, a corresponding aminoacyl-tRNA synthetase covalently links to the amino acid to the correct tRNA molecule.

Question 13

What role do the initiation factors play in protein synthesis?

Answer 13

IF-1 promotes the disassociation of the large and small ribosomal subunits.
IF-3 binds to the small ribosomal subunit and prevents it from associating with the large ribosomal subunit.
IF-2 is responsible for the binding GTP and delivering the fMet-tRNA-fMet to the initiator codon on the mRNA.

Question 14

What events bring about the termination of translation?

Answer 14

The process of termination begins when a ribosome encounters a termination codon.
Because the termination codon would be located at the A site, no corresponding tRNA
will enter the ribosome. This allows for the release factors (RF-1, RF-2, and RF-3) to
bind the ribosome.

Question 15

Compare and contrast the process of protein synthesis in bacterial and eukaryotic cells, giving similarities and differences in the process of translation in these two types of cells

Answer 15

Share the same universal code
Initiation codon in eukaryotic cells codes for methionine, in bacteria the AUG codon codes for N-formylmethionine.
Transcription takes place within the nucleus for eukaryotes, whereas most translation takes place in the cytoplasm. Therefore, transcription and translation in eukaryotes are kept temporally and spatially separate.
In bacterial cells, transcription and translation are coupled and occur nearly simultaneously.

Question 16

What are some types of posttranslational modification of proteins?

Answer 16

Frequently,
the amino-terminal methionine may be removed. Sometimes, in bacteria only the
formyl group is cleaved from the N-formylmethionine, leaving a methionine at the
amino terminal. More extensive modification occurs in some proteins that are originally
synthesized as precursor proteins. These precursor proteins are cleaved and trimmed by
protease enzymes to produce a functional protein.

Question 17

Explain how some antibiotics work by affecting the process of protein synthesis.

Answer 17

Some antibiotics, such as streptomycin, bind to the small subunit
and inhibit translation initiation. Other antibiotics, such as chloramphenicol bind to the
large subunit and block elongation of the peptide by preventing peptide bond formation.
Antibiotics such as tetracycline and neomycin bind the ribosome near the A site yet
have different effects. Tetracyclines block entry of charged tRNAs to the A site, while
neomycin induces translational errors. Finally, some antibiotics such as erythromycin,
block the translocation of the ribosome along the mRNA.