Chapter 17 Critical Thinking

A mutation prevents production of an enzyme needed for a metabolic pathway. Which classic hypothesis does this support?

One gene-one enzyme.

Why was the one gene-one enzyme hypothesis revised?

Some genes encode polypeptides that are part of larger proteins, and some genes produce functional RNA.

A Neurospora mutant grows on complete medium but not minimal medium. What is the most likely explanation?

It cannot synthesize a required nutrient.

A Neurospora mutant grows only when arginine is added. What is the likely defect?

A mutation blocks an enzyme in arginine synthesis.

Why did Beadle and Tatum test mutants on different nutrient supplements?

Different supplements revealed where each metabolic pathway was blocked.

A cell has DNA but cannot make mRNA. Which process is defective?

Transcription.

A cell makes mRNA but cannot make a polypeptide. Which process is defective?

Translation.

What is the main difference between replication and transcription?

Replication copies DNA, while transcription makes RNA from DNA.

What is the main difference between transcription and translation?

Transcription makes RNA from DNA, while translation makes protein from mRNA.

A DNA template strand is 3′-TAC CTT GGA ACT-5′. What mRNA is transcribed?

5′-AUG GAA CCU UGA-3′.

What peptide is made from 5′-AUG GAA CCU UGA-3′?

Methionine-glutamic acid-proline.

Why is UGA not included as an amino acid in the peptide from AUG GAA CCU UGA?

UGA is a stop codon.

A coding DNA strand is 5′-ATG GAA CCT TGA-3′. What mRNA is produced?

5′-AUG GAA CCU UGA-3′.

Why is the mRNA sequence nearly identical to the coding DNA strand?

The coding strand is not used as the template.

Which DNA strand determines the complementary RNA sequence?

The template strand.

Why must the RNA polymerase read the template strand 3′ to 5′?

It can synthesize RNA only 5′ to 3′.

What tRNA anticodon pairs with the mRNA codon 5′-AUG-3′?

3′-UAC-5′.

What tRNA anticodon pairs with the mRNA codon 5′-GAA-3′?

3′-CUU-5′.

What happens if a tRNA carrying methionine has an anticodon that matches UGG instead of AUG?

It would incorrectly place methionine at a tryptophan codon.

Why is aminoacyl-tRNA synthetase essential for translation accuracy?

It attaches the correct amino acid to each tRNA.

What would happen if a tRNA were loaded with the wrong amino acid but had the correct anticodon?

The wrong amino acid could be inserted into the protein.

Why can different codons specify the same amino acid without changing the protein?

The genetic code is redundant.

Why is the genetic code not ambiguous?

Each codon specifies only one amino acid.

A mutation changes GAA to GAG. What type of mutation is likely if both codons specify glutamic acid?

Silent mutation.

A mutation changes UAU to UAA. What type of mutation is this?

Nonsense mutation.

A mutation changes GAA to GUA. What type of mutation is this if glutamic acid changes to valine?

Missense mutation.

Why can a nonsense mutation be especially damaging?

It creates an early stop codon and may shorten the protein.

Why can a silent mutation sometimes still affect phenotype?

It can alter RNA processing, translation efficiency, or regulation even without changing an amino acid.

Why is an insertion of one nucleotide usually more disruptive than an insertion of three nucleotides?

One nucleotide causes a frameshift, while three add one amino acid without shifting the frame.

What happens to downstream codons after a frameshift mutation?

Most downstream codons are changed.

Why can an insertion or deletion of three nucleotides be less severe than one of one nucleotide?

It preserves the reading frame.

A sequence reads THE RED DOG ATE THE BIG CAT. What does removing the first letter model?

A frameshift mutation.

Why does a start codon establish the reading frame?

It determines where the ribosome begins grouping bases into codons.

Why can an mRNA have many codons but only one start site for a particular protein?

Translation begins at a specific start codon recognized by the initiation complex.

A eukaryotic gene has a promoter mutation that prevents transcription-factor binding. What is the likely result?

RNA polymerase II cannot efficiently initiate transcription.

What is most likely to happen if the TATA box is mutated?

Transcription initiation may be reduced or disrupted.

Why can RNA polymerase initiate RNA synthesis without a primer while DNA polymerase cannot?

RNA polymerase can begin RNA chains directly at a promoter.

A bacterial RNA polymerase reaches a terminator. What happens next?

Transcription stops and the RNA transcript is released.

A eukaryotic RNA polymerase II reaches a polyadenylation signal. What happens next?

The transcript is cleaved downstream and processed with a poly-A tail.

Why can bacterial mRNA often be translated immediately after transcription?

It usually does not require extensive RNA processing and bacteria lack a nucleus.

Why must eukaryotic pre-mRNA be processed before translation?

It needs a cap, poly-A tail, and intron removal before export and efficient translation.

A pre-mRNA retains introns after processing. What is the likely consequence?

The resulting protein may be abnormal or not produced correctly.

A mutation disrupts a splice site. What process is directly affected?

RNA splicing.

What complex removes introns from eukaryotic pre-mRNA?

The spliceosome.

Why can alternative splicing allow one gene to produce multiple proteins?

Different exon combinations can be retained in mature mRNAs.

Why can alternative splicing help explain why humans have fewer genes than expected?

One gene can generate many different protein products.

Why was the discovery of ribozymes important?

It showed that RNA can act as a biological catalyst.

Why can RNA function as an enzyme?

It can fold into shapes, contain reactive groups, and hydrogen-bond with other molecules.

What is the likely effect of removing the 5′ cap from a eukaryotic mRNA?

Reduced stability, export, and ribosome attachment.

What is the likely effect of removing the poly-A tail from a eukaryotic mRNA?

Reduced stability and less efficient translation.

A ribosome has an initiator tRNA carrying methionine in the P site. What stage is occurring?

Translation initiation.

What enters the A site during elongation?

A charged tRNA matching the next codon.

What occurs at the P site during elongation?

The growing polypeptide is held and transferred during peptide-bond formation.

What occurs at the E site during elongation?

Empty tRNA exits the ribosome.

What is the correct order of translation elongation events?

Codon recognition, peptide-bond formation, then translocation.

Why does translocation move the ribosome exactly three nucleotides?

One codon contains three nucleotides.

A stop codon enters the A site. Why does translation stop instead of adding an amino acid?

No tRNA has an anticodon for a stop codon.

What molecule recognizes a stop codon during translation?

A release factor.

Why is rRNA considered important in peptide-bond formation?

rRNA acts as a ribozyme that catalyzes the reaction.

What is the advantage of multiple ribosomes translating one mRNA simultaneously?

Many copies of the same polypeptide are made quickly.

A cell contains a polyribosome. What conclusion is most supported?

Several ribosomes are translating one mRNA at the same time.

Why are transcription and translation coupled in bacteria but not eukaryotes?

Bacteria lack a nucleus, while eukaryotes separate DNA from cytoplasmic ribosomes.

A mutation changes the DNA template strand but not the coding strand. Is this possible?

No, the strands are complementary, so a change in one changes the paired base in the other.

A mutation occurs in an intron that is removed normally. Will it always alter protein sequence?

No, it may have no effect on the final protein sequence.

A mutation occurs in an exon but does not change an amino acid. What type of mutation may it be?

A silent substitution.

A mutation creates a premature stop codon in the first exon. What outcome is most likely?

A severely shortened or nonfunctional protein.

A mutation deletes a promoter but leaves the coding region intact. What outcome is most likely?

The gene may not be transcribed.

A mutation changes an anticodon in a tRNA. What process is most directly disrupted?

Proper codon recognition during translation.

A guide RNA does not match a target DNA sequence. What is the likely CRISPR-Cas9 result?

Cas9 will not be directed efficiently to that target.

Why can CRISPR-Cas9 create unintended mutations?

Cas9 may cut DNA at similar off-target sequences.

Why are ethical concerns greater for CRISPR edits in embryos or germ cells?

The changes could be inherited by future generations.

Why does the near universality of the genetic code allow a jellyfish gene to function in another organism?

The recipient organism interprets codons in nearly the same way.

Why is a gene not always equivalent to one protein?

Alternative splicing can produce multiple polypeptides from one gene.

Why is a gene not always equivalent to one polypeptide?

Some genes code for functional RNAs instead of proteins.