Download Answers to End-of-Chapter Questions – Brooker et al ARIS site

Answers to End-of-Chapter Questions – Brooker et al ARIS site
Chapter 12
Test Yourself Questions
1. Which of the following best represents the central dogma of gene expression?
a. During transcription, DNA codes for polypeptides.
b. During transcription, DNA codes for RNA, which codes for polypeptides during translation.
c. During translation, DNA codes for RNA, which codes for polypeptides during transcription.
d. None of the above.
Answer: b. The proper sequence for gene expression in both prokaryotes and eukaryotes is
transcription followed by translation. During transcription, a portion of the DNA molecule serves as a
template for a complementary mRNA. The mRNA is then translated to produce a polypeptide.
2. Transcription of a gene begins at a site on DNA called ________ and ends at a site on DNA known
as ___________.
a. an initiation codon, termination codon
b. a promoter, the termination codon
c. an initiation codon, the terminator
d. a promoter, the terminator
e. an initiator, the terminator
Answer: d. A promoter is a DNA sequence that signals the initiation site of transcription. A terminator is
a DNA sequence that signals the end of the gene sequence. The term codon refers to RNA triplets, not
DNA.
3. The product of transcription of a structural gene is
a. tRNA.
b. mRNA.
c. rRNA.
d. polypeptide.
e. a, b and c.
Answer: b. The term structural gene refers to a DNA sequence that codes for an amino acid sequence;
therefore, the product of the transcription would be mRNA. rRNA and tRNA do not code for amino acid
sequences and would not be transcribed from a structural gene.
4. During eukaryotic RNA processing, the nontranslated sequences that are removed are called
a. exons.
b. introns.
c. promoters.
d. codons.
e. ribozymes.
Answer: b. The term “introns” refers to intervening sequences. These are not part of the final mRNA and
are removed.
5. Ribozymes are
a. the organelles where translation takes place.
b. the RNA molecules that are components of ribosomes.
c. the proteins that are components of ribosomes.
d. the portions of the pre-mRNA that are removed.
e. RNA molecules that catalyze chemical reactions.
Answer: e. Ribozymes are RNA molecules that have catalytic abilities. They function in splicing of some
introns as well as the reaction that links the amino acids during translation.
6. The _____________ is the organelle where the translation process takes place.
a. mitochondria
b. nucleus
c. ribosome
d. lysosome
e. ribozyme
Answer: c. The ribosome is the “workbench” of protein synthesis.
7. The region of the tRNA that is complementary to the triplet on the mRNA is
a. the acceptor stem.
b. the codon.
c. the peptidyl site.
d. the anticodon.
e. the adaptor loop.
Answer: d. The anticodon of the tRNA is complementary to the appropriate codon that codes for the
amino acid attached at the acceptor stem of the tRNA.
8. During the initiation step of translation, the first codon, ____ will enter the _________ and associate
with the initiator tRNA.
a. UAG, A site
b. AUG, A site
c. UAG, P site
d. AUG, P site
e. AUG, E site
Answer: d. AUG is the initiator, or start, codon. It associates with the initiator tRNA and aligns in the P
site of the ribosome.
9. The movement of the polypeptide from the tRNA in the P site to the rTNA in the A site is referred to
as
a. peptide bonding.
b. aminoacyl binding.
c. translation.
d. peptidyl transfer reaction.
e. elongation.
Answer: d.
10. The synthesis of a polypeptide occurs during which stage of translation?
a. initiation
b. elongation
c. termination
d. splicing
Answer: b
Conceptual Questions
1. Define mutation.
A change in the genetic material of an organism.
2. Explain the one gene-one enzyme hypothesis and the more modern modifications of this hypothesis.
Answer: Beadle and Tatum had the insight from their studies that a single gene controlled the synthesis
of a single enzyme. In later years it became apparent that genes coded for all proteins and that some
proteins consisted of more than one polypeptide chain. So the modern statement is one gene for each
polypeptide.
3. What is the function of an aminoacyl-tRNA synthetase?
Answer: Each of these 20 enzymes catalyzes the attachment of a specific amino acid to a specific tRNA
molecule.
Experimental Questions
1. Briefly explain how studying the pathway that leads to arginine synthesis allowed Beadle and Tatum
to conclude that one gene encodes one enzyme.
Answer: Biochemists had already established that particular enzymes are involved in a pathway to
produce arginine. Intermediates in this pathway are ornithine and citrulline. Mutants in single genes
disrupted the ability of cells to catalyze just one reaction in this pathway, thereby suggesting that a
single gene encodes a single enzyme.
2. What was the benefit of using radiolabeled amino acids in the Nirenberg and Leder experiment?
Answer: The researchers were attempting to match codons with appropriate amino acids. By labeling
one amino acid in each of the twenty tubes for each codons, the researchers were able to identify the
correct relationship by detecting which tube resulted in radioactivity on the filter.
3. Predict the results that Nirenberg and Leder would have found for the following triplets: AUG, UAA,
UAG, UGA.
Answer: The AUG triplet would have shown radioactivity in the methionine test tube. Even though AUG
acts as the start codon, it also codes for the amino acid methionine. The other three codons act as stop
codons and do not code for an amino acid. In these cases, the researchers would not have found
radioactivity trapped on filters.
Collaborative Questions
1. Discuss RNA processing in eukaryotes.
Answer: After the process of transcription, pre-mRNA is formed. This pre-mRNA consists of exons that
will be retained in mature mRNA and introns that will be removed through a process called splicing. In
some instances, alternative splicing can occur that will give rise to different proteins during translation.
Also, the pre-mRNA is capped at the 5’end. This cap enables mRNA to bind to a ribosome. Finally a
polyA tail is added to the 3’ end of the mRNA, which provides stability.
2. How can we analyze rRNA to establish evolutionary relationships between different species of
organisms?
Answer: Geneticists often explore evolutionary relationships between different species by comparing the
sequences of evolutionarily related genes. As different species evolve, they accumulate changes that
alter the sequence of those genes. After many generations, these related species contain genes that
are similar but not identical because each species will accumulate different mutations. By looking at the
similarities and differences in genes among different species, we can estimate how closely related they
are to each other in evolutionary terms.