The "universal" genetic code is now known to have exceptions. Evidence for this can be found if which of the following is true?
If UGA, usually a stop codon, is found to code for an amino acid such as tryptophan (usually coded for by UGG only).
Which of the following nucleotide triplets best represents a codon?
a triplet in the same reading frame as an upstream AUG
Which of the following provides some evidence that RNA probably evolved before DNA?
DNA polymerase uses primer, usually made of RNA.
Which of the following statements best describes the termination of transcription in prokaryotes?
RNA polymerase transcribes through the polyadenylation signal, causing proteins to associate with the transcript and cut it free from the polymerase.
Which of the following does not occur in prokaryotic eukaryotic gene expression, but does in eukaryotic gene expression?
A poly-A tail is added to the 3' end of an mRNA and a cap is added to the 5' end.
RNA polymerase in a prokaryote is composed of several subunits. Most of these subunits are the same for the transcription of any gene, but one, known as sigma, varies considerably. Which of the following is the most probable advantage for the organism of such sigma switching?
It might allow the polymerase to recognize different promoters under certain
Which of the following is a function of a poly-A signal sequence?
It codes for a sequence in eukaryotic transcripts that signals enzymatic cleavage ~1035 nucleotides away.
In eukaryotes there are several different types of RNA polymerase. Which type is involved in transcription of mRNA for a globin protein?
RNA polymerase II
Transcription in eukaryotes requires which of the following in addition to RNA polymerase?
several transcription factors (TFs)
A part of the promoter, called the TATA box, is said to be highly conserved in evolution. Which of the following might this illustrate?
Any mutation in the sequence is selected against
The TATA sequence is found only several nucleotides away from the start site of transcription. This most probably relates to which of the following?
the number of hydrogen bonds between A and T in DNA
What is a ribozyme?
an RNA with enzymatic activity
A transcription unit that is 8,000 nucleotides long may use 1,200 nucleotides to make a protein consisting of approximately 400 amino acids. This is best explained by the fact that
many noncoding stretches of nucleotides are present in mRNA.
During splicing, which molecular component of the spliceosome catalyzes the excision reaction?
Alternative RNA splicing
can allow the production of proteins of different sizes from a single mRNA.
In the structural organization of many eukaryotic genes, individual exons may be related to which of the following?
the various domains of the polypeptide product
In an experimental situation, a student researcher inserts an mRNA molecule into a eukaryotic cell after he has removed its 5' cap and poly-A tail. Which of the following would you expect him to find?
The molecule is digested by exonucleases since it is no longer protected at the 5' end.
5' UTR E1 I1 E2 I2 E3 I3 E4 UTR 3'
Which components of the previous molecule will also be found in mRNA in the cytosol?
5' I1 I2 I3 3'
When the spliceosome binds to elements of this structure, where can it attach?
to the end of an intron
Which of the following is a useful feature of introns for this model?
Introns allow exon shuffling.
Suppose that exposure to a chemical mutagen results in a change in the sequence that alters the 5' end of intron 1 (I1). What might occur?
inclusion of I1 in the mRNA
A particular triplet of bases in the coding sequence of DNA is AAA. The anticodon on the tRNA that binds the mRNA codon is
A mutant bacterial cell has a defective aminoacyl synthetase that attaches a lysine to tRNAs with the anticodon AAA instead of the normal phenylalanine. The consequence of this for the cell will be that
proteins in the cell will include lysine instead of phenylalanine at amino acid positions specified by the codon UUU.
There are 61 mRNA codons that specify an amino acid, but only 45 tRNAs. This is best explained by the fact that
the rules for base pairing between the third base of a codon and tRNA are flexible.
Which of the following is the first event to take place in translation in eukaryotes?
the small subunit of the ribosome recognizes and attaches to the 5' cap of mRNA
Which of the following is a function of a signal peptide?
to signal the initiation of transcription
When translating secretory or membrane proteins, ribosomes are directed to the ER membrane by
a signal-recognition particle that brings ribosomes to a receptor protein in the ER membrane.
What is the function of the release factor (RF)?
It binds to the stop codon in the A site in place of a tRNA
When the function of the newly made polypeptide is to be secreted from the cell where it has been made, what must occur?
Its signal sequence must target it to the ER, from which it goes to the Golgi
Suppose that a mutation alters the formation of a tRNA such that it still attaches to the same amino acid (phe) but its anticodon loop has the sequence AAU that binds to the mRNA codon UUA (that usually specifies leucine leu).
One mutated tRNA molecule will be relatively inconsequential because it will compete with many "normal" ones.
Why might a point mutation in DNA make a difference in the level of protein's activity?
It might substitute an amino acid in the active site.
Which of the following types of mutation, resulting in an error in the mRNA just after the AUG start of translation, is likely to have the most serious effect on the
a deletion of two nucleotides
What is the effect of a nonsense mutation in a gene?
It introduces a premature stop codon into the mRNA.
Which small-scale mutation would be most likely to have a catastrophic effect on the functioning of a protein?
a base deletion near the start of a gene
The most commonly occurring mutation in people with cystic fibrosis is a deletion of a single codon. This results in
a polypeptide missing an amino acid.
Which of the following mutations is most likely to cause a phenotypic change?
a single nucleotide deletion in an exon coding for an active site
In comparing DNA replication with RNA transcription in the same cell, which of the following is true only of replication?
The entire template molecule is represented in the product.
According to Beadle and Tatum's hypothesis, how many genes are necessary for this pathway?
A mutation results in a defective enzyme A. Which of the following would be a consequence of that mutation?
an accumulation of A and no production of B and C
A possible sequence of nucleotides in the template strand of DNA that would code for the polypeptide sequence phe-leu-ile-val would be
3' AAA-GAA-TAA-CAA 5'.
What amino acid sequence will be generated, based on the following mRNA codon sequence?
A peptide has the sequence NH2-phe-pro-lys-gly-phe-pro-COOH. Which of the following sequences in the coding strand of the DNA could code for this peptide?
The dipeptide that will form will be
Assume that you are trying to insert a gene into a plasmid. Someone gives you a preparation of genomic DNA that has been cut with restriction enzyme X. The gene you wish to insert has sites on both ends for cutting by restriction enzyme Y. You have a plasmid with a single site for Y, but not for X. Your strategy should be to
cut the DNA again with restriction enzyme Y and insert these fragments into the plasmid cut with the same enzyme.
How does a bacterial cell protect its own DNA from restriction enzymes?
by adding methyl groups to adenines and cytosines
What is the most logical sequence of steps for splicing foreign DNA into a plasmid and inserting the plasmid into a bacterium?
III, II, IV, V, I
A gene that contains introns can be made shorter (but remain functional) for genetic engineering purposes by using
reverse transcriptase to reconstruct the gene from its mRNA
Which of the following best describes the complete sequence of steps occurring during every cycle of PCR?
- The primers hybridize to the target DNA.
- The mixture is heated to a high temperature to denature the double-stranded target DNA.
- Fresh DNA polymerase is added.
- DNA polymerase extends the primers to make a copy of the target DNA.
2, 1, 4
Which of the following is used to make complementary DNA (cDNA) from RNA?
The reason for using Taq polymerase for PCR is that
it is heat stable and can withstand the temperature changes of the cycler
As genetic technology makes testing for a wide variety of genotypes possible, which of the following is likely to be an increasingly troublesome issue?
the need to legislate for the protection of the privacy of genetic information
Which enzyme was used to produce the molecule in Figure 20.1?
a restriction enzyme
Which of the following is a protein produced by a regulatory gene?
A mutation that inactivates the regulatory gene of a repressible operon in an E. coli cell would result in
continuous transcription of the structural gene controlled by that regulator.
The lactose operon is likely to be transcribed when
the cyclic AMP and lactose levels are both high within the cell.
Transcription of the structural genes in an inducible operon
starts when the pathway's substrate is present.
If you were to observe the activity of methylated DNA, you would expect it to
have turned off or slowed down the process of transcription
Genomic imprinting, DNA methylation, and histone acetylation are all examples of
When DNA is compacted by histones into 10-nm and 30-nm fibers, the DNA is unable to interact with proteins required for gene expression. Therefore, to allow for these proteins to act, the chromatin must constantly alter its structure. Which processes contribute to this dynamic activity?
methylation and phosphorylation of histone tails
Two potential devices that eukaryotic cells use to regulate transcription are
DNA methylation and histone modification
During DNA replication,
methylation of the DNA is maintained because methylation enzymes act at DNA sites where one strand is already methylated and thus correctly methylates daughter strands after replication.
Steroid hormones produce their effects in cells by
binding to intracellular receptors and promoting transcription of specific genes.
Transcription factors in eukaryotes usually have DNA binding domains as well as other domains that are also specific for binding. In general, which of the following would you expect many of them to be able to bind?
other transcription factors
Which of the following experimental procedures is most likely to hasten mRNA degradation in a eukaryotic cell?
removal of the 5' cap
Which of the following is most likely to have a small protein called ubiquitin attached to it?
a cyclin that usually acts in G1, now that the cell is in G2
The phenomenon in which RNA molecules in a cell are destroyed if they have a sequence complementary to an introduced double-stranded RNA is called
At the beginning of this century there was a general announcement regarding the sequencing of the human genome and the genomes of many other multicellular eukaryotes. There was surprise expressed by many that the number of protein-coding sequences was much smaller than they had expected. Which of the following could account for most of the rest?
non-protein-coding DNA that is transcribed into several kinds of small RNAs with biological function
Which of the following is characteristic of the product of the p53 gene?
It is an activator for other genes.
Which of the following best describes this phenomenon?
inherited inability to repair UV-induced mutation
One of the human leukemias, called CML (chronic myelogenous leukemia), is associated with a chromosomal translocation between chromosomes 9 and 22 in somatic cells of bone marrow. Which of the following allows CML to provide further evidence of this multistep nature of cancer?
The translocation requires breaks in both chromosomes 9 and 22, followed by fusion between the reciprocal pieces.