Molecular Biology Final Exam

Approximately how many doublings can normal human fibroblasts undergo in culture?
(A) 5
(B) 50 to 100
(C) They will go on
proliferating indefinitely.
(D) They will not double at all,
but dispersing them
culture allows them to be
studied in isolation from

(B) 50 to 100
After 50 to 100 doublings, the cells will stop growing and die.

How many genes does an E. coli have?
(A) 4,300
(B) 6,000
(C) 14,000
(D) 20,000-25,000

(A) 4,300
4,300 is the number of genes present in the bacteria E. coli.

Which of the following is a high-resolution light microscope technique for detecting interactions between proteins?
(A) FRAP (fluorescence
recovery after
photobleaching)
(B) FRET (fluorescence
resonance energy
transfer)
(C) MPEM (multi-photon
excitation m

(B) FRET (fluorescence
resonance energy transfer)
A FRET signal requires that individual fluorescent dyes carried by two separate proteins be close together in a complex. Energy transfer between fluorescent dyes is efficient only when the two fluorescent

The original atmosphere of Earth is thought to have been rich in
(A) CO2, N2, H2, H2S, and
CO.
(B) CO2, N2, O2, Ar, and Ne.
(C) CO2, N2, O2, Rn, and
Ne.
(D) CO2, N2, H2, Kr, and Xe.

(A) CO2, N2, H2, H2S, and
CO.
These gases are thought to have been the most abundant gases in the original atmosphere of Earth.

An E. coli cell under well-defined laboratory conditions divides about every
(A) 20 minutes.
(B) 2 hours.
(C) 12 hours.
(D) 24 hours.

(A) 20 minutes.
On rich laboratory medium, a typical E. coli cell will divide every 20 minutes.

The feature that most clearly separates eukaryotes from prokaryotes is the presence of _______ in eukaryotic cells.
(A) ribosomes
(B) oxidative phosphorylation
(C) DNA molecules
(D) a nucleus

(D) a nucleus
Prokaryotic cells lack a nucleus.

In fluorescence microscopy, the immediate source of the light detected is light that has been _______ the sample.
(A) absorbed by
(B) emitted by
(C) exciting
(D) scattered from

(B) emitted by
The light detected is emitted by the sample. In fluorescence, the excitation light is absorbed by the fluorescent group (electrons excited to a higher energy level), and light is then emitted as the electrons fall back to their basal energy

Transmission electron microscopy is used to
(A) observe subcellular organelles and macromolecules.
(B) view living cells in three dimensions.
(C) view fluorescently labeled proteins in cells.
(D) study in single, thin sections the shapes of whole cells.

(A) observe subcellular organelles and macromolecules.
With the use of salts and heavy metals to provide contrast, the electron microscope can be used to visualize subcellular structures in some detail.

The genome of eukaryotes consists of genes derived from
(A) archaebacteria alone.
(B) eubacteria alone.
(C) both archaebacteria and eubacteria.
(D) neither archaebacteria nor eubacteria.

(C) both archaebacteria and eubacteria.
Individual eukaryotic genes appear to have originated from either archaebacteria or eubacteria, with basic cellular metabolism genes typically coming from eubacteria. Hence, the eukaryotic genome is a mosaic with re

Microscopy, be it light or electron microscopy, is a major research tool in the molecular approach to the study of the cell. What does this tell us about the level of resolution needed to solve many problems in cell biology?

Microscopy provides a good indication of the scale at which cell biology attempts to answer questions about cell structure and function. These include questions at the subcellular level, cellular level, and tissue level. At the subcellular level, genes or

The original cell was thought to have arisen from the enclosure of self-replicating _______ by a phospholipid membrane.
(A) DNA
(B) carbohydrate
(C) protein
(D) RNA

(D) RNA
RNA has the capacity to encode information and catalytic properties that DNA does not have. Therefore, RNA can be self-replicating.

Mitochondria and chloroplasts are both thought to have become organelles within eukaryotic cells through endocytosis. Mitochondria are thought to have evolved from aerobic eubacteria, while chloroplasts can be readily related to cyanobacteria. What presen

A free-living organism has its own DNA that can be transcribed and translated into proteins within the organism. Both mitochondria and chloroplasts contain their own DNA genome that is transcribed within the organelle into RNA and translated therein into

Embryonic stem cells are different from primary cell cultures or permanent cell lines derived from a tissue in that they are capable of
(A) generating many cell types.
(B) growing and dividing.
(C) synthesizing their own DNA.
(D) transporting nutrients ac

(A) generating many cell types.
Only embryonic stem cells are capable of generating all of the cell types present in the adult organism.

Bacteriophages are
(A) a strain of gram-negative bacteria.
(B) viruses that infect bacteria.
(C) bacterial lysosomes.
(D) bacteria that phagocytose other competing bacteria.

(B) viruses that infect bacteria.
Because they use the host cellular machinery to propagate themselves, they have been used to study bacterial cell biology. For example, bacteriophage T4 was used extensively in early molecular genetic studies.

Which of the following are the most commonly used mammals for genetic studies?
(A) Humans
(B) Xenopus laevis
(C) Mice
(D) Cats

(C) Mice
Although not as easily manipulated genetically as organisms such as C. elegans and D. melanogaster, mice are the most commonly used mammals for genetic studies.

What is the theoretical limit of resolution of a light microscope used to look at a sample through oil?
(A) 0.22 ?m
(B) 0.2 nm
(C) 1 mm
(D) 0.305 ?m

(A) 0.22 ?m
This would be the value if an oil-immersion lens were used. Resolution is calculated from the following formula: Resolution = 0.61?/NA, where ? is the wavelength of light (approximately 0.5 ?m) and NA (numerical aperture) represents the size o

Chloroplasts are thought to have originated from endosymbiosis of _______ by a large host cell.
(A) anaerobic bacteria
(B) photosynthetic eubacteria such as cyanobacteria
(C) fungi such as yeast
(D) nonphotosynthetic aerobic eubacteria

(B) photosynthetic eubacteria such as cyanobacteria
Cyanobacteria, like chloroplasts, can carry out photosynthesis and are photosynthetic eubacteria. Chloroplasts are thought to have originated from them via endosymbiosis.

In contrast to animal cells, yeast or bacterial cell cultures can be grown on fairly simple media without the need to add amino acids or various vitamins and hormones. This is primarily because animal cells
(A) come from organisms that have specialized ce

(A) come from organisms that have specialized cell types.
Because of cell specialization and, as part of this, the existence of a circulatory system, animal cells can be specialized and derive nutrients from diet and vitamins and derive hormones from othe

What is the smallest number of ultracentrifugation steps needed to separate nuclei from ribosomes in a cellular lysate?
(A) 1
(B) 2
(C) 4
(D) 7

(A) 1
A separation between the two can be achieved in one step.

Which of the following is not a unicellular eukaryote?
(A) Saccharomyces cerevisiae
(B) Paramecium
(C) Archaebacteria
(D) Chlamydomonas

(C) Archaebacteria
Archaebacteria are prokaryotes.

Solid tissues in animals include epithelial tissue, _______ tissue, nervous tissue, and muscle.
(A) blood
(B) connective
(C) ground
(D) epidermal

(B) connective
Connective tissue underlies epithelial cell layers and is composed of fibroblasts as the major cell type.

How similar are the nutritional requirements for the culture of HeLa cells to those of a human?

Basal media for the cultivation of HeLa cells, a cell line of human origin, provides the essential amino acids that the cell cannot synthesize: vitamins, salts, glucose, and a range of polypeptide hormones for which serum is the source. With the exception

The source of the atmospheric oxygen necessary for the development of oxidative metabolism is thought to have been
(A) glycolysis.
(B) the formation of Earth.
(C) the breakdown of ATP.
(D) photosynthesis.

(D) photosynthesis.
Photosynthesis harnesses energy from the sun to generate glucose and oxygen; it is thought to have provided the oxygen necessary for the evolution of oxidative metabolism.

Proteins must have more than one _______ to have a quaternary structure.
(A) ? helix
(B) ? sheet
(C) polypeptide chain
(C) transmembrane segment

(C) polypeptide chain
The quaternary structure of a protein is the association of multiple polypeptide chains with one another.

The three-dimensional structure of a protein is analyzed most definitively by
(A) electron microscopy.
(B) light microscopy.
(C) subcellular fractionation.
(D) X-ray crystallography.

(D) X-ray crystallography.
X-ray crystallography is a high-resolution technique that is capable of defining the position of atoms within a protein.

The most abundant molecule in cells is
(A) aspartic acid.
(B) DNA.
(C) sucrose.
(D) water.

(D) water.
Water is indeed the most abundant molecule in cells, accounting for 70% of cellular mass, and considering its small size, an even higher fraction of the total molecules in cells.

How many common amino acids are there?
(A) 5
(B) 10
(C) 20
(D) 25

(C) 20

Passive transport across a membrane refers to
(A) transport into the interior of a cell.
(B) transport out of a cell.
(C) transport in the energetically favorable direction.
(D) simple diffusion across membranes, without the help of proteins such as chann

(C) transport in the energetically favorable direction.
Passive transport refers to the transport of a molecule in the energetically favorable direction, taking into consideration concentration and electrochemical gradients via membrane channels or carrie

Introducing a double bond into a fatty acid has the effect of putting a(n) _______ into the conformation of the molecule.
(A) amino acid bulge
(B) kink
(C) reverse spiral
(D) reinforcing rod

(B) kink
The double bond introduces a kink in the hydrocarbon chain.

Transmembrane proteins are water-insoluble. Why?

Hydrophobic portions of transmembrane proteins are "dissolved" in the lipid bilayer of the membrane. These portions are not water-soluble; hence the whole protein is water-insoluble. To solubilize a transmembrane protein, detergent must be used. The bindi

Suppose you are using a proteomics approach to identify proteins that migrate to specific positions in a two-dimensional gel. You cut an individual protein out of the gel and use trypsin to digest it into peptides. You then determine the charge-to-mass ra

For a given peptide, the charge-to-mass ratio is the sum of the contributions of the individual amino acids composing that peptide. The charge and mass contribution for each amino acid will depend on whether the amino acid is at the N- or C-terminus of th

Protein interaction networks may be characterized by all of the following approaches
(A) gentle isolation followed by mass spectrometry.
(B) shotgun sequencing of random DNA clones.
(C) screening for protein interactions in vitro.
(D) the yeast two-hybrid

(B) shotgun sequencing of random DNA clones.
Gentle isolation of protein complexes followed by mass spectrometry is a standard approach for characterizing protein interactions.

Which of the following is not one of the four major classes of organic molecules in the cell?
(A) Carbohydrates
(B) Lipids
(C) Water
(D) Proteins

(C) Water
Although water is a major cell constituent, accounting for 70% of the cell's mass, it is not an organic molecule. Nucleic acids are the fourth major class of organic molecules in the cell.

Which of the following bases is not found in DNA?
(A) Adenine
(B) Cytosine
(C) Thymine
(D) Uracil

(D) Uracil
Uracil is a pyrimidine base found in RNA, but not in DNA.

In the most complete sense, "proteomics" refers to the large-scale analysis of
(A) cell proteins.
(B) protein localization.
(C) protein interaction networks.
(D) All of the above

(D) All of the above

Transport across biological membranes is speeded by what class of biological molecules?
(A) Carbohydrates
(B) Lipids
(C) Nucleic acids
(D) Proteins

(D) Proteins
Transporters in membranes are proteins of either the channel- or carrier-protein class.

What is the major carbohydrate-storage molecule in plants?
(A) Starch
(B) Cellulose
(C) Glycogen
(D) Deoxyribonucleic acid

(A) Starch
Starch is a polysaccharide composed of glucose residues in the ? configuration. The principal linkage is between carbon 1 of one glucose and carbon 4 of a second glucose.

About 50% of the mass of most biological membranes consists of lipids and about 50% consists of proteins. Therefore,
(A) the membranes contain fewer molecules of lipid than of protein.
(B) the membranes contain equal numbers of lipid and protein molecules

(C) the membranes contain more molecules of lipid than of protein.
Lipids are much lower in molecular weight than membrane proteins are, and hence in terms of molecular abundance there are many more molecules of lipid than of protein.

Cholesterol, a membrane lipid in animals, has a chemical structure similar to
(A) estradiol.
(B) phosphatidylinositol.
(C) thymine.
(D) triacylglycerol.

(A) estradiol.
Cholesterol is a steroid and a precursor to steroid hormones such as estradiol.

Molecules that are partly water-soluble and partly water-insoluble are
(A) hydrophilic.
(B) amphipathic.
(C) hydrophobic.
(D) allosteric.

(B) amphipathic.
An example of an amphipathic molecule is a phospholipid, which has a long hydrocarbon chain (the water-insoluble part) attached to a polar head group (the water-soluble part).

Lipids with unsaturated fatty acids
(A) decrease the fluidity of membranes.
(B) increase the charge associated with the inner face of a membrane.
(C) increase the fluidity of membranes.
(D) are present only on the inner side of the plasma membrane.

(C) increase the fluidity of membranes.
Double bonds produce kinks in fatty-acid chains, causing them to pack irregularly, which increases the fluidity of membranes.

Which of the following classes of amino acids is buried within the folded structure of the protein?
(A) Acidic
(B) Basic
(C) Nonpolar
(D) Polar

(C) Nonpolar
Nonpolar amino acids are hydrophobic and are in the interior of proteins.

In the fluid mosaic model of biological membrane structure, transmembrane proteins are
(A) embedded nearly randomly in the lipid bilayer.
(B) almost completely surrounded by membrane lipid.
(C) segregated into large protein clusters or rafts.
(D) weakly h

(A) embedded nearly randomly in the lipid bilayer.
Typically, only small, ?-helical portions of the protein are embedded in the membrane, and individual proteins are distributed fairly randomly within the membrane bilayer.

What is the effect of a ?-barrel on the permeability of a membrane?
(A) It decreases permeability.
(B) It increases permeability.
(C) It has no effect.
(D) ?-barrels are never found in membranes.

(B) It increases permeability.
A ?-barrel is a protein structural feature that spans a membrane, creating a pore-like opening. The presence of this feature therefore increases the permeability of a membrane.

Whereas small uncharged molecules can diffuse through the hydrophobic core of a phospholipid bilayer, a larger polar substance such as glucose must enter cells by
(A) binding to a non-phospholipid such as cholesterol.
(B) binding to the carbohydrate porti

(D) binding to carrier proteins that facilitate the passage of specific molecules across membranes.
Through binding to a carrier protein, a transporter, glucose is placed in a hydrophilic environment and not exposed to the hydrophobic membrane lipids.

Which of the following is not an analytical instrument approach that can be applied to meet the goals of proteomics?
(A) Atomic absorption analysis of the metal content of plant tissues
(B) High-throughput microscope analysis of the distribution of GFP fu

(A) Atomic absorption analysis of the metal content of plant tissues
Metal analysis is a separate field in molecular cell biology.

Water is a polar molecule and can form hydrogen bonds. How is this property of water an important factor in determining protein structure?

Hydrophilic amino acids tend to be exposed on the surface of proteins, where their hydrophilic side chains can both form hydrogen bonds and have charged interactions with water.

Mass spectrometry is currently the major tool used in proteomics because of its ability to assign very precise
(A) mass-to-charge ratios to peptides, which can then be matched to predicted values for peptides in a complete protein database.
(B) mass-to-ch

(A) mass-to-charge ratios to peptides, which can then be matched to predicted values for peptides in a complete protein database.
The mass-to-charge ratio of individual peptides is compared to predicted values in a complete protein database to identify pr

A disulfide bond is formed between _______ residues.
(A) cysteine
(B) glycine
(C) methionine
(D) tyrosine

(A) cysteine
Each cysteine (Cys, C) has an SH (sulfhydryl) group. Following oxidation, two cysteine residues are then linked to form a disulfide bond.

Phospholipids consist of a 3-carbon core to which fatty acids and a phosphate group are linked. The most common 3-carbon core is
(A) dihydroxyacetone.
(B) glyceraldehyde
(C) glycerol.
(D) serine.

(C) glycerol.
Glycerol has three hydroxyl groups and is the 3-carbon core to which phosphate and two fatty acids are added in the four most common phospholipids.

Why should GFP-tagged proteins rather than chemically tagged proteins be used for global analysis of protein localization?

Because the fluorescence of GFP is inherent to its amino acid sequence, recombinant DNA techniques can be used to place a GFP sequence at either the N- or C-terminus of any protein. Large-scale (i.e., high-throughput) screening for the localization of rec

The ? (alpha) helix is an example of which level of protein structure?
(A) Primary
(B) Quaternary
(C) Secondary
(D) Tertiary

(C) Secondary
Secondary structure refers to the arrangement of amino acids within a localized region, and an ? helix is a coiled structure of often relatively limited length.

Which of the following techniques is used to inactivate a gene by altering the DNA that encodes it?
(A) Homologous recombination
(B) Antisense nucleic acid blocks
(C) Antibody microinjection
(D) Introduction of dominant inhibitory mutants

(A) Homologous recombination
Mutant versions of a gene can be made to replace the endogenous, wild-type copy by homologous recombination.

The two strands of DNA in the double helix are held together by
(A) interactions between the sugar residues of each chain.
(B) interactions between phosphate residues of each strand.
(C) hydrogen bonds between the bases of each strand.
(D) covalent bonds

(C) hydrogen bonds between the bases of each strand.
Hydrogen bonds between adenines and thymines and between cytosines and guanines of opposite strands hold the double helix together.

Which of the following serves as an adaptor between amino acids and mRNA during translation?
(A) rRNA
(B) DNA
(C) tRNA
(D) siRNA

(C) tRNA
Transfer RNAs (tRNAs) serve as adaptor molecules that align amino acids along the mRNA template. Each amino acid is attached by a specific enzyme to its appropriate tRNA, which contains a recognition sequence that base pairs to a complementary se

Which of the following correctly represents the distinction between the terms "genotype" and "phenotype"?
(A) Phenotype is the genetic composition of an organism, while genotype is the physical appearance.
(B) There is no difference between a genotype and

(D) The genotype is the genetic composition of an organism, whereas the phenotype is its physical appearance.
For example, a gene specifying seed color can be denoted (Y) for yellow or (y) for green. Yellow (Y) is said to be dominant, and green (y) is sai

The process by which proteins are made from RNA templates is called
(A) transcription.
(B) translation.
(C) transposition.
(D) interpretation.

(B) translation.
Translation is the process by which the RNA sequence (made up of nucleotides) is translated into the protein sequence (made of up amino acids).

A restriction enzyme with a four-base recognition site would cleave DNA with a statistical frequency of once every
(A) 4 base pairs.
(B) 256 base pairs.
(C) 4096 base pairs.
(D) 65.5 kilobase.

(B) 256 base pairs.
Given that the genetic code makes use of four bases, the sequence would randomly occur every 4 � 4 � 4 � 4 base pairs (every 256 base pairs).

Which of the following techniques can be used to identify proteins that interact with one another?
(A) Polymerase chain reaction
(B) Yeast two-hybrid system
(C) Southern blotting
(D) Northern blotting

(B) Yeast two-hybrid system
cDNAs derived from the mRNAs of two proteins that are thought to interact are joined to two distinct domains of a protein that stimulates expression of a target gene in yeast. These hybrid cDNA molecules are then transformed in

All of the following are examples of cloning vectors except
(A) plasmids.
(B) cosmids.
(C) yeast artificial chromosomes (YACs).
(D) bacteriophages.

(D) bacteriophages.
Bacteriophage is a virus that infects E. coli. However, some of the DNA sequences isolated from bacteriophage are used in cloning vectors.

Which of the following is not necessary in a plasmid vector to facilitate successful cloning?
(A) Origin of replication
(B) Gene-conferring resistance to antibiotic
(C) Restriction site
(D) IRES (internal ribosomal entry site)

(D) IRES (internal ribosomal entry site)
Antibiotic resistance genes are required and included as a means of selecting bacteria carrying the plasmid. Selection is accomplished by plating bacteria on the antibiotic to which resistance is conferred. Only ba

The characterization of restriction endonucleases was a key step in the development of recombinant DNA technology. What is the function of these enzymes?
(A) Synthesis of complementary strands of DNA from a DNA template
(B) Synthesis of RNA from a DNA tem

(D) Cleavage of DNA at specific sequences
These enzymes, identified in bacteria, cleave DNA at distinct recognition sites four to eight base pairs in length. They can be used to cleave a DNA molecule at unique sites.

Which of the following correctly describes a cDNA molecule?
(A) A DNA copy of an RNA sequence synthesized by the enzyme reverse transcriptase
(B) The nucleic acid portion of a ribosome
(C) A DNA molecule capable of independent replication in a host cell
(

(A) A DNA copy of an RNA sequence synthesized by the enzyme reverse transcriptase
Reverse transcriptases catalyze the synthesis of DNA from an RNA template. These enzymes can be used experimentally to synthesize a DNA copy of an RNA sequence. The resultin

The enzyme that synthesizes DNA using RNA templates is called
(A) DNA polymerase.
(B) RNA polymerase.
(C) ribozymes.
(D) reverse transcriptase.

(D) reverse transcriptase.
Reverse transcriptase is found in RNA tumor viruses and catalyzes the synthesis of a DNA copy of the RNA genome. The DNA copy can then be stably integrated into the host genome, an event that can initiate the development of canc

In experiments using polymerase chain reactions (PCR), it is often more difficult to amplify thorough regions of DNA that are high in GC content versus those regions that are either lower in GC content or are AT-rich. Based on your knowledge of DNA struct

In base pairing between strands of DNA, there are three hydrogen bonds between each G-C base pair and only two hydrogen bonds between A-T base pairs. In PCR, heat is used to melt the hydrogen bonds between strands of DNA. The energy required to melt regio

Transgenic plants are easier to produce than transgenic animals because
(A) plants can be grown more easily from single cultured cells into which recombinant DNA has been introduced.
(B) plant DNA can be cloned more easily.
(C) plant cells can be transfor

(A) plants can be grown more easily from single cultured cells into which recombinant DNA has been introduced.
Plant cells exhibit the ability to completely regenerate a new plant from single cells much more frequently than animal cells do, so it is relat

Transgenic mice carry a foreign gene
(A) only in their germ line cells.
(B) only in their somatic cells.
(C) in some cells (both germ line and somatic) but not in others�transgenic mice are chimeras.
(D) in all of their cells.

(D) in all of their cells.
When a transgenic mouse is made, a foreign gene is introduced into a fertilized egg, which then goes on to develop into an entire mouse, and thus all cells in the mouse carry the gene.

Which of the following correctly outlines the process of RNA interference, beginning with a double-stranded RNA molecule?
(A) Unwinding of siRNA; cleavage by Dicer; association with RISC; pairing with target mRNA; mRNA cleavage
(B) Reverse transcription o

(C) Cleavage by Dicer; association with RISC; unwinding of siRNA; pairing with target mRNA; mRNA cleavage
Double-stranded RNAs are cleaved into short interfering RNAs by the enzyme Dicer. The siRNAs associate with the RNA-induced silencing complex (RISC)

The SDS in SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of a protein is used to
(A) neutralize the charge of a protein so that charge is not a factor in its rate of migration.
(B) denature the protein and give it an overall negative charge.
(C) cross

(B) denature the protein and give it an overall negative charge.
This eliminates the differences in shape between proteins as factors in their migration rates so that they migrate according to size.

The distance between linked genes can be estimated by the frequency of
(A) recombination.
(B) mutation.
(C) meiosis.
(D) transformation.

(A) recombination.
Recombination between two genes is less likely for genes that are close together than for those that are far apart.

Which of the following terms refers to a collection of clones that contain all the genomic or mRNA sequences of a particular cell type?
(A) DNA microarray
(B) Recombinant DNA library
(C) Expression vector
(D) Yeast two-hybrid system

(B) Recombinant DNA library
DNA libraries consist of a collection of clones that carry recombinant vectors containing all the genomic or mRNA sequences of a particular cell type.

EcoRI and NotI are two DNA restriction enzymes that recognize the specific sequences GAATTC and GCGGCCGC, respectively. Which of these restriction enzymes would cleave DNA most frequently and why?

EcoRI. The specific sequences recognized by EcoRI would be expected to occur once in every 46 base pairs, based on four possible nucleotides in a specific sequence of six. NotI would occur less frequently�only once in 48 base pairs, based on four possible

How many molecules would be produced from two molecules of DNA following five rounds of PCR amplification?
(A) 10
(B) 32
(C) 64
(D) One billion

(C) 64
Each starting molecule of DNA would give rise to 32 (i.e., 25) progeny molecules after five rounds of PCR.

What technique would you use to determine if the level of a specific mRNA had been increased in response to an inducer?
(A) Northern blotting
(B) Southern blotting
(C) Western blotting
(D) Immunoprecipitation

(A) Northern blotting
Northern blotting uses a radioactive probe to detect cellular RNAs separated out by size on a blot. The amount of radioactivity reflects the amount of RNA in the sample and so can be used to determine the effect of the inducer.