RepetoDoes one codon normally specify several different amino acids within the standard genetic code?
No; each sense codon specifies one amino acid.
Cell biology essentials
450 questions in English on membranes, metabolism, signaling, cell division and gene expression.
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How can a cell-surface signal change gene expression?
A signaling pathway can alter transcription factor activity or location.
What enzyme class executes much of the protein cleavage in animal apoptosis?
Caspases, which are proteases.
What does a growth factor generally signal to a responsive cell?
To grow, survive or progress toward division, depending on the receptor and context.
What are the usual products when ATP is hydrolyzed to ADP?
ADP and inorganic phosphate.
What bases pair with adenine and guanine in standard double-stranded DNA?
Adenine pairs with thymine; guanine pairs with cytosine.
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Every card in the deck, in study order.
Question Answer
Does one codon normally specify several different amino acids within the standard genetic code? No; each sense codon specifies one amino acid.
How can a cell-surface signal change gene expression? A signaling pathway can alter transcription factor activity or location.
What enzyme class executes much of the protein cleavage in animal apoptosis? Caspases, which are proteases.
What does a growth factor generally signal to a responsive cell? To grow, survive or progress toward division, depending on the receptor and context.
What are the usual products when ATP is hydrolyzed to ADP? ADP and inorganic phosphate.
What bases pair with adenine and guanine in standard double-stranded DNA? Adenine pairs with thymine; guanine pairs with cytosine.
What happens to introns during standard pre-mRNA splicing? They are removed from the transcript.
In which direction does RNA polymerase synthesize RNA? 5′ to 3′.
Which side of the Golgi sorts many products for their onward destinations? The trans side, including the trans-Golgi network.
Are chromosomes usually still made of two chromatids after normal meiosis I? Yes; homologs have separated, but sister chromatids remain joined.
Which part of an amino acid chiefly distinguishes it from other amino acids? Its side chain, or R group.
What problem can the spindle checkpoint delay anaphase to prevent? Separation before chromosomes are properly attached to the spindle.
From which ribosomal site does a deacylated tRNA usually leave? The E site.
During which stage does meiotic crossing over occur? Prophase I.
Why is a membrane phospholipid called amphipathic? It has a hydrophilic head region and hydrophobic tail regions.
What does self-renewal mean for a stem cell population? Cell division can produce cells that retain stem cell properties.
What is synapsis in meiotic prophase I? The close pairing of homologous chromosomes.
What is a kinetochore's role during mitosis? It connects a chromosome to spindle microtubules and participates in segregation control.
What molecule binds CAP to favor activation of lac operon transcription? cAMP.
How does the 5′ cap help a eukaryotic mRNA function? It protects the RNA and helps with export and translation initiation.
What happens to a molecule when it is oxidized in a redox reaction? It loses electrons.
How can an external signal initiate the extrinsic apoptotic pathway? It activates a cell-surface death receptor that recruits caspase-activating machinery.
Why is primase needed to start ordinary DNA replication? It makes an RNA primer that provides a free 3′ hydroxyl for DNA polymerase.
What does a transcriptional activator generally do? It increases transcription of its target genes.
What is a polyribosome or polysome? Several ribosomes translating the same mRNA simultaneously.
In a simple dehydration reaction joining two monomers, what small molecule is released? Water.
What does diploid mean? Having two homologous sets of chromosomes.
What does NADH commonly carry between metabolic reactions? High-energy electrons that can be transferred to other molecules.
Why can raising temperature eventually reduce enzyme activity rather than increase it? High temperatures can disrupt the enzyme's folded structure.
What does hydrophilic mean for a molecule or molecular region? It interacts favorably with water, often because it is polar or charged.
How can ATP hydrolysis drive an otherwise unfavorable cellular reaction? The reactions are coupled so their combined free-energy change is favorable.
Which DNA strand does RNA polymerase use to specify a new RNA sequence? The template strand.
What reaction does a protein phosphatase catalyze? Removal of phosphate groups from proteins by hydrolysis.
What do integrins link across the animal cell membrane? Extracellular matrix components to intracellular structures and signaling machinery.
Why can a single DNA base substitution leave a protein's amino acid sequence unchanged? The altered codon can still specify the same amino acid.
What is a tissue in a multicellular organism? An organized group of cells working together in a shared function.
What bond type connects lipid chains to glycerol in typical archaeal membrane lipids? Ether bonds.
What reaction does a protein kinase catalyze? Transfer of a phosphate group, usually from ATP, to a protein.
How many cells make up a unicellular organism? One cell carries out the organism's life functions.
For each ATP used, how many sodium and potassium ions does the typical animal sodium–potassium pump move? Three sodium ions out and two potassium ions in.
What happens to a typical animal cell placed in a hypertonic solution? It loses water and shrinks.
How does binding an inducer to the lac repressor promote transcription? It reduces the repressor's ability to bind the operator.
Why does diacylglycerol remain associated with the membrane after it is generated? It is hydrophobic and retains lipid tails.
What reduced electron carriers are produced per turn of the citric acid cycle? Three NADH and one FADH2.
Why can bacterial ribosomes begin translation before transcription of an mRNA finishes? No nuclear envelope separates the two processes.
Which protein is the principal subunit of microfilaments? Actin.
Why can a low surface-area-to-volume ratio constrain a cell's size? Less membrane area is available for exchange per unit of cytoplasm.
What directly powers secondary active transport of a solute against its gradient? Downhill movement of another solute along its electrochemical gradient.
Do photosynthetic plant cells also carry out mitochondrial respiration? Yes; photosynthesis and mitochondrial respiration serve different metabolic roles.
Which motor protein interacts with actin to generate contraction? Myosin.
Under the same ordinary agarose-gel conditions, which DNA fragments usually travel farther: smaller or larger ones? Smaller fragments.
Can eukaryotic gene expression be regulated after transcription? Yes; RNA processing, RNA stability, translation and protein turnover can all be regulated.
How can changing cyclin abundance help time cell-cycle transitions? It changes the activity of cyclin–CDK complexes.
How can a receptor that signals without its normal ligand contribute to cancer? It can send persistent growth-promoting signals even when an external stimulus is absent.
What does a proteasome do to its protein substrates? It unfolds and degrades them into smaller peptides.
What chemical group does an activated receptor tyrosine kinase add to target tyrosines? A phosphate group.
Which two domains contain prokaryotic organisms? Bacteria and Archaea.
What is the general role of anabolic pathways? They build complex molecules, usually requiring energy input.
What happens to chromosome compaction as a cell enters prophase? Chromosomes condense and become more visibly distinct.
During activation, what nucleotide replaces GDP on a heterotrimeric G protein's alpha subunit? GTP.
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How does fusion of two haploid gametes affect chromosome-set number? It restores the diploid state.
Where do phospholipid hydrophobic tails face in a membrane bilayer? Toward one another in the bilayer interior.
What energy source drives oxygenic photosynthesis? Light.
What is a promoter? A DNA region where transcription machinery assembles to initiate transcription.
In a symporter, how do the coupled solutes move relative to one another? They move in the same direction across the membrane.
What does fermentation regenerate so that glycolysis can continue? NAD+ from NADH.
What is a proto-oncogene in normal cells? A gene whose regulated product helps promote growth or cell-cycle progression.
What is added to the 3′ end of most eukaryotic nuclear mRNAs after cleavage? A poly-A tail.
What does an ER translocon provide for a growing secretory protein? A passage through the ER membrane.
Compared with heterochromatin, how is euchromatin generally organized? It is less compact and more accessible to transcription machinery.
What is a microbial biofilm? A microbial community embedded in an extracellular matrix, often attached to a surface.
What becomes of pyruvate in lactate fermentation? It is reduced to lactate while NADH is oxidized to NAD+.
What can an appropriate polyubiquitin chain mark a cytosolic protein for? Degradation by the proteasome.
What is a centrosome's main organizing role in many animal cells? It acts as a major microtubule-organizing center.
How can p53 respond to DNA damage in a cell that retains this pathway? It can promote cell-cycle arrest, repair responses or apoptosis, depending on the damage and context.
Is there normally an S phase between meiosis I and meiosis II? No; chromosomes are not replicated again between the divisions.
Which plant compartment performs many digestive functions carried out by animal lysosomes? The lytic vacuole.
How can cholesterol help an animal membrane avoid becoming too rigid at low temperatures? It interferes with tight packing of phospholipid tails.
How do desmosomes help mechanically stressed animal tissues? They anchor neighboring cells together and distribute tensile forces.
What attraction forms a hydrogen bond between two water molecules? A partially positive hydrogen on one molecule is attracted to the partially negative oxygen on another.
What distinguishes a stem cell from a fully differentiated cell in its developmental potential? A stem cell can generate more specialized cell types within its permitted range.
What is the operator in a regulated bacterial operon? A DNA site to which a regulatory protein such as a repressor binds.
Which potentially damaging molecule is produced by many peroxisomal oxidation reactions? Hydrogen peroxide.
What inorganic molecule supplies carbon for the Calvin cycle? Carbon dioxide.
What drives net simple diffusion of an uncharged solute across a permeable membrane? Its concentration gradient, from higher toward lower concentration.
How does a cis double bond change the shape of a fatty acid chain? It introduces a bend that hinders tight packing.
During open mitosis in typical animal cells, what happens to the nuclear envelope in prometaphase? It breaks down so spindle microtubules can access chromosomes.
Does DNA replication alone turn a diploid cell into a tetraploid cell? No; DNA amount doubles, but the number of homologous chromosome sets remains two.
What does microscope magnification describe? How much larger the image appears than the object.
What are sister chromatids before their separation in mitosis? The two replicated copies of a chromosome joined by cohesion.
How does the charge of many histone regions favor DNA binding? Their positive charge attracts negatively charged DNA.
Why include a negative control in a cell biology experiment? To reveal background effects or signals that occur without the tested factor.
What is the cytosol of a eukaryotic cell? The aqueous portion of the cytoplasm outside membrane-bound organelles.
Why should treatment and control groups differ only in the variable being tested when feasible? It reduces alternative explanations for an observed difference.
How does cholesterol restrain membrane fluidity at higher temperatures? It limits some phospholipid movement.
Must every cell in a green plant contain chloroplasts? No; many nonphotosynthetic cells, such as many root cells, lack chloroplasts.
Which interactions stabilize alpha helices and beta sheets in a polypeptide backbone? Hydrogen bonds between backbone groups.
What does induced fit describe in enzyme action? Substrate binding changes the enzyme's shape to favor catalysis.
What happens to a plant cell's plasma membrane during plasmolysis? It pulls away from the cell wall as the cell loses water.
Why can the same ligand produce different responses in different cell types? The cells can express different receptors, signaling components or target proteins.
How do mitochondrial cristae support oxidative phosphorylation? They increase inner membrane area for respiratory complexes and ATP synthase.
What directs many large nuclear proteins to nuclear import receptors? A nuclear localization signal in the protein.
Which cell-wall polymer is characteristic of bacteria but absent from archaeal walls? Peptidoglycan.
What is an enzyme's active site? The region that binds substrate and carries out catalysis.
What is the small ribosomal subunit's central role in translation? Positioning and decoding the mRNA with tRNAs.
What ATP-equivalent yield is made directly per turn of the citric acid cycle? One GTP or ATP, depending on the enzyme form and cell.
What two components make up the proton-motive force across the inner mitochondrial membrane? A proton concentration difference and an electrical potential difference.
What does each daughter DNA duplex contain after semiconservative replication? One parental strand and one newly synthesized strand.
When a secretory vesicle fuses with the plasma membrane, which side becomes exposed outside the cell? The side that faced the vesicle lumen.
What is a protein's primary structure? Its amino acid sequence.
Which face of the Golgi receives material arriving from the ER? The cis face.
In an antiporter, how do the coupled solutes move relative to one another? They move in opposite directions across the membrane.
Why is correct tRNA charging essential for translation accuracy? The ribosome reads the anticodon, so a wrongly attached amino acid can be incorporated.
What is a bacterial plasmid? A DNA molecule that can replicate separately from the main chromosome.
What happens to a vesicle membrane during exocytosis? It fuses with the plasma membrane, releasing lumenal cargo outside.
What two main components make up chromatin? DNA and associated proteins.
How far do paracrine signals usually act? Locally on nearby cells.
What is shared between atoms in a covalent bond? One or more pairs of electrons.
How does a labeled nucleic acid probe recognize a target sequence? By complementary base pairing.
Why must cells terminate signaling pathways after a stimulus ends? To limit the response's duration and maintain sensitivity to later signals.
In an illuminated chloroplast, where do protons accumulate relative to the thylakoid membrane? Inside the thylakoid lumen.
Why can a plant cell become turgid rather than burst in a hypotonic environment? Its cell wall resists expansion and allows pressure to build.
What three components make up a typical nucleotide? A nitrogenous base, a pentose sugar and one or more phosphate groups.
Where does an allosteric regulator bind relative to an enzyme's active site? At a separate regulatory site that can alter enzyme activity.
Which filament system is especially prominent just beneath the plasma membrane in the cell cortex? Actin filaments.
In the standard E. coli replication model, which polymerase removes RNA primers and replaces them with DNA? DNA polymerase I.
Why is a standard PCR mixture heated during each denaturation step? To separate the DNA template strands.
Which cytoskeletal polymers form the main structural elements of the mitotic spindle? Microtubules.
How can chromatin remodeling alter gene expression? It changes access of transcription machinery to DNA.
How many carbon dioxide molecules are released per complete turn of the citric acid cycle? Two.
What is an operon in the usual bacterial model? A group of genes controlled together and transcribed from a shared promoter.
What is the smallest unit that can carry out the basic functions of life? A cell.
What ion can the endoplasmic reticulum store for intracellular signaling? Calcium ions.
How do motile eukaryotic cilia generally differ from flagella in length and number? Cilia are usually shorter and more numerous; flagella are usually longer and fewer.
Does phosphorylation always activate a target protein? No; it can activate, inhibit or otherwise alter the target depending on the protein and site.
What happens to pH when hydrogen ion concentration increases? pH decreases.
What does microscope resolution describe? The ability to distinguish nearby objects as separate.
What does fluid mean in the fluid mosaic model of a membrane? Many lipids and proteins can move laterally within the membrane.
What are alleles? Alternative versions of a gene at the same locus.
Are free and ER-bound cytosolic ribosomes permanently different kinds of ribosome? No; their location depends on the protein being synthesized.
Where does pyruvate oxidation occur in a typical animal cell? In the mitochondrial matrix.
Which filament and motor form the contractile ring in typical animal cytokinesis? Actin filaments and myosin.
Where is a plant cell wall relative to the plasma membrane? Outside the plasma membrane.
Can binding to an extracellular matrix alter an animal cell's behavior? Yes; membrane receptors can relay the binding into intracellular signals.
Why can loss of cell-cycle checkpoint control threaten genome stability? Cells may divide with damaged DNA or segregate chromosomes incorrectly.
What separates during mitotic anaphase? Sister chromatids separate and move toward opposite poles.
Where do many newly made secretory proteins begin folding and forming disulfide bonds? In the ER lumen.
How does facilitated diffusion differ from simple diffusion through the bilayer? It uses a membrane transport protein while still moving down the solute's gradient.
What structure forms between daughter nuclei during cytokinesis in higher plant cells? A cell plate that develops into the new separating wall and membranes.
Which solutes chiefly determine a solution's tonicity relative to a cell? Solutes that do not readily cross that cell's membrane.
If an intron is retained in an mRNA, must the resulting protein be unchanged? No; retention can alter the coding sequence, introduce a stop signal or otherwise affect expression.
According to cell theory, where do new cells come from? Pre-existing cells through cell division.
How can a water-soluble signal affect the cell without entering it? It binds a surface receptor that transduces the signal across the membrane.
What does light absorption at photosystem I do to electrons arriving from the transport chain? It raises their energy again for transfer toward NADP+ reduction.
Why is bacterial horizontal gene transfer different from ordinary inheritance during cell division? DNA can move between existing cells rather than only from parent to daughter.
What is the main role of antenna pigments in a photosystem? They collect light energy and transfer excitation toward the reaction center.
What internal component supplies the template for telomerase to extend a chromosome end? An RNA template carried by telomerase.
Why does prompt engulfment of apoptotic fragments limit damage to nearby cells? It prevents widespread release of intracellular contents into the tissue.
While synthesizing DNA 5′ to 3′, in which direction does a polymerase read its template? 3′ to 5′.
What does IP3 commonly trigger by binding receptors on the ER? Release of calcium ions into the cytosol.
How many nucleotides make up a standard mRNA codon? Three.
What happens to exons during standard pre-mRNA splicing? They are joined together in the mature RNA.
Why are respiration pathways connected to biosynthetic pathways? Their intermediates can supply building blocks for molecules such as amino acids and lipids.
What occurs during G1 before DNA replication? Cell growth and preparation for DNA synthesis.
Where do most citric acid cycle reactions occur in a typical animal cell? In the mitochondrial matrix.
How can gap junctions transmit signals directly between animal cells? They let ions and small signaling molecules pass between neighboring cytoplasms.
How do microtubules contribute to eukaryotic ciliary beating? Motor-driven sliding between adjacent doublets is converted into bending.
What is the extracellular matrix of an animal tissue? A network of secreted proteins and carbohydrates outside the cells.
What does polymerase chain reaction amplify in its standard form? A selected DNA region bounded by primer-binding sites.
Why must most of the Calvin cycle's carbon intermediates remain in the cycle? They regenerate RuBP so further carbon fixation can occur.
What bond joins successive amino acids in a polypeptide? A peptide bond.
What fuel do cytoskeletal motor proteins typically hydrolyze to generate movement? ATP.
What is bacterial transformation as a route of genetic exchange? Uptake of DNA from the environment.
Why does a hormone usually affect only certain cells even when many cells encounter it? Responsive cells have the appropriate receptor and signaling machinery.
In which direction does the ribosome move along an mRNA? 5′ to 3′.
What is a major role of molecular chaperones in protein maturation? They help proteins fold and limit inappropriate aggregation.
Why can membrane-bound compartments help a eukaryotic cell run different reactions? They maintain different local conditions and separate otherwise incompatible processes.
What does a polycistronic bacterial mRNA contain? Coding information for more than one protein.
How does the central vacuole help support a mature plant cell? Water in the vacuole contributes to turgor pressure against the cell wall.
What does DNA ligase seal during replication? Nicks in the sugar–phosphate backbone between adjacent DNA segments.
Why can a brief rise in cytosolic calcium serve as a signal? Resting cytosolic calcium is kept low, so the rise can activate calcium-sensitive proteins.
Which of the three major cytoskeletal filament classes has the largest diameter? Microtubules.
Which eukaryotic RNA polymerase transcribes tRNA genes and 5S rRNA genes? RNA polymerase III.
What distinguishes the DNA-containing region of a prokaryotic cell from a eukaryotic nucleus? It is not enclosed by a nuclear envelope.
Which enzyme attaches carbon dioxide to RuBP in the Calvin cycle? Rubisco.
What do single-strand binding proteins do during DNA replication? Stabilize exposed single-stranded DNA and prevent premature re-pairing.
Which eukaryotic RNA polymerase transcribes nuclear protein-coding genes? RNA polymerase II.
Which molecule is the source of oxygen released during oxygenic photosynthesis? Water.
What process copies a DNA sequence into RNA? Transcription.
What happens when transcription terminates? RNA synthesis stops and the transcript is released.
In which direction do DNA polymerases extend a new DNA strand? 5′ to 3′, by adding nucleotides to its 3′ end.
How do microvilli improve an absorptive cell's exchange capacity? They increase its plasma membrane surface area.
Is an enzyme consumed as a net reactant in each catalytic cycle? No; it is regenerated and can catalyze another cycle.
Which ribosomal subunit contains the peptidyl-transferase center? The large subunit.
How does a microRNA help identify an mRNA for regulation? By base pairing with a complementary target sequence.
Why is the citric acid pathway called a cycle? Its starting acceptor, oxaloacetate, is regenerated.
What does activation energy represent? The energy barrier that reactants must overcome to reach the transition state.
Which cytoskeletal structures serve as tracks for long-distance intracellular vesicle movement? Microtubules.
In an ATP-producing chloroplast, which way do protons pass through ATP synthase? From the thylakoid lumen into the stroma.
What happens around each set of chromosomes during telophase of open mitosis? A nuclear envelope re-forms.
What can pass directly through animal gap junction channels? Ions and some small molecules between adjacent cells.
What is the general role of a tumor-suppressor gene's normal product? To restrain inappropriate proliferation or protect processes such as genome maintenance.
How do coat proteins help a transport vesicle form? They help select cargo and bend the donor membrane into a bud.
What is the net ATP yield from glycolysis per glucose molecule? Two ATP.
What kind of cargo is taken up by phagocytosis? Large particles, such as cellular debris or whole microorganisms.
How are endocrine signals carried to distant target cells in animals? Through the circulation.
What happens to a protein during denaturation? Its normal folded structure is disrupted, often reducing or eliminating its function.
What is an origin of replication? A DNA region where replication begins.
Which two products of the light-dependent reactions power the Calvin cycle? ATP and NADPH.
What mitochondrial protein can help activate the intrinsic apoptotic pathway after release into the cytosol? Cytochrome c.
What does proofreading by a replicative DNA polymerase remove? An incorrectly paired nucleotide at the growing strand's 3′ end.
How can an activated oncogene alter cell behavior? It can drive growth-promoting signals too strongly or at inappropriate times.
Why repeat a cellular experiment with independent biological samples? To assess variability and whether the result is reproducible.
How does anaerobic respiration differ from fermentation? Anaerobic respiration uses an electron transport chain with a terminal acceptor other than oxygen; fermentation does not use such a chain.
Which cytoskeletal filament system drives many amoeboid cell protrusions? Actin filaments.
What does hydrolysis use to split a covalent bond in a biological polymer? A water molecule.
Why does rough endoplasmic reticulum look rough in electron micrographs? Ribosomes are attached to its cytosolic surface.
Which regulatory protein must bind a typical cell-cycle CDK for it to become active? A cyclin.
How can a protein's activity change without changing its amino acid sequence or abundance? Post-translational modifications such as phosphorylation can alter its activity.
What modification is added to the 5′ end of most eukaryotic nuclear mRNAs? A modified guanosine cap.
What aspect of a specimen is emphasized by scanning electron microscopy? Surface structure and topography.
What is the general association between histone acetylation and chromatin accessibility? Acetylation often favors more accessible chromatin and transcription.
What major assembly process occurs in the nucleolus? Ribosomal RNA processing and assembly of ribosomal subunits.
Why is a water molecule polar? Its electrons are shared unevenly, leaving oxygen partially negative and the hydrogens partially positive.
What complex removes most introns from eukaryotic pre-mRNA? The spliceosome, made of RNAs and proteins.
How many chromosomes are normally present in a human egg or sperm nucleus? 23.
What carbon-containing products are formed from pyruvate during alcoholic fermentation in yeast? Ethanol and carbon dioxide.
What two-carbon group enters the citric acid cycle carried by coenzyme A? An acetyl group.
What coat protein is commonly involved in receptor-mediated endocytosis at the plasma membrane? Clathrin.
Where is ATP made during the usual glycolysis-plus-fermentation pathway? In glycolysis by substrate-level phosphorylation.
Which region of the endoplasmic reticulum is prominent in lipid synthesis? The smooth ER.
What highly branched polysaccharide stores glucose in animal cells? Glycogen.
Why can inserting one nucleotide within a coding region alter many downstream amino acids? It changes how downstream nucleotides are grouped into codons.
What mechanical stress do intermediate filaments chiefly help cells resist? Tension that would stretch or tear them.
What makes two chromosomes a homologous pair? They carry corresponding genes at corresponding loci, though their alleles may differ.
How many rounds of DNA replication normally occur before the two meiotic divisions? One round.
Which coat is associated with carriers leaving the ER for the Golgi pathway? COPII.
What does antiparallel mean for the two strands of a DNA double helix? They run in opposite 5′-to-3′ directions.
What determines the approximate boundaries of a standard PCR product? The positions and orientations of the two primers.
What property allows many steroid hormones to reach intracellular receptors? Their lipid-soluble nature allows them to cross the plasma membrane.
In many mammalian promoters, what is increased DNA methylation generally associated with? Reduced transcription.
What two components determine an ion's electrochemical gradient? Its concentration difference and the electrical potential difference across the membrane.
What does ligand binding change in a ligand-gated ion channel? The probability that its pore is open, altering ion flow.
What separates in anaphase II of normal meiosis? Sister chromatids.
How many lipid bilayers enclose a typical eukaryotic nucleus? Two: the inner and outer nuclear membranes.
What chromosome region organizes formation of the kinetochore? The centromere.
How is ATP made by substrate-level phosphorylation? A phosphate is transferred directly from a phosphorylated substrate to ADP.
What does a KDEL signal help do for a soluble resident ER protein that escapes to the Golgi? Retrieve it to the ER.
Where does glycolysis occur in a typical eukaryotic cell? In the cytosol.
Which ribosomal site holds the tRNA carrying the growing polypeptide before peptide transfer? The P site.
Why does enlarging a blurry microscope image not necessarily reveal more cellular detail? Enlargement does not improve the image's resolving power.
What building blocks make a triacylglycerol molecule? One glycerol linked to three fatty acids.
Why can a thermodynamically favorable reaction still proceed very slowly? It may have a large activation-energy barrier.
How many NADH molecules does glycolysis produce per glucose molecule? Two NADH.
Which coat helps retrieve resident ER proteins from the Golgi pathway back to the ER? COPI.
What does a positive Gibbs free-energy change indicate for a reaction's forward direction? It requires a net free-energy input under those conditions.
In ATP-producing mitochondria, which way do protons pass through ATP synthase? From the intermembrane space back into the matrix.
If a spherical cell grows without changing shape, what happens to its surface-area-to-volume ratio? The ratio decreases because volume increases faster than surface area.
How do many bacteria reproduce by binary fission? They copy their DNA and partition the cell into two daughter cells.
What recognizes a stop codon and promotes release of the finished polypeptide? A release factor.
What does it mean that the genetic code is degenerate? Several different codons can specify the same amino acid.
How does endocytosis bring material into a cell? The plasma membrane folds inward and encloses material in an internal vesicle.
What shared feature of mitochondria and chloroplasts supports their bacterial ancestry? They contain their own DNA and ribosomes.
What structures regulate exchange through the nuclear envelope? Nuclear pore complexes.
How does cytokinesis usually partition a typical animal cell? A contractile ring constricts the membrane into a cleavage furrow.
What two processes directly balance cell number in a renewing tissue? Cell production and cell loss.
What is an enzyme cofactor? A nonprotein component needed for the activity of some enzymes.
What are telomeres? Specialized repetitive DNA–protein structures at the ends of linear chromosomes.
Why must water be oxidized at photosystem II? To replace electrons lost from its reaction-center chlorophyll.
Which crosses a pure lipid bilayer more readily: oxygen or a sodium ion? Oxygen, because it is small and nonpolar.
What does aneuploidy describe? An abnormal number of individual chromosomes rather than an entire extra chromosome set.
What key event occurs at a photosystem reaction center? An excited electron is transferred to an electron acceptor.
What is the common axonemal arrangement in motile eukaryotic cilia? Nine outer microtubule doublets surrounding two central microtubules, the 9 + 2 pattern.
What does a negative Gibbs free-energy change indicate about a reaction under stated conditions? The reaction is thermodynamically favorable in the forward direction.
Why is the ATP yield from complete glucose oxidation not a fixed universal number? It depends on coupling efficiency, transport costs and how cytosolic reducing equivalents enter mitochondrial metabolism.
Does breaking a chemical bond by itself release energy? No; bond breaking requires energy. Net energy release depends on the overall reaction, including new interactions formed.
What does combinatorial control mean for a eukaryotic gene? Its expression depends on the combined action of multiple regulatory factors.
If transcription and translation rates per mRNA stay constant, how can longer mRNA survival affect protein output? It can increase total protein production from that transcript.
What does bacterial quorum sensing use accumulated signals to assess? The local abundance or density of signaling cells.
When does a protein have quaternary structure? When multiple polypeptide subunits assemble into a functional complex.
What happens to a chlorophyll electron when the pigment absorbs an appropriate photon? The electron is raised to a higher energy state.
What bond links sugar units in a polysaccharide? A glycosidic linkage.
Which chromatids usually exchange segments during meiotic crossing over? Nonsister chromatids of homologous chromosomes.
What is an enhancer in eukaryotic gene regulation? A regulatory DNA sequence that can increase transcription when bound by appropriate factors.
What is cellular differentiation? Development of specialized cellular structures and functions.
In the standard E. coli model, which sugar conditions favor strong lac operon expression? Lactose is available and glucose is scarce.
Why is receptor-mediated endocytosis selective for particular cargo? Cargo binds specific cell-surface receptors before uptake.
What major wall feature characterizes typical Gram-positive bacteria? A thick peptidoglycan layer outside the plasma membrane.
What three-carbon sugar phosphate is the net carbohydrate product exported from the Calvin cycle? Glyceraldehyde-3-phosphate, or G3P.
Do all Bcl-2-family proteins inhibit apoptosis? No; the family includes both apoptosis-promoting and apoptosis-inhibiting members.
What amino acid does AUG specify in the standard genetic code? Methionine.
What developmental range does pluripotent describe for a mammalian stem cell? The ability to generate the cell types of the adult body.
What additional membrane surrounds the thin peptidoglycan layer of typical Gram-negative bacteria? An outer membrane.
Where are nuclear genes transcribed in a typical eukaryotic cell? In the nucleus.
What is apoptosis in animal cell biology? A regulated cell-death process that dismantles a cell in an organized way.
What is a ligand in cell signaling? A molecule that binds a receptor or another specific binding protein.
What does haploid mean? Having one complete set of chromosomes.
What three products are formed when one pyruvate is oxidized by the pyruvate dehydrogenase complex? Acetyl-CoA, carbon dioxide and NADH.
What intracellular protein class is activated by a G protein-coupled receptor? A heterotrimeric G protein.
Which eukaryotic RNA polymerase makes the large precursor containing most ribosomal RNAs? RNA polymerase I.
What does cellular homeostasis maintain? Relatively stable internal conditions despite changes in the surroundings.
What is a mutation at the DNA sequence level? A lasting change in the nucleotide sequence.
In linear photosynthetic electron flow, which acts first: photosystem I or photosystem II? Photosystem II, despite its higher number.
At the same temperature, how do cis-unsaturated tails generally affect membrane fluidity? They increase fluidity by preventing tight lipid packing.
Does an epigenetic change necessarily alter the DNA base sequence? No; gene activity can change without changing the underlying sequence.
What five-carbon molecule accepts carbon dioxide in the Calvin cycle? Ribulose-1,5-bisphosphate, or RuBP.
What event produces independent assortment of different chromosome pairs during meiosis? The independent orientations of homologous pairs at metaphase I.
What is a protein's tertiary structure? The overall three-dimensional shape of one polypeptide chain.
In the common mammalian cotranslational pathway, what recognizes an emerging ER signal sequence? The signal recognition particle, or SRP.
Which reduced carrier is produced when NADP+ accepts photosynthetic electrons? NADPH.
What are Okazaki fragments? Short DNA segments synthesized on the lagging strand.
In the standard E. coli lac model, what happens to cAMP when glucose is scarce? cAMP rises.
What does saturated mean for a fatty acid's hydrocarbon chain? It contains no carbon–carbon double bonds.
Which ribosomal site usually receives an incoming charged tRNA during elongation? The A site.
How does a carrier differ from an open channel during solute transport? A carrier binds solute and changes conformation; an open channel provides a passage.
Are the 5′ and 3′ untranslated regions of an mRNA translated into its protein? No; they remain in the mRNA but lie outside the translated coding region.
What separates in anaphase I of normal meiosis? Homologous chromosomes.
What extra conversion is needed before standard PCR can analyze an RNA sequence? Reverse transcription of RNA into complementary DNA.
How does an enzyme increase a reaction's rate? It lowers the activation-energy barrier.
What is the final electron acceptor in mitochondrial aerobic respiration? Molecular oxygen.
What is a monosaccharide? A single sugar unit, such as glucose.
What does bidirectional replication mean at a typical bacterial replication origin? Two replication forks move away from the origin in opposite directions.
What is cellulose's main structural role in plants? It forms reinforcing fibers in cell walls.
What carries DNA between bacteria during transduction? A bacterial virus, or bacteriophage.
What distinguishes bacterial conjugation from uptake of free environmental DNA? DNA transfer occurs through direct contact between cells.
What does mitosis divide, compared with cytokinesis? Mitosis divides the nucleus; cytokinesis divides the cytoplasm.
What microtubule arrangement forms a typical animal centriole? Nine sets of microtubule triplets arranged in a ring.
What enzyme class terminates cAMP signals by converting cAMP to AMP? Cyclic nucleotide phosphodiesterases.
How does a pathway's end product reduce its own further production in feedback inhibition? It inhibits an earlier enzyme in the pathway.
What does the G2 checkpoint assess before entry into mitosis? Whether DNA replication is complete and major DNA damage is resolved.
What is the general role of catabolic pathways? They break down molecules and can release usable energy.
What happens to a typical animal cell placed in a strongly hypotonic solution? It gains water, swells and may burst.
Does glycolysis directly require molecular oxygen as a reactant? No; it can proceed without oxygen if NAD+ is regenerated.
What is the immediate product of transcribing a gene? An RNA molecule, not a protein.
What polysaccharide is a major glucose reserve in plants? Starch.
Is ubiquitin necessarily destroyed with a proteasome's substrate? No; ubiquitin is removed and can be reused.
How does chemiosmosis couple a membrane gradient to ATP synthesis? Protons flow down their electrochemical gradient through ATP synthase, driving ATP formation.
Why can carbon form the backbones of many different biological molecules? It can form four covalent bonds, including bonds with other carbon atoms.
How many chromatids are present in a paired, replicated homologous chromosome pair? Four chromatids.
Does measuring abundant mRNA alone establish that its protein is abundant? No; translation efficiency and protein degradation also affect protein abundance.
What distinguishes a coenzyme from an inorganic enzyme cofactor? A coenzyme is an organic molecule.
What is a pre-mRNA? An initial eukaryotic protein-coding transcript that has not completed processing.
What covalent linkage joins adjacent nucleotides within a DNA strand? A phosphodiester linkage.
What does replicative DNA helicase do? It separates the two DNA strands at the replication fork.
Does eukaryotic cytoplasm include organelles outside the nucleus? Yes; cytoplasm includes cytosol and structures suspended in it, excluding the nucleus.
What does G0 describe in the cell cycle? A nondividing state outside the active cycle that may be temporary or long-lasting.
Which structure separates the interior of every cell from its surroundings? The plasma membrane.
How many haploid nuclei usually result from one diploid nucleus completing meiosis? Four haploid nuclei.
What intrinsic reaction helps switch an activated G protein alpha subunit off? Hydrolysis of bound GTP to GDP.
What part of a tRNA pairs with an mRNA codon? Its anticodon.
Where in chloroplasts does the Calvin cycle occur? In the stroma.
Does a plant cell wall replace the need for a selectively permeable plasma membrane? No; the plasma membrane still controls exchange with the cytoplasm.
What happens to a molecule when it is reduced in a redox reaction? It gains electrons.
What connects the cytoplasm of adjacent plant cells through their cell walls? Plasmodesmata.
Why are normal meiotic products usually genetically different from one another? Crossing over and independent assortment create different allele combinations.
What major genome-related event defines S phase? DNA replication.
What carbon-containing products result from one glucose molecule completing glycolysis? Two three-carbon pyruvate molecules.
How does the RNA sequence synthesized by RNA polymerase compare with the corresponding DNA coding-strand sequence, both written 5′ to 3′? It matches that strand's sequence except that RNA uses uracil in place of thymine.
What happens to chromosome-set number during a normal mitotic division? It is preserved in each daughter nucleus.
Where do chromosomes align during metaphase? Near the spindle equator, forming the metaphase plate.
How can one gene produce different mature mRNAs by alternative splicing? Different combinations of exons are joined.
Which system can digest whole damaged organelles: the proteasome or the autophagy–lysosome system? The autophagy–lysosome system.
What cellular information does transmission electron microscopy chiefly reveal in thin sections? Internal ultrastructure as electrons pass through the specimen.
What mechanical property does collagen provide to many extracellular matrices? Tensile strength.
What two opposing rates determine a protein's cellular abundance? Its rate of synthesis and its rate of degradation.
If oxygen becomes unavailable to an otherwise oxygen-dependent mitochondrial chain, why does electron flow slow or stop? Its terminal electron acceptor is missing.
Where is the electron transport chain of aerobic respiration in a typical eukaryotic cell? In the inner mitochondrial membrane.
How does nucleotide excision repair handle a bulky DNA lesion? It removes a short damaged DNA segment, then fills and seals the gap using the intact strand as a template.
Which protein kinase is commonly activated by cAMP? Protein kinase A, or PKA.
What net water movement is expected for a cell in an isotonic solution? No sustained net water movement, although water continues moving both ways.
What is rRNA's role in a ribosome? It provides structural and catalytic functions.
Where can respiratory electron transport occur in bacteria that lack mitochondria? In the plasma membrane or associated internal membrane systems.
What common DNA lesion can ultraviolet light create between neighboring pyrimidines? A covalent pyrimidine dimer, such as a thymine dimer.
What do general transcription factors help RNA polymerase II do? Assemble at a promoter and initiate transcription.
If the inner mitochondrial membrane becomes freely permeable to protons, what happens to its ability to drive ATP synthesis? The proton-motive force dissipates, reducing ATP synthesis by ATP synthase.
What enzyme attaches the appropriate amino acid to a tRNA? An aminoacyl-tRNA synthetase.
What is a chiasma? A visible connection between homologs at a site of crossing over.
Which protein heterodimers build microtubules? Alpha- and beta-tubulin dimers.
Can a viral genome consist of RNA rather than DNA? Yes; different viruses use DNA or RNA genomes.
If one experiment agrees with a cellular hypothesis, does that prove the hypothesis universally? No; it supports the hypothesis under the tested conditions without excluding every alternative.
What is osmosis? Net water movement across a selectively permeable membrane driven by a difference in water availability.
What pH environment supports many lysosomal digestive enzymes? An acidic lumen.
What determines how an mRNA sequence is grouped into successive codons during translation? The reading frame established at initiation.
Why can removal of the terminal RNA primer leave an unfilled gap on a linear chromosome? There is no upstream 3′ hydroxyl from which ordinary DNA polymerase can fill that end.
What product forms when oxygen accepts electrons and protons at the end of the mitochondrial respiratory chain? Water.
Do cyanobacteria need chloroplasts to carry out oxygenic photosynthesis? No; they use their own photosynthetic membranes.
How can an enhancer far from a promoter influence transcription? DNA looping can bring bound regulatory proteins into contact with promoter-associated machinery.
What distinguishes an integral membrane protein from a peripheral one? An integral protein is embedded in the lipid bilayer; a peripheral protein associates with its surface.
What problem does topoisomerase relieve ahead of a replication fork? Torsional strain and excessive DNA supercoiling caused by unwinding.
How can activation of a few initiator caspases produce extensive apoptotic cleavage? They activate downstream caspases, amplifying the proteolytic cascade.
Why are viruses not classified as cells? They lack cellular organization and cannot reproduce independently of host cellular machinery.
What does a transcriptional repressor generally do when bound at its regulatory site? It reduces transcription of its target genes.
What does catalase help remove in peroxisomes? Hydrogen peroxide, converting it to water and oxygen.
How does a chemical buffer resist changes in pH? It can accept or release hydrogen ions as conditions change.
How is the leading strand synthesized relative to the movement of its replication fork? Continuously in the same overall direction as fork movement.
What is the basic structure of a nucleosome? DNA wrapped around a core of histone proteins.
What molecule chiefly catalyzes peptide-bond formation in the ribosome? Ribosomal RNA.
How can a signaling cascade amplify a small initial signal? Each activated component can activate or generate many downstream molecules.
What does the Golgi do to many proteins received from the ER? It modifies and sorts them for delivery to cellular destinations.
What does the G1 checkpoint assess before commitment to DNA replication? Whether conditions, cell resources and DNA integrity permit progression.
Which DNA bases are purines? Adenine and guanine.
How does active retinoblastoma protein help restrain entry into S phase? It limits transcriptional programs needed for DNA replication.
What two common outcomes can microRNA-associated complexes cause for target mRNAs? Reduced translation and increased mRNA degradation.
Can a functional gene have an RNA rather than a protein as its final product? Yes; examples include rRNA and tRNA genes.
What is the nucleoid of a bacterial cell? The region containing its chromosome without a surrounding nuclear envelope.
How can two cell types with essentially the same genome make different proteins? They express different sets of genes and regulate their products differently.
What does pinocytosis take up from the extracellular space? Fluid and dissolved substances.
Where in chloroplasts do the light-dependent reactions occur? In the thylakoid membranes.
What major preparation occurs during G2? Preparation for mitosis after DNA replication.
Why do large eukaryotic chromosomes use many replication origins? Multiple starting points allow their long DNA molecules to be copied in parallel.
What directly supplies energy to an ATP-driven primary active transporter? ATP hydrolysis.
Does an enzyme change the equilibrium position of the reaction it catalyzes? No; it speeds approach to equilibrium without changing the equilibrium position.
Why do nonpolar molecular regions tend to cluster in water? Clustering reduces their contact with water.
What enzyme converts ATP into the second messenger cAMP? Adenylyl cyclase.
Why does light-independent not mean that the Calvin cycle works independently of light-generated products? It needs ATP and NADPH supplied by the light reactions.
In respiring mitochondria, in which direction does the electron transport chain pump protons? From the matrix into the intermembrane space.
Why can a change in pH alter enzyme activity? It can change charges and interactions needed for substrate binding and catalysis.
What nucleotide form supplies monomers for DNA synthesis? Deoxyribonucleoside triphosphates, or dNTPs.
Must protein folding wait until translation is finished? No; many proteins begin folding as the chain emerges from the ribosome.
At diffusion equilibrium, do molecules stop moving? No; movement continues, but there is no net movement in either direction.
Why is the lagging strand synthesized discontinuously? DNA polymerase works only 5′ to 3′, opposite the fork's overall movement on that template.
What is a second messenger in an intracellular signaling pathway? A small intracellular molecule or ion that relays a signal from a receptor.
What type of error does DNA mismatch repair correct after replication? Incorrect base pairs that escaped polymerase proofreading.
What makes a signal autocrine? The secreting cell itself can respond to the signal.
In which direction does a ribosome build a polypeptide? From its amino terminus toward its carboxyl terminus.
Which meiotic division reduces the number of chromosome sets from two to one? Meiosis I.
How can an animal cell deliver a damaged organelle to lysosomes? By enclosing it in an autophagosome that fuses with a lysosome.
What membrane change commonly accompanies necrotic cell injury? Loss of plasma membrane integrity with leakage of cellular contents.
What is a major role of a bacterial sigma factor? It helps RNA polymerase recognize appropriate promoters.
How does compact heterochromatin generally affect access to DNA? It limits access by transcription machinery.
Which sugar distinguishes DNA nucleotides from RNA nucleotides? DNA contains deoxyribose; RNA contains ribose.
On which side of an animal plasma membrane are its carbohydrate chains exposed? The extracellular side.
Is one circular chromosome a universal rule for all bacteria? No; it is a common arrangement, but bacterial chromosome number and shape vary.
Are growth of an individual cell and increase in cell number the same process? No; growth increases cell mass or size, whereas division produces additional cells.
How does abundant tryptophan repress the E. coli trp operon in the basic repressor model? Tryptophan binds the repressor, enabling it to bind the operator.
Why is cancer usually not explained by one universal mutation present in every tumor? Different tumors accumulate different combinations of alterations affecting growth and survival.
Does RNA polymerase need an existing primer to begin transcription? No; it can initiate an RNA chain without a primer.
What two-carbon carrier product from fatty acid breakdown can enter the citric acid cycle? Acetyl-CoA.
What is a lysosome's main role in a typical animal cell? Digestion and recycling of macromolecules and cellular material.
What do UAA, UAG and UGA specify in the standard genetic code? Translation termination rather than an amino acid.
In the basic E. coli lac operon model, what does the repressor do when inducer is absent? It binds the operator and strongly limits transcription.
In a typical agarose gel, which direction does DNA migrate in an electric field? Toward the positive electrode because DNA is negatively charged.
Where does a competitive inhibitor bind in the simplest competitive-inhibition model? At the active site, competing with the substrate.
If a cell's sodium gradient dissipates, what happens to sodium-driven uptake of another solute against its gradient, all else equal? The driving force for that uptake decreases.
What is nondisjunction during cell division? Failure of homologous chromosomes or sister chromatids to separate properly.
What do epithelial tight junctions restrict? Passage of substances between adjacent cells.
What does FtsZ help organize in many dividing bacteria? The division site and the ring-associated machinery that constricts the cell.
What do aquaporin channels chiefly transport? Water.
Which common microscopy approach can observe living cells using visible light? Light microscopy.
Which three phases make up interphase in a typical proliferating eukaryotic cell? G1, S and G2.
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