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Unit 1: Chemistry of Life

Topic 1.1 · Which one fitsA runner cools down as sweat evaporates from her skin. Which property of water explains the cooling, and what about water's structure causes it?

Water's high heat of vaporization. Polar water molecules are held together by hydrogen bonds, which must be broken for water to evaporate, so evaporating sweat carries a lot of thermal energy away and helps keep body temperature steady.
Review: 1.1 Structure of Water and Hydrogen Bonding
Topic 1.1 · Which one fitsWater rises up the narrow tubes of a plant's stem, with water molecules clinging to each other and to the tube walls. Which two properties of water are at work?

Cohesion, water molecules sticking to each other, and adhesion, water sticking to the walls. Both come from hydrogen bonds formed by polar water molecules.
Review: 1.1 Structure of Water and Hydrogen Bonding
Topic 1.2 · Which one fitsBacteria are grown where their only phosphorus source is radioactive phosphate. Which kinds of biological molecules use phosphorus as a building block, so the label ends up in their structure?

Nucleic acids (DNA and RNA) and phospholipids, a type of lipid, because phosphorus is used to build both. Proteins are not built from phosphorus, so to label proteins a researcher would use radioactive sulfur, which is used in building proteins.
Review: 1.2 Elements of Life
Topic 1.3 · Which one fitsA digestive enzyme breaks a polysaccharide into single sugars. Which reaction is this, and what happens to water at each bond?

Hydrolysis. Water is added across the bond between two monomers: its hydrogen ion joins one monomer and its hydroxyl group joins the other, which cleaves the covalent bond.
Review: 1.3 Introduction to Macromolecules
Topic 1.3 · Spot the mistakeA student writes: "Dehydration synthesis builds polymers by adding a water molecule between each pair of monomers." What is wrong?

It is backward. Dehydration synthesis removes a hydrogen from one monomer and a hydroxyl group from the other, so the equivalent of a water molecule is lost as the covalent bond forms. Adding water breaks bonds; that is hydrolysis.
Review: 1.3 Introduction to Macromolecules
Topic 1.4 · Say it in contextTwo polysaccharides are built from the same monosaccharide, but one is a straight chain and the other is highly branched. What does this show about carbohydrate polymers?

The same monomers can be covalently linked in different arrangements, so complex carbohydrates can be linear or branched. A polymer's properties depend on how its monomers are assembled, not only on which monomer it uses.
Review: 1.4 Carbohydrates
Topic 1.4 · Know the termWhat are the monomers of polysaccharides, and what kind of bond joins them?

Monosaccharides, the simple sugars. They are joined by covalent bonds, formed by dehydration synthesis, into complex carbohydrates.
Review: 1.4 Carbohydrates
Topic 1.5 · Which one fitsFat A has fatty acid tails with only single bonds between carbons. Fat B's tails each have three carbon-carbon double bonds. Which is more likely to be liquid at room temperature, and why?

Fat B. Double bonds put kinks in the carbon chains, so the tails cannot pack tightly. The more unsaturated a lipid is, the more liquid it is at room temperature.
Review: 1.5 Lipids
Topic 1.5 · Which one fitsA nonpolar molecule made by one gland travels in the blood and changes growth and energy metabolism in distant cells. Which kind of lipid is it most likely to be?

A steroid. Steroids act as hormones that support growth and development, energy metabolism and homeostasis. Fats mainly store energy, and phospholipids build membranes.
Review: 1.5 Lipids
Topic 1.6 · Work it outOne DNA strand reads 5' ATGC 3'. Write the complementary strand in the 5' to 3' direction.

5' GCAT 3'. A pairs with T and C with G, giving TACG lined up 3' to 5' against the original. Because the strands are antiparallel, reading the new strand 5' to 3' reverses it.
Review: 1.6 Nucleic Acids
Topic 1.6 · Which one fitsDuring nucleic acid synthesis, a new nucleotide is joined to a growing strand. To which end is it added?

The 3' end. The incoming nucleotide forms a covalent bond with the 3' hydroxyl of the last sugar, so every nucleic acid grows in the 5' to 3' direction.
Review: 1.6 Nucleic Acids
Topic 1.7 · Say it in contextA mutation swaps one hydrophobic amino acid buried inside a folded protein for one with an ionic R group. Why might the protein stop working when only one amino acid changed?

R group interactions determine how the chain folds. A charged R group cannot join the hydrophobic interactions around it, so the tertiary structure, and with it the protein's shape, can change. Shape determines function.
Review: 1.7 Proteins
Topic 1.7 · Spot the mistakeA student says alpha-helices and beta-pleated sheets form from hydrogen bonds between R groups. What is wrong?

Secondary structures form from hydrogen bonds between atoms of the polypeptide backbone, not the R groups. Interactions among R groups, such as hydrophobic interactions, ionic interactions and disulfide bridges, build the tertiary structure.
Review: 1.7 Proteins

Unit 2: Cells

Topic 2.1 · Which one fitsA pancreas cell secretes large amounts of a digestive enzyme, a protein. Which two parts of the endomembrane system would you expect to be especially abundant in it?

Rough ER, whose membrane-bound ribosomes carry out protein synthesis, and the Golgi complex, which modifies the new proteins and packages them for trafficking. Transport vesicles carry the protein between them and to the plasma membrane.
Review: 2.1 Cell Structure and Function
Topic 2.1 · Spot the mistake"Ribosomes are found only in eukaryotes, because they belong to the endomembrane system." What is wrong with this?

Ribosomes are non-membrane structures made of rRNA and protein, so they are not part of the endomembrane system. They are found in cells of all forms of life, which reflects the common ancestry of all known life.
Review: 2.1 Cell Structure and Function
Topic 2.2 · Work it outCompare the surface area-to-volume ratio of a cube-shaped cell 2 units on a side with one 4 units on a side.

The 2-unit cell: SA = 6(2^2) = 24 and V = 2^3 = 8, so the ratio is 3. The 4-unit cell: SA = 96 and V = 64, so the ratio is 1.5. Doubling the side halves the ratio, so the larger cell exchanges materials less efficiently for its volume.
Review: 2.2 Cell Size
Topic 2.2 · Which one fitsA shrew and an elephant. Which loses heat faster per unit of body mass, and which has the higher metabolic rate per unit of mass?

The shrew, on both counts. A smaller body has a higher surface area-to-volume ratio, so it exchanges proportionally more heat with the environment, and smaller organisms typically have a higher metabolic rate per unit body mass.
Review: 2.2 Cell Size
Topic 2.3 · Which one fitsIn a plasma membrane, which part of each phospholipid faces the watery cytosol and outside, and which part faces the middle of the bilayer?

The polar, hydrophilic phosphate heads face the aqueous environment inside and outside the cell. The nonpolar, hydrophobic fatty acid tails face each other in the interior of the membrane.
Review: 2.3 Plasma Membrane
Topic 2.3 · Which one fitsA protein spans the membrane. What kind of amino acid side groups would you expect on the band of its surface that touches the fatty acid tails?

Nonpolar, hydrophobic side groups. Hydrophobic regions form the surface that meets the membrane interior, while hydrophilic regions are exposed to the cytosol or sit inside the protein.
Review: 2.3 Plasma Membrane
Topic 2.4 · Which one fitsRank O₂, H₂O and Na⁺ by how easily each crosses a membrane made only of phospholipids, and say why.

O₂ crosses freely, because it is small and nonpolar. H₂O, small, polar and uncharged, passes in small amounts. Na⁺ barely crosses at all: the nonpolar tails block ions, which need channel or transport proteins.
Review: 2.4 Membrane Permeability
Topic 2.4 · Which one fitsA plant cell and an animal cell are both placed in distilled water. Water rushes into both, but only the animal cell bursts. Which structure makes the difference?

The plant's cell wall. It provides a structural boundary and protection from osmotic lysis, so the swelling plant cell pushes against the wall instead of bursting. Animal cells have no cell wall.
Review: 2.4 Membrane Permeability
Topic 2.5 · Which one fitsA cell keeps taking up a nutrient even though the nutrient is already more concentrated inside than outside. The uptake stops when ATP production is blocked. Passive or active transport?

Active transport. Moving molecules from low to high concentration requires a direct input of energy. Passive transport only moves molecules down their gradient, from high to low concentration.
Review: 2.5 Membrane Transport
Topic 2.5 · Which one fitsA white blood cell folds its plasma membrane around a bacterium and pulls it inside in a vesicle. Which process is this, and does it need energy?

Endocytosis, and yes. Taking in large particles by folding the membrane into new vesicles requires energy, as does exocytosis, its reverse, where vesicles fuse with the membrane to release material.
Review: 2.5 Membrane Transport
Topic 2.6 · Which one fitsPotassium ions flow out of a nerve cell down their concentration gradient through a protein pore, with no ATP used. What kind of transport is this?

Facilitated diffusion through a channel protein. Charged ions like K⁺ and Na⁺ cannot cross the hydrophobic interior on their own, and moving them down the gradient takes no energy input. The ion movement can also polarize the membrane.
Review: 2.6 Facilitated Diffusion
Topic 2.7 · Work it outFind the solute potential of a 0.1 M NaCl solution at 27 degrees C in an open beaker, using R = 0.0831 L·bars/(mol·K). What is its water potential?

NaCl ionizes into two particles, so i = 2, and T = 27 + 273 = 300 K. Solute potential = −(2)(0.1)(0.0831)(300) = −4.99 bars. In an open beaker the pressure potential is 0, so the water potential is also −4.99 bars.
Review: 2.7 Tonicity and Osmoregulation
Topic 2.7 · Which one fitsA cell has a water potential of −3 bars and is placed in a solution with a water potential of −5 bars. Which way does water move, and is the solution hypotonic or hypertonic to the cell?

Water moves out of the cell into the solution, from higher water potential (−3 bars) to lower (−5 bars). The solution is hypertonic to the cell.
Review: 2.7 Tonicity and Osmoregulation
Topic 2.7 · Spot the mistakeA student says water moves by osmosis toward the side with the lower solute concentration, because that side has more water. What is wrong?

It is backward. Water moves from regions of low solute concentration to regions of high solute concentration, which is from high water potential to low. Adding solute lowers water potential and draws water in.
Review: 2.7 Tonicity and Osmoregulation
Topic 2.8 · Say it in contextA poison stops ATP production in a nerve cell. Over time the Na⁺ and K⁺ gradients across its membrane run down. Why?

The Na⁺/K⁺ pump uses metabolic energy from ATP to establish and maintain these gradients and the membrane potential. Without ATP the pump stops, and ions keep moving down their gradients through channels until the gradients fade.
Review: 2.8 Mechanisms of Transport
Topic 2.8 · Spot the mistake"A cell with plenty of ATP can actively transport ions across its membrane even without any membrane proteins." What is wrong?

Membrane proteins are necessary for active transport. ATP supplies the energy, but a transport protein such as the Na⁺/K⁺ pump has to carry the ions across the hydrophobic membrane interior.
Review: 2.8 Mechanisms of Transport
Topic 2.9 · Say it in contextWhy does a eukaryotic cell keep its digestive enzymes inside lysosomes instead of free in the cytosol?

Compartmentalization. Membranes keep specific enzymatic reactions separate, which minimizes competing interactions; free hydrolytic enzymes would digest the cell's own molecules.
Review: 2.9 Cell Compartmentalization
Topic 2.10 · Which one fitsMitochondria and chloroplasts each have a double membrane and their own DNA. Which explanation for their origin do these features support?

Endosymbiosis: these organelles evolved from once free-living prokaryotic cells that came to live inside another cell.
Review: 2.10 Origins of Cell Compartmentalization
Topic 2.10 · Spot the mistake"Prokaryotes have no internal organization at all, because they lack membrane-bound organelles." What is wrong?

Prokaryotes typically lack membrane-bound organelles, but they still have internal regions with specialized structures and functions. Eukaryotes differ in keeping internal membranes that partition the cell into specialized regions.
Review: 2.10 Origins of Cell Compartmentalization

Unit 3: Cellular Energetics

Topic 3.1 · Say it in contextAn energy diagram shows the same reaction with and without an enzyme. The peak is much lower with the enzyme. What exactly has the enzyme changed?

The activation energy. An enzyme is a biological catalyst that speeds the reaction by lowering the energy needed to get it started; the energy of the reactants and of the products stays the same.
Review: 3.1 Enzymes
Topic 3.1 · Which one fitsAn enzyme acts on molecule X but not on molecule Y, which is almost the same size but carries an opposite charge in one region. Why?

The substrate's shape and charge must both be compatible with the enzyme's active site to form an enzyme-substrate complex. The opposite charge keeps Y from fitting the active site properly.
Review: 3.1 Enzymes
Topic 3.2 · Which one fitsAdding much more substrate restores the reaction rate when inhibitor A is present, but not when inhibitor B is present. Which is the competitive inhibitor?

A. A competitive inhibitor binds reversibly to the active site, so extra substrate can outcompete it. B binds an allosteric site and changes the enzyme's activity, and more substrate cannot undo that.
Review: 3.2 Environmental Impacts on Enzyme Function
Topic 3.2 · Say it in contextAn enzyme's rate climbs steadily from 10 to 37 degrees C, then falls sharply by 50 degrees C. Explain both parts of the curve.

Rising temperature speeds up molecules, so enzymes and substrates collide more often and the rate rises until the optimal temperature. Above it, the heat disrupts hydrogen bonds and changes the enzyme's structure, so the rate drops.
Review: 3.2 Environmental Impacts on Enzyme Function
Topic 3.2 · Spot the mistake"Once an enzyme is denatured, it can never work again." What is wrong?

In some cases denaturation is reversible, and the enzyme can regain activity once conditions return. Denaturation is a change to the protein's structure, and some enzymes can recover that structure.
Review: 3.2 Environmental Impacts on Enzyme Function
Topic 3.3 · Which one fitsAn inhibitor blocks the third enzyme in a five-step metabolic pathway. Which molecule builds up, and what happens to the pathway's final product?

The product of step 2, which is the reactant for step 3, builds up, and the final product falls. In a sequential pathway the product of each step is typically the reactant for the next.
Review: 3.3 Cellular Energy
Topic 3.3 · Say it in contextGlycolysis runs in the cells of archaea, bacteria and eukaryotes alike. What does this suggest?

Common ancestry. Core metabolic pathways such as glycolysis and oxidative phosphorylation are conserved across all three domains, which is best explained by inheritance from a shared ancestor.
Review: 3.3 Cellular Energy
Topic 3.4 · Which one fitsA chloroplast is broken open and its stroma separated from its thylakoids. Which part can make ATP and NADPH in light, and which part can build carbohydrate from CO₂?

The thylakoids make ATP and NADPH in light, because their membranes hold the photosystems, the electron transport chain and ATP synthase. The stroma builds carbohydrate from CO₂ in the Calvin cycle, if it is given ATP and NADPH.
Review: 3.4 Photosynthesis
Topic 3.4 · Say it in contextA chemical makes thylakoid membranes leaky to protons, but electrons still flow through the transport chain. What happens to ATP production, and why?

It drops. ATP synthase makes ATP from the flow of protons back down their gradient (chemiosmosis). If protons leak across the membrane, the gradient cannot build, so photophosphorylation slows or stops.
Review: 3.4 Photosynthesis
Topic 3.4 · Spot the mistake"Photosynthesis first evolved in plants, which then gave Earth its oxygen." What is wrong?

Photosynthesis first evolved in prokaryotes. Evidence supports cyanobacterial photosynthesis as the source of the oxygenated atmosphere, and prokaryotic pathways were the foundation of eukaryotic photosynthesis.
Review: 3.4 Photosynthesis
Topic 3.5 · Say it in contextA poison stops electrons from passing to oxygen at the end of the mitochondrial electron transport chain. Predict the effect on the proton gradient and on ATP made by oxidative phosphorylation.

Electron flow stops, so protons are no longer pumped and the gradient across the inner membrane runs down. With no proton flow through ATP synthase, oxidative phosphorylation stops.
Review: 3.5 Cellular Respiration
Topic 3.5 · Which one fitsYeast sealed in a flask with no oxygen keep making ATP and give off alcohol. Which process lets them keep going?

Fermentation. It allows glycolysis to proceed without oxygen and produces organic molecules such as alcohol or lactic acid. Only the small amount of ATP from glycolysis is made.
Review: 3.5 Cellular Respiration
Topic 3.5 · Spot the mistake"The electron transport chain makes ATP directly as electrons pass along it." What is wrong?

The electron transport chain builds a proton gradient across the inner mitochondrial membrane. ATP synthase then makes ATP as protons flow back through it, which is chemiosmosis.
Review: 3.5 Cellular Respiration
Topic 3.5 · Say it in contextIn some tissues of an endotherm, protons flow back into the matrix without passing through ATP synthase. What does this produce, and how does it help the animal?

Heat. Decoupling oxidative phosphorylation from electron transport releases the energy as heat instead of storing it in ATP, which endotherms can use to regulate body temperature.
Review: 3.5 Cellular Respiration

Unit 4: Cell Communication and Cell Cycle

Topic 4.1 · Which one fitsCell A releases a chemical that affects only cells a short distance away. Cell B releases a chemical carried in the blood to a different cell type across the body. What kind of signaling does each use?

A uses a local regulator, which targets cells in the vicinity of the signaling cell. B uses long-distance signaling, such as a hormone traveling in the bloodstream.
Review: 4.1 Cell Communication
Topic 4.2 · Which one fitsA small nonpolar signaling molecule changes gene expression in its target cells, but the cells have no surface receptor for it. Where is its receptor most likely to be?

Inside the cell, in the cytoplasm or nucleus. A small nonpolar ligand can cross the plasma membrane and bind a receptor there.
Review: 4.2 Introduction to Signal Transduction
Topic 4.2 · Spot the mistake"In a G protein-coupled receptor pathway, the ligand enters the cell and carries the signal all the way to its target." What is wrong?

The ligand binds the receptor's ligand-binding domain outside the cell. The intracellular domain then changes shape, and enzymes and second messengers such as cAMP relay and amplify the signal inside.
Review: 4.2 Introduction to Signal Transduction
Topic 4.3 · Say it in contextA mutation leaves a relay protein in a growth-signaling pathway stuck in its active form, even with no ligand present. Predict the effect.

The pathway stays on. Every component downstream keeps receiving the signal, so the cell keeps making its response, here growth, without any signal arriving.
Review: 4.3 Signal Transduction Pathways
Topic 4.3 · Say it in contextA drug binds a receptor's ligand-binding domain and blocks the ligand from binding. Predict the cell's response when the ligand is present.

Little or no response. The receptor cannot change shape to start transduction, so the downstream components are never activated. A chemical that interacts with any component of a pathway can inhibit it.
Review: 4.3 Signal Transduction Pathways
Topic 4.4 · Which one fitsBody temperature rises during exercise, sweating cools the body, and temperature falls back toward its set point. Positive or negative feedback?

Negative feedback. The response reduces the initial stimulus and returns the system to its set point, which is how homeostasis is maintained.
Review: 4.4 Feedback
Topic 4.4 · Which one fitsDuring childbirth, contractions trigger release of a hormone that causes stronger contractions, which release more hormone, until the baby is born. Which type of feedback?

Positive feedback. Each response intensifies the stimulus, moving the system further from its starting point until the process is complete.
Review: 4.4 Feedback
Topic 4.5 · Which one fitsA cell's DNA content doubles, and each chromosome now has two sister chromatids joined at a centromere. Which phase of the cell cycle just ended?

S phase of interphase, when DNA replicates. G1 comes before it, when the cell grows and duplicates organelles, and G2 follows, with protein synthesis and centrosome replication.
Review: 4.5 Cell Cycle
Topic 4.5 · Say it in contextA drug prevents spindle fibers from forming in dividing cells. What can those cells no longer do in mitosis?

Line up chromosomes at the equator in metaphase and pull sister chromatids to the poles in anaphase. Without spindle fibers, the complete genome cannot be divided between two daughter cells.
Review: 4.5 Cell Cycle
Topic 4.6 · Say it in contextA cell makes cyclin, but a mutation keeps cyclin from binding its cyclin-dependent kinase. Predict the effect on the cell cycle.

The cell stalls at a checkpoint. Interactions between cyclins and cyclin-dependent kinases control progression through the cycle, so without them the cell cannot move on to the next stage.
Review: 4.6 Regulation of Cell Cycle
Topic 4.6 · Which one fitsA mutation disables a checkpoint, so cells with damaged DNA keep dividing instead of stopping. What disease can this disruption lead to?

Cancer. Disruptions to the cell cycle can lead to cancer. When the checkpoint works, a damaged cell is held back from dividing, or may undergo apoptosis.
Review: 4.6 Regulation of Cell Cycle

Unit 5: Heredity

Topic 5.1 · Which one fitsHomologous chromosomes are pulled to opposite poles while each pair of sister chromatids stays attached. Which stage is this?

Anaphase I of meiosis. Homologs separate in meiosis I; sister chromatids separate later, in anaphase II, and in mitosis.
Review: 5.1 Meiosis
Topic 5.1 · Work it outA cell with a diploid number of 8 chromosomes divides by mitosis, and another by meiosis. How many cells result from each, and how many chromosomes does each cell have?

Mitosis: 2 genetically identical cells with 8 chromosomes each. Meiosis: 4 haploid cells with 4 chromosomes each, genetically different from each other.
Review: 5.1 Meiosis
Topic 5.2 · Say it in contextOne pair of homologs fails to separate in meiosis I, and meiosis II then runs normally. What happens to the four gametes?

None is haploid. Two gametes get an extra copy of that chromosome and two get no copy. This is nondisjunction: when separation fails, gametes are no longer haploid.
Review: 5.2 Meiosis and Genetic Diversity
Topic 5.2 · Which one fitsA chromosome in a gamete carries some alleles from the person's mother and some from the father along its length. Which event made that combination, and when?

Crossing over, in prophase I, when non-sister chromatids exchange genetic material. It increases genetic diversity among the gametes.
Review: 5.2 Meiosis and Genetic Diversity
Topic 5.3 · Work it outTwo AaBb plants are crossed, and the genes are on different chromosomes. What fraction of the offspring is expected to be aabb?

1/16. Each gene assorts independently, and P(aa) = 1/4 for each, so P(aa and bb) = 1/4 × 1/4 = 1/16.
Review: 5.3 Mendelian Genetics
Topic 5.3 · Which one fitsA tall pea plant of unknown genotype is crossed with a short plant (tall is dominant). About half the offspring are short. What is the tall parent's genotype?

Heterozygous, Tt. Crossing with a homozygous recessive plant is a test cross: a TT parent would give only tall offspring, while Tt × tt gives about half tall and half short.
Review: 5.3 Mendelian Genetics
Topic 5.3 · Spot the mistakeTwo heterozygous parents have three children, all with the dominant trait. "Their fourth child is sure to show the recessive trait." What is wrong?

Each fertilization is an independent event. The chance of a recessive child is 1/4 for every child, no matter what earlier children were like.
Review: 5.3 Mendelian Genetics
Topic 5.4 · Work it outA cross is expected to give a 3:1 ratio. Of 400 offspring, 280 show the dominant trait and 120 the recessive. Find chi-square and decide at p = 0.05, with a critical value of 3.84.

Expected: 300 and 100. Chi-square = (280 − 300)^2/300 + (120 − 100)^2/100 = 1.33 + 4.00 = 5.33. That exceeds 3.84, so reject the null hypothesis: the data differ from the predicted 3:1 ratio.
Review: 5.4 Non-Mendelian Genetics
Topic 5.4 · Work it outTwo linked genes are tested in a cross, and 170 of 1,000 offspring are recombinant. How far apart are the genes on the chromosome?

17 map units. Map distance is the recombination frequency as a percent: 170 / 1,000 = 0.17, or 17 percent.
Review: 5.4 Non-Mendelian Genetics
Topic 5.4 · Work it outA woman is a carrier of a recessive X-linked allele, and her partner does not have it. What is the probability that a son of theirs shows the trait?

1/2. A son gets his only X from his mother, and she passes the allele on half the time. His father gives him a Y, so the father's X does not matter for sons.
Review: 5.4 Non-Mendelian Genetics
Topic 5.4 · Which one fitsA trait passes from an affected mother to all her children, but affected fathers never pass it on. Which inheritance pattern fits?

Non-nuclear inheritance through mitochondrial DNA. In animals, mitochondria are usually transmitted by the egg and not the sperm, so these traits are typically maternally inherited.
Review: 5.4 Non-Mendelian Genetics
Topic 5.5 · Which one fitsGenetically identical cuttings of one plant grow blue flowers in acidic soil and pink flowers in basic soil. What does this show?

Phenotypic plasticity. Environmental conditions influence gene expression, so one genotype can produce different phenotypes.
Review: 5.5 Environmental Effects on Phenotype

Unit 6: Gene Expression and Regulation

Topic 6.1 · Work it outA sample of double-stranded DNA is 30 percent adenine. What percent of its bases are guanine?

20 percent. Adenine pairs with thymine, so T is also 30 percent, leaving 40 percent for G and C, which pair with each other and so split it evenly.
Review: 6.1 DNA and RNA Structure
Topic 6.2 · Say it in contextA cell lacking working ligase replicates its DNA. Which new strand is left in pieces, and why that one?

The lagging strand. DNA polymerase makes it discontinuously, in fragments, and ligase joins them. The leading strand is made continuously, so it needs no joining.
Review: 6.2 DNA Replication
Topic 6.2 · Spot the mistake"DNA polymerase can start a new strand anywhere on the template, in either direction." What is wrong?

DNA polymerase needs an RNA primer to start, and it synthesizes DNA only in the 5' to 3' direction. That is why one strand is made continuously and the other in fragments.
Review: 6.2 DNA Replication
Topic 6.3 · Work it outA template DNA strand reads 3' TAC GGA 5'. What mRNA does RNA polymerase make from it?

5' AUG CCU 3'. RNA polymerase reads the template 3' to 5' and builds mRNA 5' to 3', pairing A with U, T with A and G with C.
Review: 6.3 Transcription and RNA Processing
Topic 6.3 · Which one fitsOne eukaryotic gene produces one version of a protein in muscle cells and a different version in nerve cells. Which RNA processing step makes this possible?

Alternative splicing. Introns are cut out and exons are spliced together in different combinations, giving different mature mRNAs from the same gene.
Review: 6.3 Transcription and RNA Processing
Topic 6.4 · Say it in contextThe coding sequence of a human gene, with its introns already removed, is put into bacteria, and the bacteria make the human protein with the correct amino acid sequence. What does this show?

Nearly all living organisms use the same genetic code, so bacteria read the human codons as the same amino acids. That shared code is evidence for the common ancestry of all living organisms.
Review: 6.4 Translation
Topic 6.4 · Say it in contextA drug blocks reverse transcriptase. Why does it stop a retrovirus from making new viruses?

A retrovirus must copy its RNA genome into DNA using reverse transcriptase. That DNA integrates into the host genome and is then transcribed and translated to build new viruses, so blocking the copying stops every later step.
Review: 6.4 Translation
Topic 6.5 · Which one fitsA bacterium transcribes the genes for breaking down a sugar only when that sugar is present. Is this set of genes constitutive, inducible or repressible?

Inducible. The genes are switched on in response to the sugar. Constitutive genes are expressed all the time, and a repressible system is normally on until something turns it off.
Review: 6.5 Regulation of Gene Expression
Topic 6.5 · Which one fitsA chemical tag added to a gene's DNA silences the gene without changing its base sequence, and the tag can later be removed. What kind of change is this?

An epigenetic change: a reversible modification of DNA or histones that affects gene expression without changing the DNA sequence.
Review: 6.5 Regulation of Gene Expression
Topic 6.6 · Say it in contextA mutation in a stretch of DNA far upstream of a gene sharply lowers its transcription, though the gene's coding sequence is unchanged. How?

The mutation likely changed an enhancer. Transcription factors bind enhancer or promoter sequences to help RNA polymerase start transcription, and these sequences can be upstream or downstream of the gene.
Review: 6.6 Gene Expression and Cell Specialization
Topic 6.6 · Say it in contextCells given a small RNA whose bases match one gene's mRNA make much less of that gene's protein. What does this show?

Certain small RNA molecules regulate gene expression. Here the small RNA reduces how much protein is made from that gene, even though the gene itself is unchanged.
Review: 6.6 Gene Expression and Cell Specialization
Topic 6.7 · Which one fitsWhich usually changes a protein more: a single-nucleotide insertion near the start of a gene, or a single-nucleotide substitution near its end?

The insertion. It is a frameshift mutation, so every codon after it is read in the wrong frame and the amino acid sequence changes from that point on. A substitution changes at most one codon.
Review: 6.7 Mutations
Topic 6.7 · Spot the mistake"Bacteria exposed to an antibiotic mutate so that they become resistant." What is wrong?

Mutations arise randomly, from replication or repair errors and from radiation or reactive chemicals, not because the bacteria need them. The antibiotic selects bacteria that already carry resistance, and those survive and reproduce.
Review: 6.7 Mutations
Topic 6.7 · Which one fitsA bacterium gains a resistance gene when another bacterium transfers DNA to it through direct cell-to-cell contact. Which process is this?

Conjugation. Taking up DNA from the surroundings is transformation, and a virus carrying DNA between cells is transduction. All of them increase genetic variation in prokaryotes.
Review: 6.7 Mutations
Topic 6.8 · Which one fitsDNA fragments are run on a gel. Which fragments end up farthest from the wells?

The smallest. Gel electrophoresis separates DNA fragments by size and charge: negatively charged DNA moves toward the positive end, and short fragments move through the gel fastest.
Review: 6.8 Biotechnology
Topic 6.8 · Which one fitsA lab has a tiny sample of DNA and needs millions of copies of one short region of it. Which technique fits?

PCR, the polymerase chain reaction. Repeated cycles of denaturing the DNA, annealing primers and extending new strands amplify the region.
Review: 6.8 Biotechnology

Unit 7: Natural Selection

Topic 7.1 · Which one fitsBeetle A lives three years and leaves 10 offspring that survive to reproduce. Beetle B lives one year and leaves 30 offspring that survive to reproduce. Which beetle is more fit?

Beetle B. Evolutionary fitness is measured by reproductive success, not by how long an individual survives.
Review: 7.1 Introduction to Natural Selection
Topic 7.1 · Spot the mistake"Each giraffe stretched its neck to reach higher leaves, and that is why giraffe necks got longer." What is wrong?

Individuals do not evolve by effort. Necks already varied; in competition for limited food, giraffes with longer necks were more likely to survive and leave more offspring, so the trait became more common over generations.
Review: 7.1 Introduction to Natural Selection
Topic 7.2 · Say it in contextA moth population has light and dark individuals. After pollution darkens tree bark, dark moths become more common over many generations. Explain.

The changed environment applied a new selective pressure. On dark bark the dark phenotype raised fitness, so dark moths survived and reproduced more. Natural selection acts on phenotypic variation already in the population.
Review: 7.2 Natural Selection
Topic 7.3 · Which one fitsFor generations, farmers keep seeds only from the wild mustard plants with the largest leaves. Over time the crop looks very different from its wild ancestor. Which process is this?

Artificial selection. Humans, not the environment, choose which variants reproduce, and that changes variation in the species.
Review: 7.3 Artificial Selection
Topic 7.4 · Which one fitsA storm randomly kills most of a beetle population, and the survivors' allele frequencies differ from the original population. Which process is this?

The bottleneck effect, a type of genetic drift. The allele frequencies changed by chance when the population shrank, not because some alleles helped survival.
Review: 7.4 Population Genetics
Topic 7.5 · Work it outIn a population in Hardy-Weinberg equilibrium, 36 of 400 individuals show a recessive trait. How many individuals are expected to be heterozygous?
1. q^2 = 36/400 = 0.09, so q = 0.3 and p = 1 − 0.3 = 0.7. Heterozygotes are 2pq = 2(0.7)(0.3) = 0.42, and 0.42 × 400 = 168.
Review: 7.5 Hardy–Weinberg Equilibrium
Topic 7.5 · Check the conditionsEach year, a few individuals from a neighboring population join a small island population, and individuals choose mates of their own color. Which Hardy-Weinberg conditions are broken?

Three. No migration (new individuals arrive), random mating (mates are chosen by color) and a large population size (it is small). Allele or genotype frequencies can change.
Review: 7.5 Hardy–Weinberg Equilibrium
Topic 7.5 · Spot the mistake"In any population, a dominant allele will rise in frequency until it replaces the recessive one." What is wrong?

Dominance does not change allele frequencies. If all Hardy-Weinberg conditions held, both frequencies would stay the same every generation. Frequencies change through selection, drift, migration, mutation or non-random mating.
Review: 7.5 Hardy–Weinberg Equilibrium
Topic 7.6 · Which one fitsA human arm, a bat wing and a whale flipper have the same arrangement of bones but do different jobs. What kind of evidence is this, and what does it support?

Morphological homology. The shared structure, despite different functions, is evidence of common ancestry.
Review: 7.6 Evidence of Evolution
Topic 7.6 · Say it in contextA protein differs by 2 amino acids between species A and B, and by 15 between species A and C. What do the data suggest?

A and B share a more recent common ancestor than A and C. Fewer differences in protein amino acid sequences point to a closer evolutionary relationship.
Review: 7.6 Evidence of Evolution
Topic 7.7 · Which one fitsA newly found single-celled organism has membrane-bound organelles, linear chromosomes and genes containing introns. Which group does it belong to, and what do these features show?

Eukaryotes. Membrane-bound organelles, linear chromosomes and genes with introns are structural and functional evidence for the common ancestry of all eukaryotes.
Review: 7.7 Common Ancestry
Topic 7.8 · Say it in contextA chemotherapy drug shrinks a tumor at first, but the tumor regrows and the drug no longer affects it. Explain in terms of evolution.

The tumor cells varied. The drug killed the sensitive cells, and the resistant ones survived and multiplied. Evolution of resistance to chemotherapy drugs, like resistance to antibiotics and pesticides, shows evolution is still going on.
Review: 7.8 Continuing Evolution
Topic 7.9 · Which one fitsBody structure suggests species X is closest to species Y, but DNA sequences put X closest to Z. Which result usually carries more weight in building the tree?

The DNA data. Molecular data typically give more accurate and reliable evidence than morphological traits, and a tree is a hypothesis that is revised as new evidence comes in.
Review: 7.9 Phylogeny
Topic 7.9 · Which one fitsOne diagram has branch lengths calibrated by fossils and a molecular clock; another shows only the branching order. Which is a phylogenetic tree and which a cladogram?

The calibrated one is a phylogenetic tree, which shows amount of change over time. The cladogram shows relationships but no time scale and no measure of difference between groups.
Review: 7.9 Phylogeny
Topic 7.9 · Say it in contextIn a cladogram of five animals, one animal lacks every shared derived character that the other four have. What is it, and what does the node joining it to the rest represent?

It is the out-group, the lineage least closely related to the others. The node where its branch splits from the rest represents the most recent common ancestor of the out-group and the other four.
Review: 7.9 Phylogeny
Topic 7.10 · Which one fitsTwo frog populations live in the same pond, but one breeds in early spring and the other in late summer. What kind of reproductive isolation is this, and what kind of speciation could it lead to?

A pre-zygotic barrier, since it prevents mating, and so fertilization, between them. Because the populations overlap geographically, any speciation that results is sympatric.
Review: 7.10 Speciation
Topic 7.10 · Which one fitsA river changes course and splits a squirrel population in two. Thousands of years later, the two groups cannot interbreed. Which type of speciation is this?

Allopatric speciation, which happens in populations that are geographically isolated from each other.
Review: 7.10 Speciation
Topic 7.10 · Say it in contextA horse and a donkey can mate and produce a mule, but mules are sterile. Under the biological species concept, are horses and donkeys one species?

No. A species is a group that can interbreed to produce viable, fertile offspring. Mule sterility is a post-zygotic mechanism that keeps the two species reproductively isolated.
Review: 7.10 Speciation
Topic 7.11 · Say it in contextA new disease reaches two populations of the same species, one with high genetic diversity and one with very little. Which is more likely to persist, and why?

The genetically diverse one. It is more likely to include individuals that can withstand the disease, while a population with little genetic diversity is at risk of decline or extinction.
Review: 7.11 Variations in Populations
Topic 7.12 · Work it outEarth formed about 4.6 billion years ago, was too hostile for life until about 3.9 billion years ago, and the earliest fossils date to 3.5 billion years ago. What window does this give for the origin of life?

Between about 3.9 and 3.5 billion years ago: after Earth became able to support life and before the earliest fossil evidence of it.
Review: 7.12 Origins of Life on Earth
Topic 7.12 · Know the termThe RNA world hypothesis proposes that RNA was the earliest genetic material. What does it assume about how genetic continuity was kept, what replication required, and whether proteins acted as catalysts?

Genetic continuity was assured by RNA replicating itself, base pairing was necessary for that replication, and genetically encoded proteins were not involved as catalysts.
Review: 7.12 Origins of Life on Earth

Unit 8: Ecology

Topic 8.1 · Which one fitsClassify each response as behavioral or physiological: a lizard moves into the shade at midday, and a person shivers in the cold.

The lizard's move is behavioral, and shivering is physiological. Organisms respond to changes in their environment through both kinds of mechanism.
Review: 8.1 Responses to the Environment
Topic 8.1 · Say it in contextWolves that hunt in a coordinated pack bring down prey that a lone wolf could not. How does this behavior relate to fitness?

Cooperative behavior tends to increase the fitness of the individual and the survival of the population. Natural selection favors behaviors that increase survival and reproductive success.
Review: 8.1 Responses to the Environment
Topic 8.2 · Say it in contextA drought kills much of the grass in a grassland. Predict the effect on the herbivores and the predators that eat them.

Both are likely to decline. A drop in the biomass of producers in an area reduces the energy available to every trophic level above them, so primary and secondary consumers shrink in number or size.
Review: 8.2 Energy Flow Through Ecosystems
Topic 8.2 · Which one fitsAt a deep-sea vent with no sunlight, bacteria build organic molecules using energy from small inorganic molecules in the water. What are these bacteria?

Chemosynthetic autotrophs. They capture energy from inorganic molecules rather than from sunlight, which can happen even in the absence of oxygen.
Review: 8.2 Energy Flow Through Ecosystems
Topic 8.2 · Spot the mistake"Energy cycles through an ecosystem, just like carbon and nitrogen do." What is wrong?

Matter and nutrients cycle between organisms and the environment through biogeochemical cycles. Energy flows through ecosystems instead: it enters, mostly as sunlight captured by producers, and is not recycled.
Review: 8.2 Energy Flow Through Ecosystems
Topic 8.3 · Work it outA population has 120 births and 80 deaths in a year. Using dN/dt = B − D, how fast is it changing?

It grows by 40 individuals per year: dN/dt = 120 − 80 = 40.
Review: 8.3 Population Ecology
Topic 8.3 · Work it outA population of 500 grows exponentially with a maximum per capita growth rate of 0.1 per year. What is its growth rate now?

50 individuals per year. dN/dt = r_max × N = 0.1 × 500 = 50. Unconstrained reproduction gives exponential growth, so the rate keeps rising as N grows.
Review: 8.3 Population Ecology
Topic 8.4 · Work it outA population with a maximum per capita growth rate of 0.2 and a carrying capacity of 1,000 grows logistically. Find dN/dt when N = 250 and when N = 950.

At 250: 0.2 × 250 × (750/1,000) = 37.5 per unit time. At 950: 0.2 × 950 × (50/1,000) = 9.5. Growth slows as the population nears its carrying capacity.
Review: 8.4 Effect of Density on Populations
Topic 8.4 · Which one fitsA disease spreads faster as a deer herd gets more crowded, while a hard frost kills the same share of the herd whatever its size. Which factor is density-dependent?

The disease. Its effect grows with population density. The frost is density-independent, because its effect does not depend on how crowded the herd is.
Review: 8.4 Effect of Density on Populations
Topic 8.5 · Work it outA plot has 10 individuals of species A, 10 of B and 20 of C. Find Simpson's diversity index, using Diversity Index = 1 − Σ(n/N)^2, where n is the number of one species and N is the total.

0.625. N = 40, so Diversity Index = 1 − Σ(n/N)^2 = 1 − [(10/40)^2 + (10/40)^2 + (20/40)^2] = 1 − (0.0625 + 0.0625 + 0.25) = 0.625. Values closer to 1 mean higher diversity.
Review: 8.5 Community Ecology
Topic 8.5 · Which one fitsBarnacles attach to a whale and get carried to new feeding waters, while the whale is neither helped nor harmed. Which relationship is this?

Commensalism: one species benefits and the other is unaffected. In mutualism both benefit; in parasitism one benefits at the other's expense.
Review: 8.5 Community Ecology
Topic 8.6 · Say it in contextSea otters are few in number, but when they are removed, sea urchins multiply and the kelp forest collapses. What role do the otters play?

They are a keystone species. A keystone species has an effect on the ecosystem out of proportion to its abundance, and removing it often makes the ecosystem collapse.
Review: 8.6 Biodiversity
Topic 8.7 · Which one fitsA pesticide is barely detectable in lake water but reaches high concentrations in the fish-eating birds at the top of the food chain. What is this called?

Biomagnification. It is one of the human impacts that can drive changes in ecosystems and lead to extinctions. Eutrophication, from nutrient runoff, is another.
Review: 8.7 Disruptions in Ecosystems
Topic 8.7 · Say it in contextA plant with no predators or competitors in its new range is brought to an island. Predict its effect on native plants.

It is likely to spread and outcompete native species. An invasive species can exploit a new niche free of predators or competitors, or outcompete natives for resources.
Review: 8.7 Disruptions in Ecosystems
Topic 8.7 · Say it in contextWhere a certain disease is common, Aa individuals survive it better than both AA and aa individuals. What happens to the a allele in that population?

It stays in the population even if aa is harmful. This is heterozygote advantage: the heterozygous genotype has higher relative fitness than either homozygote, so selection keeps both alleles.
Review: 8.7 Disruptions in Ecosystems