AP BiologyCommon mistakes
Common AP Biology mistakes, explained
The wrong ideas behind the questions students miss most. Each one takes about a minute: predict, see it fail, then check yourself on a new question.
Unit 1: Chemistry of Life
- A bigger molecule doesn't mean new elements“When a molecule gets longer or gains double bonds, it picks up new elements.”
- A hydrogen bond links a hydrogen to an O or N“A hydrogen bond is a hydrogen on one molecule bonding to a hydrogen on another.”
- A rare element can still limit growth“Carbon, hydrogen, oxygen and nitrogen are most of a cell, so a 1 percent element can't limit growth.”
- An H or an O isn't enough for a hydrogen bond“Any molecule with oxygen or hydrogen atoms can form hydrogen bonds.”
- Boiling and melting run on latent heat“Evaporation and steam take so much energy because water has a high specific heat.”
- Breaking a bond takes energy. Forming one releases it.“Bonds store energy, so breaking a bond releases the energy inside it.”
- Building a bond releases water. Breaking one uses it.“Breaking a bond releases water, and forming one uses it up.”
- Charged R groups face the water“Opposite charges attract, so charged R groups pair up in the core and nonpolar ones sit outside.”
- Denaturing leaves the backbone whole“When heat or acid denatures a protein or DNA, it breaks the molecule into its building blocks.”
- Density doesn't decide what mixes with water“Whether something dissolves or mixes in water depends on whether it's heavier or lighter than water.”
- Dissolving salt or sugar doesn't change it“When something dissolves, the water reacts with it and turns it into a new substance.”
- DNA and RNA differ only in the sugar and one base“Thymine, uracil, ribose and deoxyribose are all nucleotide parts, so any of them can go in DNA or RNA.”
- Each bond gives off one water“Each monomer gives off one water, so 6 monomers joined together means 6 waters.”
- Each bond is named for what it joins“A bond name from one macromolecule, like peptide bond, works for the others too.”
- Every base pair is one purine and one pyrimidine“Partner bases are the same kind, so A and T are both purines, and two like bases can pair.”
- Fats are built like polymers but aren't polymers“Fats are built by dehydration synthesis, so they're polymers like starch.”
- Folding and denaturing don't reorder amino acids“Folding, heat, pH or crowding can shuffle a protein's amino acids into a new order.”
- Heavier doesn't mean stronger“The heavier molecule should boil higher or hold on tighter, because it has more mass.”
- Name the protein level by what's bonded to what“A hydrogen bond means secondary, R groups mean tertiary, and a big shape means quaternary.”
- Not every bond in a protein or DNA is in the chain“If two parts of a protein or DNA are bonded, that bond must be part of the chain.”
- Not every macromolecule has nitrogen, phosphorus or sulfur“All the macromolecules are made of the same elements, so a missing element should affect them all.”
- Not every sugar difference is alpha versus beta“When two carbohydrates behave differently, one has alpha linkages and the other beta.”
- Oil doesn't push water away“Hydrophobic molecules repel water. Oil actively pushes water away.”
- Only molecules with a phosphate group have phosphorus“Phospholipids and DNA have phosphorus, so fats, steroids and big proteins must too.”
- Oxygen and size don't add energy per gram“A molecule with more oxygen, or a bigger one with more atoms or phosphates, holds more energy.”
- Phosphorus doesn't mean DNA. Nitrogen doesn't mean protein.“If a molecule has phosphorus, it's a nucleic acid. If it has nitrogen, it's a protein.”
- Polar doesn't mean charged“Water's oxygen is negative, so water is charged and polar groups attract by ionic bonds.”
- Read the variable the experiment changed“Any pattern I can spot in the table is something the experiment showed.”
- Same formula doesn't mean same molecule“Molecules with the same formula are the same thing and act alike.”
- Same sugar and size don't make an enzyme fit“If two molecules are the same size and the same sugar, one enzyme should work on both.”
- Saturated tails pack tighter than kinked ones“Unsaturated tails pack tighter, since double bonds grip neighbors. Straight tails just slide past each other.”
- Separating molecules leaves the bonds inside them whole“When water boils or DNA strands come apart, the strong bonds inside the molecules break.”
- Starch, glycogen, cellulose and chitin do different jobs“They're all glucose chains, so any of them can store fuel, in plants and animals alike.”
- The backbone is sugar and phosphate“The bases are bonded to each other in a line, and that chain holds the strand together.”
- The monomer is the whole repeating unit“Starch breaks into maltose, so that's its monomer. DNA is written in A, T, C and G, so bases are its monomers.”
- The phospholipid head is the end that likes water“The tails love water and the head avoids it, so the heads hide in the middle of the membrane.”
- Uneven sharing doesn't make ions“Oxygen takes the electrons from hydrogen, so water is made of ions.”
- Water sticking to itself isn't water sticking to a surface“Water climbs a narrow tube because it sticks to itself. The tube is just a container.”
- Why ice has the most hydrogen bonds“Cold weakens hydrogen bonds, so ice and cold water have fewer of them.”
Unit 2: Cells
- A bigger cell has more cytoplasm to feed“A bigger cell has more cytoplasm, so it can take in and handle more.”
- A bigger cell of the same shape has less surface per volume“If a cell keeps its shape, its surface area to volume ratio stays the same as it grows or shrinks.”
- A bigger total can still be less per unit“A cell with more membrane takes in more, so it must be better supplied.”
- A carrier only takes what fits its binding site“A carrier will move any molecule of the same kind and size as its usual one.”
- A changed rate doesn't prove membrane damage“If a treatment changes how fast something enters a cell, it must have damaged the membrane.”
- A channel doesn't make a membrane leaky“Adding or opening a channel makes the membrane leakier, so more of everything gets through.”
- A factor that stayed the same can't explain a difference“Cold slows enzymes, so the cold must be why one tube ran slower.”
- A fast rate doesn't mean a pump is working“If a solute crosses fast, or its rate keeps climbing, it must be actively transported.”
- A flat uptake rate means the carriers are full“When the uptake rate levels off, the gradient must be gone.”
- A longer cell isn't always a bigger cell“The cell that stretches farther is the bigger cell.”
- A missing product doesn't show which step broke“If the end product is missing, the enzyme that makes it must be broken.”
- A protein can bind a molecule without moving it“If a membrane protein grabs a molecule, it must be moving it across the membrane.”
- A result means little without its control“The treated group changed a lot, so the treatment caused the change.”
- At equilibrium, molecules keep crossing“Once the concentrations are equal, molecules stop crossing the membrane.”
- Bacteria do the same jobs without organelles“A cell needs the organelle our cells use for a job, so no mitochondria means no electron transport.”
- Big cells run short because of distance, not material“Big blocks exchange slowly because their insides are thicker and harder to get through.”
- Binding alone doesn't name a membrane protein's job“A protein that grabs a molecule is an enzyme, and one that holds on is an anchor.”
- Binding triggers the carrier. It doesn't pay for the move“Binding to the carrier gives the solute the energy to cross.”
- Blaming an ingredient the data rule out“The ingredient that sounds most likely is the cause, as long as it was in the treatment that worked.”
- Changing together doesn't make it the cause“If something changed at the same time as the result, it must have caused it.”
- Channels and carriers don't pick a direction“A glucose carrier moves glucose into the cell. That's the way it always goes.”
- Channels are open pores, carriers change shape“A channel grabs each molecule and flips it across. A carrier is an open tunnel.”
- Check the machinery before you blame ATP“If a cell can't do something, it must be low on ATP. If it does a lot, it has lots of energy.”
- Chemistry decides first, size second“Small molecules get through membranes and into proteins more easily, so size predicts what they do.”
- Cold slows diffusion too“If uptake drops in the cold, it must be a pump. Diffusion uses no energy, so it shouldn't care.”
- Each solute follows its own gradient“Solutes move away from the side with more dissolved stuff in total.”
- Endocytosis brings things in, exocytosis sends them out“Endocytosis is when a vesicle fuses with the membrane and dumps its cargo outside.”
- Forgetting the ion that crosses with it“The sugar gets in by itself, through its own transport protein.”
- Freshwater fish dump water. Saltwater fish save it.“A saltwater fish makes lots of urine to get rid of salt, and a freshwater fish drinks to stay hydrated.”
- Ions and large polar molecules need a protein to cross“Glucose and Na+ are small and dissolve in water, so they slip right through the lipids.”
- Low activity doesn't mean little enzyme“Low enzyme activity means the sample has only a little of that enzyme.”
- Low ATP doesn't always mean a step is blocked“If a drug makes ATP output drop, it must have stopped the electron transport chain or ATP synthase.”
- Membrane molecules drift sideways but rarely flip“The membrane is fluid, so proteins and lipids can flip from one face to the other whenever they like.”
- Membranes aren't held together by covalent bonds“The lipids and proteins in a membrane are covalently bonded together, like one big molecule.”
- Mitochondria and chloroplasts run mostly on nuclear genes“Mitochondria and chloroplasts have their own DNA, so they make most of their own proteins.”
- Mitochondria were engulfed. The nuclear envelope folded in.“Mitochondria came from the cell membrane folding inward, and the nucleus came from an engulfed cell.”
- Mixing up what the stroma and the thylakoids do“Sugar gets built inside the thylakoids, and H+ piles up in the stroma to make ATP.”
- More solute does not always mean water moves in“Water moves into the cell with more solute, or out of the cell with more pressure.”
- Moved isn't the same as made“There's more of the protein in the membrane now, so the cell made more.”
- Not every carrier moves two things at once“A transport protein always moves two substances together, one riding along with the other.”
- Not every organelle was once a bacterium“Any organelle with two membranes, lots of folded membrane or ribosomes must have been an engulfed cell.”
- Not every transport protein costs ATP“If a protein moves something across the membrane, the cell is spending ATP.”
- On intact cells, a protease reaches only the outer face“A protease added to the cells can cut any part of the protein, inside or out.”
- One cell can have two kinds of ribosome“Every ribosome in a eukaryotic cell is the same kind, wherever it sits.”
- Organelles are where enzymes do their work“An organelle is basically a container that holds things for the cell until they're needed.”
- Osmosis doesn't cost the cell any ATP“Cells spend ATP, or run a pump, to move water in and out.”
- Passive transport can't pile up a solute“With enough carriers or channels, a cell can keep pulling a solute in past the outside level.”
- Proteins enter the ER while they're being made“A protein gets made first. Then the cell sends it into the ER if it belongs there.”
- Proteins get changed after the ribosome makes them“Any change to a protein, like a sugar or a phosphate, is put on at the ribosome while it's made.”
- Pumps split ATP. Cotransporters run on a gradient.“Anything that moves a solute uphill splits ATP, and a pump can run on an ion gradient.”
- Read the sign before the size“The bigger the digits, the bigger the value, so -0.8 is more than -0.3.”
- Salts split into ions, and every ion counts“0.10 M is 0.10 M, so NaCl counts like glucose and i = 1.”
- Secreted proteins start on the rough ER, not the smooth ER“The smooth ER makes proteins, or a protein passes through the smooth ER before the rough ER.”
- Shape and pressure come from the plant cell wall“The plasma membrane keeps a plant cell in shape and stops it from bursting.”
- Simple or facilitated diffusion? Check for the protein“If it moves from more to less with no ATP, it's simple diffusion.”
- Size alone doesn't set the surface area-to-volume ratio“The smaller cell always has the higher surface area-to-volume ratio. Same volume means same surface.”
- Some carriers move two solutes in opposite directions“When a carrier moves two solutes together, they both cross in the same direction.”
- Some molecules cross a membrane without a protein“Nothing gets through a membrane unless a channel or carrier protein lets it through.”
- The calcium in a signal often comes from the ER“When a signal raises calcium in a cell, the calcium has to come in from outside the cell.”
- The H+ gradient is across the inner membrane“The outer mitochondrial membrane holds the H+ gradient and the ATP synthase.”
- The membrane can't tell what the cell needs“The membrane lets in the molecules the cell needs, like glucose, and keeps out the ones it doesn't.”
- The membrane decides what enters a plant cell“The cell wall controls what gets into a plant cell.”
- The membrane's oily core is what stops ions“The charged heads on the surface repel ions, trap them, or pull them through.”
- The nucleolus makes ribosomes. Ribosomes make proteins.“The nucleolus is where the cell makes its proteins.”
- The sodium-potassium pump sends sodium out“The pump brings sodium in and sends potassium out, or it moves 2 sodium for every 3 potassium.”
- Twice as far takes four times as long“If a molecule has to diffuse twice as far, it takes twice as long.”
- Water moves toward the solute“Water flows to the side with less solute, where there's already more water.”
- What really keeps a membrane protein from flipping“A membrane protein can't flip because something holds it, like an anchor, its size, or its sugars.”
- Which cargo needs a vesicle“A big molecule just needs a wide enough channel, and a small one could ride in a vesicle.”
Unit 3: Cellular Energetics
- A buffer holds pH where it is“A buffer pushes the pH up, feeds the reaction, or keeps the temperature steady.”
- A lower barrier doesn't make an uphill reaction go“Enzymes and ATP do the same job. They lower the activation energy, which lets an uphill reaction go.”
- A plateau isn't the fastest the system can go“Once the curve levels off, the system is going as fast as it ever can.”
- A rate that levels off means every enzyme is busy“When the rate levels off, the substrate is used up and the reaction is done.”
- A slow enzyme isn't always a denatured one“If an enzyme slows down or stops working, it must have denatured.”
- An enzyme's optimum fits the place it works“All of an organism's enzymes work best at its body's conditions, or at its habitat's.”
- Change the variable you're asked about, and nothing else“Any two groups that differ in only one thing make a fair test of the question.”
- Cold slows an enzyme, but too much heat unfolds it“Cold and heat harm an enzyme the same way, so both losses reverse or neither does.”
- Count the gaps between readings, not the readings“Four readings means the change happened four times.”
- Each control answers a different question“The boiled tube shows what a full reaction looks like. The controls all check the same thing.”
- Electron flow and ATP synthesis are separate steps“When electrons move down the chain, that movement itself makes the ATP.”
- Enzymes don't get used up“The reaction slowed down because the enzyme got used up.”
- Enzymes lower the barrier. They add no energy.“An enzyme speeds up a reaction by giving the molecules the energy to react.”
- Enzymes speed reactions without changing the energy released“An enzyme changes how much energy a reaction releases, making it more favorable.”
- Every energy transfer loses some as heat“Energy is conserved, so all of it can pass to the next step as useful energy.”
- Flip the membrane and the H+ and ATP sides flip too“H+ and ATP end up on the same sides as in a normal cell, whatever the setup.”
- Heat from ATP can't power an uphill reaction“The heat from splitting ATP is what powers a cell's uphill reactions.”
- In a chloroplast, H+ pile up inside the thylakoids“Like a mitochondrion, a chloroplast builds its H+ gradient between its two outer membranes.”
- Ions pile up on the side the pump sends them to“The electron transport chain pumps H+ into the matrix, so that's where H+ collects.”
- It's usually the enzyme that heat and pH change“Heat slowed the reaction by destroying the substrate, leaving the enzyme nothing to work on.”
- Light reactions make ATP. The Calvin cycle uses CO2.“The Calvin cycle makes ATP, and the light reactions use the CO2.”
- Making lactate or ethanol recycles NAD+ and makes no ATP“When there's no oxygen, turning pyruvate into lactate or ethanol is what makes the ATP.”
- Measure activation energy from the reactants“Activation energy is how high the peak sits on the energy diagram.”
- More substrate only beats a competitive inhibitor“More substrate can overcome a noncompetitive inhibitor, but a competitive one stays strong.”
- Needed doesn't mean used up“If adding something makes a reaction go, it must be getting used up to make the product.”
- Not all photosynthesis gives off oxygen“Anything that does photosynthesis splits water and releases oxygen.”
- Not every inhibitor blocks the active site“Anything that slows an enzyme down must be blocking its active site.”
- Only reduction spends NADPH in the Calvin cycle“Fixation is the big step, so it must be the one that burns the ATP and NADPH.”
- Oxygen catches electrons at the end of the chain“Oxygen combines with glucose's carbons to make CO2, the way a fire burns fuel.”
- Photosynthesis gives off O2. Respiration gives off CO2.“A plant doing photosynthesis uses up O2, gives off CO2, or breaks down sugar for energy.”
- The chain pumps protons out. ATP synthase lets them in“ATP synthase pumps protons out of the matrix, and that's how it makes ATP.”
- The O2 plants release comes from water“Plants take in CO2 and give off O2, so the O2 must be oxygen pulled off the CO2.”
- The partner reaction has to release enough energy“If an endergonic step is coupled to any exergonic reaction, the step will run.”
- Used energy turns into heat“When a cell or animal uses energy, the energy is gone.”
- When an enzyme is blocked, its substrate piles up“If an enzyme is blocked, the product it makes will build up.”
- Where each stage of respiration happens“The Krebs cycle happens between the two mitochondrial membranes, where all the protons are.”
Unit 4: Cell Communication and Cell Cycle
- A cell can get the signal and still not respond“If the cell didn't respond, the signal was never made or never reached the receptor.”
- A cell's response can outlast its signal“Once the signal molecule is gone, the response inside the cell stops right away.”
- A checkpoint pauses the cycle. Repair enzymes fix the DNA“The checkpoint fixes damaged DNA, or keeps it from getting damaged.”
- A G protein is on with GTP and off with GDP“When a signal comes, the G protein splits GTP for energy, or a kinase adds a phosphate to GDP.”
- A growing response isn't feedback by itself“If a response keeps growing, speeding up or lasting, that's positive feedback.”
- A hormone falls when the job is done“A hormone level drops because the gland has used up its supply.”
- A mutation can make a protein more active“A mutation always damages a protein, so it works less or stops working.”
- A phosphate switches the next kinase on“When a kinase adds a phosphate to a protein, it shuts that protein off.”
- A resting cell sits in G0, outside the cycle“A cell that isn't dividing is paused partway through the cycle, like in G2.”
- A signal is only as fast as its steps“Once a signal binds, the cell responds right away, whatever the pathway.”
- A signal's range comes from how fast it is removed“How far a signal reaches depends on how much the cell releases, how big it is, or how it binds its receptor.”
- A small messenger like cAMP carries the signal inside“The hormone, or its receptor, carries the signal into the cell.”
- A stuck-on step doesn't need the signal above it“Blocking the signal or the receptor stops every step below it, even one that's stuck on.”
- A tumor suppressor is a brake, and cancer loses it“A mutated tumor suppressor starts pushing the cell to divide, like an oncogene.”
- Blocking the off switch keeps a signal on“If a drug blocks an enzyme in a signaling pathway, the response gets weaker.”
- CDK is the enzyme. Cyclin is the switch.“Cyclin is the enzyme that drives the cell cycle, and CDK levels rise and fall to time each phase.”
- Cells can signal without touching“Cells have to touch, or be wired together, to affect each other.”
- Doubling the signal doesn't double the response“Twice the signal, or twice as many receptors filled, means twice the response.”
- Each cyclin-CDK pair runs its own step“Cyclin-CDK is one switch for the whole cycle, so losing any cyclin stops everything at the start.”
- Each stage of a cascade multiplies the signal“Each stage of a cascade adds its molecules to the total, so 10 and then 10 makes 20.”
- Feedback brings the variable back to the set point“After a spike, the body resets its target to the new level and holds the variable there.”
- How much signal arrives changes the response“If a signal reaches a cell, the cell gives its full response. If it doesn't, nothing happens.”
- Kinases add phosphates. Phosphatases remove them.“A phosphatase adds the phosphate, so blocking it switches a pathway off.”
- Losing a cell cycle brake makes cells divide more“If a checkpoint protein is lost, the cell can't get past the checkpoint and stops dividing.”
- Many signals work without entering the cell“A signal has to get inside the cell to change anything.”
- Mitosis and cytokinesis are two steps“When the nucleus divides, the cell divides with it, in one event.”
- Mitosis doesn't halve the chromosomes“When a cell divides, each new cell gets half of the chromosomes.”
- Mixing up the sensor, control center and effector“The part that reacts to a change, like a sweat gland, must be what sensed it.”
- Mixing up the stages of mitosis“The sisters split in metaphase, and the nuclear envelope breaks down later, around anaphase.”
- More signal can't get past a broken step“If one step in a pathway is broken, a strong enough signal will still get the message through.”
- Negative feedback undoes a change. Positive adds to it.“Positive feedback brings things back to normal, and negative feedback makes things worse.”
- Neighbors can signal without secreting anything“A cell signaling a neighbor must release a molecule that drifts over to a receptor on it.”
- Not every cell response turns on a gene“When a cell gets a signal, it responds by turning genes on and making new proteins.”
- Not every second messenger acts like cAMP“Every second messenger is like cAMP. An enzyme makes it, it floats in the cytosol, and it turns on a kinase.”
- Resetting a pathway doesn't take new proteins“To switch a pathway off and ready it for the next signal, the cell has to destroy and remake its proteins.”
- Signals multiply in the relay, not at the receptor“A signal gets amplified at the surface, because each signal molecule switches on many receptors.”
- Some feedback loops make the change bigger“A feedback loop always brings a value back to its set point.”
- Some receptors are inside the cell“Receptors sit in the plasma membrane, so every signal binds the outside of the cell.”
- T cells find infections by what cells display“A T cell spots an infection by sensing the virus itself, free or inside a cell.”
- The cell decides what a signal does“A signal carries its own instructions, so every cell it reaches does the same thing.”
- The cell with the receptor is the one that responds“The cell that makes a signal is the one that responds to it.”
- The double mutant tells you the order“The single mutants tell me the order, so I don't need the strain with both changes.”
- The receptor decides which cells respond“A hormone acts on the cells it reaches, or the ones it can get inside.”
- The stages of cell division don't take equal time“Each stage of mitosis or the cell cycle takes about the same time.”
- Thinking DNA gets copied right before division“The DNA gets copied in G2 or during mitosis, right before the cell splits.”
- Thinking GTP and phosphates power the next step“Adding a phosphate or GTP gives a protein energy, and that energy powers the next step.”
- Touch alone doesn't signal. It takes the right protein.“If a cell touches other cells or gets crowded, that contact is the signal that sets off its response.”
- Transduction is the relay inside the cell“Transduction turns the signal into action, so the cell's final action is transduction.”
- When a gap is too big to be chance“My results are off, but that's probably just chance or measurement error.”
- Which checkpoint checks what“Any DNA problem stops the cell at G1/S, spindle trouble stops it before mitosis, and any cyclin runs any step.”
Unit 5: Heredity
- A change in one lifetime isn't natural selection“The trait shifted after the environment changed, so selection must have favored it.”
- A dominant trait can be rare“If most individuals have a trait, it must be dominant.”
- A smooth range in a trait takes many genes“One gene with incomplete dominance can make a smooth range, like human height.”
- A trait passed only by the mother isn't from a nuclear gene“The mother carries the allele, so all her offspring get it from her nucleus.”
- A trait that skips both parents is recessive“Two black mice had a white pup, so white must be the dominant color.”
- Copying DNA doesn't double the chromosomes“Copying DNA doubles the chromosomes, since two chromatids are two chromosomes.”
- Copying DNA doesn't mix alleles“When DNA is copied before meiosis, the copies can come out with new mixes of alleles.”
- Count only the boxes that fit the phenotype“Half of Rr x Rr offspring are Rr, so a round one I pick is Rr half the time.”
- Crossing over happens in prophase I“Chromosomes swap pieces while the DNA is copied, or when they line up in the middle at metaphase.”
- Crossing over or independent assortment?“Crossing over and independent assortment both shuffle alleles, so either can explain any new mix.”
- Crossing over trades pieces both ways“In crossing over, one chromatid passes a piece of its DNA over to the other.”
- Dividing recombinants by the wrong total“Recombination frequency is recombinants divided by parentals, or by every offspring the cross made.”
- Every gene and offspring needs its own factor“Once I've found the chance for the gene that seems to matter, that's my answer.”
- Extra classes don't mean many genes“More than two kinds of offspring, or an odd ratio, means the trait is polygenic.”
- Fertilization adds two sets of chromosomes“When two gametes fuse, the zygote ends up with the same number of chromosomes as each gamete.”
- Gamete combinations come from the pairs“I get gamete combinations by counting chromosomes, squaring the pairs, or using 2 to the chromosome number.”
- Homologs split only in anaphase I“Homologs pair at metaphase, or they have already separated once they pair or line up.”
- In Aa × Aa, Aa is twice as likely as AA or aa“Two heterozygotes make AA, Aa and aa offspring, so each genotype is a third.”
- Maternal and paternal chromosomes don't travel as sets“In meiosis I, all the mother's chromosomes go to one pole and all the father's go to the other.”
- Meiosis copies DNA once, then divides twice“Each division in meiosis needs its own round of DNA replication first.”
- Meiosis halves and shuffles the chromosomes“Meiosis copies the cell, so the new cells keep the same chromosome number and alleles.”
- Meiosis I splits the pairs, meiosis II the sisters“Meiosis I splits the sister chromatids, then meiosis II separates the homologous pairs.”
- Meiosis takes two divisions“Meiosis I halves the chromosome number, so its two cells are the finished gametes.”
- Multiply for AND, add for OR“Chances that must both happen get added, and chances where either one counts get multiplied.”
- Nondisjunction in meiosis I spoils all four gametes“Nondisjunction gives the same abnormal gametes in either division.”
- One cell's extra chromosome is its partner's missing one“Nondisjunction adds or deletes a chromosome, and the other cells come out normal.”
- One crossover changes two of the four chromatids“When homologs cross over, all four chromatids trade pieces, so every gamete from that cell is recombinant.”
- One cue can be enough to trigger a response“A response only happens when all of its cues show up together.”
- One gene can shape many traits“Each trait needs its own gene, so traits that show up together must come from separate linked genes.”
- One primary oocyte makes one egg, not four“Egg formation works like sperm formation, so one cell makes four eggs.”
- Parents pass on alleles, not their blood type“A parent hands down their blood type, and O goes with anything.”
- Plasticity is how much a trait shifts between environments“The genotype with the biggest values is the one that responds most to its environment.”
- Pooling sons and daughters hides X linkage“I can count all the offspring together. The total ratio shows how the gene is inherited.”
- Same alleles don't always make the same trait“Same alleles mean the same trait, so any difference must come from different alleles.”
- Segregation is one gene. Independent assortment is two.“When a gene's alleles split evenly into gametes, that's independent assortment.”
- Siblings differ by reshuffling, not by new mutations“Sperm, eggs and siblings differ mostly because of copying errors in their DNA.”
- Sister chromatid swaps don't make new allele combinations“Any exchange between chromatids shuffles alleles, even an exchange between two sister chromatids.”
- Sister chromatids are copies. Homologs are partners.“A chromosome's two sister chromatids are the copy from my mom and the copy from my dad.”
- Splitting sister chromatids doesn't mix alleles“Pulling sister chromatids apart in meiosis II gives each gamete a new mix of alleles.”
- The chromosome number halves in meiosis I“The cell stays diploid until meiosis II, where the number halves.”
- The dominant alleles don't have to share a chromosome“In a double heterozygote, both dominant alleles sit on one chromosome and both recessives on the other.”
- The environment switches genes on and off“If the environment changes a trait, it must have changed the alleles.”
- The parents set the ratio“Any cross with one dominant allele gives 3:1, and any two-gene cross gives 9:3:3:1.”
- Traits come from genes and environment together“A trait is either genetic or environmental. If the environment can change it, it isn't really genetic.”
- When the exposure happens can matter more than how long“The longer exposure should cause the bigger effect, whenever it happened.”
Unit 6: Gene Expression and Regulation
- A = T is about two strands, not one“Any DNA strand has equal A and T, and equal G and C, since they pair.”
- A 1-base deletion can hurt more than a 3-base one“Bigger deletions change the protein more, and a deletion near the end barely matters.”
- A base pair's width comes from its rings“A base pair fits the helix if it has the right number of hydrogen bonds, like A-T's 2 or G-C's 3.”
- A circle gives one piece per cut“Every cut adds a piece, so a plasmid cut once should give 2 fragments.”
- A cloning enzyme has to pass three checks“If an enzyme opens the plasmid where the gene goes, it's the one to use.”
- A linear DNA duplex has two 5' ends, and a circle has none“A double-stranded DNA molecule has one 5' end, whatever its shape.”
- A methyl tag doesn't change the DNA sequence“Methyl groups on a gene change or damage its bases, so it's a mutation.”
- A mutation doesn't always hurt the enzyme“Any mutation in an enzyme's gene makes the enzyme work worse.”
- A partner strand runs the opposite way“To write the partner, I pair each base and keep the same order, 5' to 3', left to right.”
- A slowdown isn't always a gene switched off“If an enzyme's output drops, the cell must have turned its gene off.”
- A small number isn't zero“If a reading drops to a tiny number, the process has stopped.”
- A third-base change isn't always silent“The genetic code is redundant, so a change to one base, especially the third, won't change the protein.”
- A tRNA comes in at A and leaves from E“A new tRNA comes in at E, for entry, or at P, where the chain is.”
- Activators turn genes up, repressors turn them down“Losing an activator should raise a gene's output, and losing a repressor should lower it.”
- An enhancer boosts transcription but can't start it“The enhancer is where RNA polymerase binds and transcription begins.”
- An enhancer can work far from its gene“Control DNA only works right next to its gene, facing the right way.”
- Apply the exon rules before you count mRNAs“Every exon is kept or skipped, so I multiply by 2 for each exon.”
- Bacteria and humans read codons the same way“Each organism has its own genetic code, so a codon can mean one amino acid in us and another in bacteria.”
- Bacteria can pick up genes from other species“A cell can only get a gene from its parent, so a gene stays in one family line.”
- Bacteria can't read a eukaryotic promoter“A gene's own promoter switches it on wherever it goes, so it will work in bacteria too.”
- Bacteria translate mRNA as it's made. Eukaryotes wait.“Bacteria cap, tail and splice mRNA like our cells do, or eukaryotes translate it as it's made.”
- Binary fission doesn't shuffle genes“When bacteria divide, crossing over and independent assortment mix up their genes.”
- Cap, splicing and tail each have their own job“The 5' cap marks where introns get cut, and the signal near the end is one more splice site.”
- Check the row that could prove you wrong“My explanation fits these rows, so it explains the data.”
- Dividing faster doesn't make a new kind of cell“A cell that divides faster or grows more becomes a different kind of cell.”
- DNA and RNA differ in one sugar and one base“DNA and RNA use the same parts, so either can have thymine or uracil, ribose or deoxyribose.”
- DNA and RNA grow only at the 3' end“A new nucleotide can join a DNA or RNA strand at its 5' end, or at either end.”
- DNA polymerase can't start a strand on its own“DNA polymerase can start a new strand anywhere on the template, as long as nucleotides are around.”
- DNA's negative charge is on the backbone“DNA is negative because of its bases, the part that carries the code.”
- Each new base pairs with the one across from it“The partner strand, or the RNA made from a gene, has the same bases as the strand it's built on.”
- Each replication step needs the one before it“Priming, sealing and unwinding can happen in any order, as long as they all happen.”
- Each RNA has its own job“Any of the three RNAs can build the ribosome, carry amino acids or get a cap and tail.”
- Every band in a DNA mix needs a source“If all of a suspect's bands show up in the mixed sample, that suspect could be the other contributor.”
- Follow the broken piece to where it lands“If a chromosome loses a piece, it's a deletion. If one gains a piece, it's a duplication.”
- Four PCR cycles apart means 16 times the DNA“A sample that crosses the threshold 4 cycles sooner started with about 4 times the DNA.”
- Histones and extra DNA don't make a cell eukaryotic“If a cell has histones or more than one DNA molecule, it must be a eukaryote.”
- Inducers pull repressors off. Corepressors put them on.“Lactose helps the lac repressor bind, and tryptophan pulls the trp repressor off the DNA.”
- Introns are cut out of the RNA copy“Introns get cut out of the DNA, or skipped while the gene is being copied.”
- Miscounting hydrogen bonds in DNA base pairs“I count the hydrogen bonds on each strand, and I'm never sure which pair gets three.”
- Mixing up helicase, topoisomerase and ligase“Helicase unwinds DNA, so it also undoes the twisting. Ligase takes out the primers.”
- Mixing up histone acetylation and DNA methylation“Acetylating histones packs DNA tighter, and methylating DNA turns a gene on.”
- More of the same protein is not a new protein“Making more or less of a protein changes the protein. A lower band on a gel means less of it.”
- Neighboring genes can be read in opposite directions“One strand is the template for every gene, so all genes are read the same way.”
- Old DNA strands stay whole when DNA is copied“When DNA is copied, the old DNA gets broken up and mixed into every new strand.”
- One extra base changes every codon after it“Adding or deleting one base only changes the codon where it happens.”
- One mismatch rules a person out“If most of the alleles agree, the two DNA profiles match.”
- One swapped base changes only one codon“When one base is swapped, the reading frame shifts and every codon after it changes.”
- Operons are bacterial. Eukaryotic genes work differently“A human gene has an operator like the lac operon, and an operon makes one mRNA per gene.”
- Polymerase binds the promoter, not the operator“The operator is where RNA polymerase binds, and the promoter is where the repressor sits.”
- Regulatory mutations change the amount of protein“A mutation that changes a trait must have changed the protein's amino acids or shape.”
- RNA is processed before it leaves the nucleus“The mRNA leaves the nucleus first, and then it gets capped, given a tail and spliced.”
- RNA pairs with the template and matches the coding strand“The RNA has the same sequence as its template, so the strand that reads like the RNA is the template.”
- RNA polymerase doesn't pick which genes turn on“RNA polymerase or the ribosome decides which genes a cell expresses.”
- RNA polymerase reads its template 3' to 5'“RNA polymerase reads the template 5' to 3', the same way the sequence is written.”
- Small molecules change genes through a protein“The signal molecule binds the DNA itself and switches the gene on or off.”
- Specialized cells keep their genes and switch most off“Cells specialize by losing genes they don't need, so liver and nerve cells have different genes.”
- The codon is on the mRNA, the anticodon on the tRNA“The three bases on the tRNA are the codon, so I can look them up in the codon table.”
- The DNA closes up again after transcription“Once a gene is transcribed, its DNA stays unwound, with the RNA holding the strands apart.”
- The mature mRNA has no introns“The finished mRNA is the same as the transcript, with the introns still sitting between the exons.”
- The new nucleotide brings its own energy“Every building step needs its own ATP, so DNA polymerase spends one per nucleotide.”
- The operator doesn't make the repressor“The operator is the gene that makes the repressor.”
- The promoter, cap and tail never become protein“The promoter, cap and poly-A tail are part of the code, so they become protein.”
- The ribosome reads 5' to 3' and adds to the C end“The ribosome reads mRNA 3' to 5', or adds new amino acids to the chain's first end.”
- Trace each double digest piece back“Both short single digest pieces start from the same end, so I just subtract.”
- Translation ends at the first stop codon“The ribosome skips a stop codon, or a tRNA reads it, and translation keeps going.”
- Translation starts at the first AUG“The ribosome starts reading at the first base of the mRNA, where the cap is.”
Unit 7: Natural Selection
- A clade is one whole branch of the tree“Species that look alike or sit close together on a tree make a clade, even if I leave a relative out.”
- A factor that helps isn't always required“If adding something makes the result much bigger, the result must need it.”
- A gap that survives the same conditions is inherited“Two populations differ because their surroundings changed each one as it grew.”
- A place name doesn't make it biogeography“If an observation says where something lives or was found, it's biogeography.”
- A regrown population still lacks its lost alleles“The population grew back, so its genetic diversity came back too.”
- A result both ideas predict decides nothing“It matches my hypothesis, so it supports mine over the other.”
- A self-made membrane only holds things in“Once a membrane closes around molecules, you have a cell that can copy and fuel itself.”
- A shared trait usually comes from one ancestor“Related species that share a trait each evolved it separately, or got it from another species.”
- A small gap between groups can be just noise“If one group's average is higher, even a little, the treatment made a difference.”
- A species gets only the changes on its own path“If an ancestor gained a trait, or a close relative has it, this species has it too.”
- A survival edge only spreads if it's inherited“If individuals with a trait survive or reproduce more, the population shifts toward that trait.”
- A trait starts at the ancestor of the species that share it“A common trait goes back to the root. A new one starts at the newest split.”
- A trait that wins in one place can lose in another“Some traits are just better, so a favored trait stays the fitter one wherever the organism lives.”
- Adding up to 1 doesn't prove Hardy-Weinberg“The counts add up, p + q = 1 and p holds steady, so the population is in equilibrium.”
- After selection, divide by who's left“A variant's new share is its survivors over the starting total, or just its survival rate.”
- Allele frequencies aren't genotype frequencies“If 40% of the alleles are a, then 40% of the plants are aa.”
- An allele that loses sometimes doesn't have to vanish“If an allele does badly in some genotypes or some years, selection will wipe it out.”
- An old trait or an old absence doesn't mark a clade“If two species both have a trait, or both lack one, they must be each other's closest relatives.”
- An organelle's ancestor isn't the host cell's ancestor“If a gene in a eukaryote matches a bacterium, the eukaryote must descend from that bacterium.”
- Artificial selection is about who gets to breed“Any time people change an organism's genes on purpose, that's artificial selection.”
- Asexual lines still get new mutations“New mutations only come from meiosis, so a clone copies its DNA perfectly every time.”
- Big differences can hide a shared ancestor“If two groups are this different, they must have started out separately.”
- Both went up, but not by the same amount“If two numbers moved the same way, they moved by about the same amount.”
- Branching off first doesn't make a species primitive“The species that branches off first, or looks simplest, is the one the others came from.”
- Count gene copies, not individuals“To get an allele's frequency, I divide by the number of individuals.”
- Count trait changes on branches“Each species with the trait gets its own change, or they all share one.”
- DNA differences alone don't give you a date“If two species differ at 20 sites in a gene, they split about 20 million years ago.”
- Don't add a hidden cost to a trait that's winning“Every advantage has a catch, so a helpful trait will be held back by some cost.”
- Drift is strong in small populations and weak in big ones“Drift is just chance, and chance acts everywhere, so it moves every population about equally.”
- Embryos don't replay their ancestors as adults“A human embryo passes through the adult forms of its ancestors, first a fish, then a reptile.”
- Equilibrium doesn't mean equal alleles“At Hardy-Weinberg equilibrium, the alleles should be 50:50, or the genotypes 1:2:1.”
- Evolution has no goal“Evolution keeps making species better, moving them toward the traits they'll need.”
- Find the column the result follows“I can compare the rows by whichever column sounds most important.”
- Fitness is counted in offspring that breed“The variant that makes the most offspring, or survives one stage best, is the fittest.”
- Fitness is counted in offspring that breed“The fittest individual is the one that lives longest or is strongest and healthiest.”
- Fossils can't show evolution happening now“Fossils and shared bones prove evolution, so they show it's still going on today.”
- Gene flow can wash out local adaptation“Gene flow just brings in new alleles, so it gives a population more variation for selection to use.”
- Gene flow, drift and inbreeding don't make new alleles“An allele that shows up after migration, drift or inbreeding was made by that event.”
- Genetic drift has no direction“Genetic drift pushes an allele's frequency steadily up or down over the generations.”
- Genotype proportions don't carry over“If 20% of the parents are Aa, then 20% of their offspring will be Aa too.”
- Gradual or punctuated? Read every interval“A big total change means gradual. A jump or a flat stretch means punctuated.”
- Hardy-Weinberg only holds when its conditions do“Hardy-Weinberg says allele frequencies stay the same, so I can apply it to any population.”
- Healthy-looking doesn't mean genetically varied“A population that looks big and healthy must have a lot of genetic variation.”
- Heterozygotes are 2pq, not pq“Heterozygotes are p times q. With p = 0.6 and q = 0.4, that makes Bb 0.6 × 0.4 = 0.24.”
- Hybrid fitness decides how a hybrid zone ends“Lots of hybrids means the two will merge, or the hybrids will become a new species.”
- Identical genes mean shared weaknesses“A population of genetically identical individuals is stronger, since they're all equally healthy.”
- Inbreeding pairs up alleles. It doesn't erase or damage them“Inbreeding damages DNA or wipes out good alleles, so the allele frequencies change.”
- Individuals don't adapt. Populations do.“When the environment changes, each organism adapts and passes the change on to its offspring.”
- Life shares one genetic code because it was inherited“Every cell reads AAA as lysine because no other code could work.”
- Looking alike doesn't mean closely related“If two species look alike or do the same job, they must share a recent common ancestor.”
- Match the gene's speed to the age of the split“More differences make a clearer tree, so the fastest-changing gene is always best.”
- Meeting by chance doesn't make mating random“If individuals just mate with whoever they happen to meet, mating is random.”
- Meiosis shuffles alleles. Mutation makes new ones.“Meiosis makes new alleles. That's why siblings from the same parents turn out different.”
- Mixing groups? Weight each one by its share“When two groups mix, I average their allele frequencies or add the newcomers' share to the old frequency.”
- Mixing up directional, stabilizing and disruptive selection“The biggest changes are at both ends, so it must be disruptive selection.”
- More change doesn't mean an older split“The lineage that has changed the most must have branched off the longest ago.”
- More common doesn't mean selected“If an allele got more common, selection must have favored it.”
- More differences mean an older split“Species with more sequence differences are more closely related, or split more recently.”
- Mutation makes new alleles but barely moves their frequency“When an allele gets more common, stays common, or differs between populations, mutation is the cause.”
- Mutations don't show up because they're needed“Faced with a threat like an antibiotic, an organism develops the mutation it needs.”
- Name the barrier by where reproduction fails“If two species make almost no hybrids, that alone tells me which barrier is at work.”
- Not every cell is like yours“Every living thing has a nucleus and mitochondria and needs oxygen, like my cells do.”
- Not every changed structure is vestigial“If a structure got shorter, disappeared or does something new, it's vestigial.”
- One environment can't show how genes act everywhere“If two strains differ in one environment, they'll differ the same way everywhere.”
- Pick the history with the fewest changes“Any history that ends with the right species having the trait is as good as another.”
- Picking p², 2pq or q² for the wrong genotype“p², 2pq and q² all come from p and q, so whichever one I work out answers the question.”
- Proteins didn't have to come first“Only proteins can catalyze reactions, so proteins had to come before RNA.”
- Resistance doesn't switch off when the drug stops“Once the antibiotic is gone, the bacteria don't need their resistance, so they switch back.”
- Selection can move the mean or the spread“I can tell whether selection happened from one number, like whether the average changed.”
- Selection can only favor variants that already exist“When the environment changes, selection makes the new traits a population needs.”
- Selection doesn't need any type to die out“If every type is still surviving and breeding, selection isn't happening.”
- Selection needs something that copies itself first“Life had to start with a membrane, a way to capture energy, or DNA.”
- Selection on traits does change allele frequencies“Selection only acts on what an organism looks like, so allele frequencies stay the same.”
- Sharing a habitat doesn't make species relatives“Species that share a kind of habitat are probably close relatives, and ones that don't probably aren't.”
- Species are set by who breeds with whom“Looks and location decide whether two groups are one species.”
- Species can split without a wall between them“A new species can only form after a river, mountain or ocean splits a population in two.”
- Taking the most fossils as the first fossils“Something first appeared where its evidence gets plentiful and clear.”
- The most common type isn't always the fittest“Most of the population has this trait, so it must be the best adapted one.”
- Traits gained during life aren't inherited“If a parent gains a trait during its life, like bigger muscles, its offspring are born with it.”
- Traits spread when their carriers leave more offspring“A trait spreads because it helps the species survive.”
- Two strong varieties can share the same weak spot“Pair the two varieties that each resist the most, and the field is covered.”
- Use only what the study measured“A sensible explanation is a fine answer, even if the study never measured that factor.”
- Your closest living relative is a cousin“Of two closest living relatives, one evolved from the other, like humans from chimps.”
Unit 8: Ecology
- A growing population adds more each day“A steady growth factor means the same gain every day, so I add that gain or multiply the factor by the days.”
- A response comes after its cause“If something is always high when a condition shows up, it must have caused that condition.”
- A species can change others it never eats“If two species aren't directly linked in a food web, a change in one won't affect the other.”
- A steady count doesn't mean nothing is changing“If a population stays the same size, the same individuals are just staying alive.”
- A total is how many times how much each“The group where each one does the most, or the biggest group, must make the most.”
- Add respiration back to get gross photosynthesis“The O2 a plant gives off in the light is the photosynthesis number, and I always subtract respiration.”
- An empty patch refills only if animals can reach it“A big patch of good habitat will get its species back on its own, and being cut off only protects it.”
- Animals learn to ignore a harmless repeat“If an animal stops reacting, it must be worn out or its senses must be damaged.”
- Animals moving in and out count too“Population change is just births minus deaths, so animals moving in or out don't change it.”
- Atoms get recycled. Energy doesn't.“Decomposers recycle everything, so energy goes back to plants like nutrients do.”
- Bigger populations don't always grow faster“The bigger a population gets, the more it adds, so it grows fastest right before its limit.”
- Breeding earlier makes a population grow faster“If each female has the same number of young, the populations grow at the same rate.”
- Carrying capacity belongs to the habitat“Each species has its own carrying capacity, so it levels off at the same size wherever it lives.”
- Competing species can share a habitat“If two species compete for the same food, one of them has to drive the other out.”
- Complex can be innate. Automatic can be learned.“Complex behavior must be learned, and anything every animal does automatically must be innate.”
- Daily rhythms keep going in the dark“Daily rhythms happen because light switches activity on and off each day.”
- Density dependence doesn't wait for a crowding point“Density only matters once a population gets crowded enough to cross some threshold.”
- Density dependence is about crowding“A yearly factor is density dependent, and a random one, like a fire, is density independent.”
- Eating or living on another species isn't competition“When one organism hurts or depends on another, the two are competing.”
- Energy eaten isn't all absorbed“Whatever an animal eats ends up in its body, so ingested and assimilated energy are equal.”
- Exponential growth means a steady ratio“A population is exponential when its curve is steep, when it doubles, or when it adds a lot each year.”
- Fastest growth comes before the biggest population“A population is at its biggest when it's growing fastest.”
- Fewer deaths doesn't mean a lower death rate“If fewer animals died this year, the death rate went down.”
- Find the keystone by what happens when it's gone“The keystone is the most common species, or the biggest one, or the one at the top of the food chain.”
- Find when in life most of the deaths happen“If two curves start and end at the same numbers, or both have one steep drop, they're the same type.”
- Genetic diversity is not species diversity“Diversity means lots of species, so one species can't be very diverse.”
- Give each partner its own sign“If one species gains and the other is fine, that's mutualism. A small cost still counts as commensalism.”
- Life history traits come in matched sets“When animals have lots of babies, most of them survive. Species that care for their young bounce back fastest.”
- Living in the same place isn't competing“If two species live in the same place, they must be competing.”
- Living together doesn't mean helping each other“If two species live together or share the same food, they must be helping each other.”
- Lots of young doesn't mean a population is growing“A population that's producing lots of young must be growing.”
- Mixing up population, community and ecosystem“Community and ecosystem both mean the living things in an area, so I can swap them.”
- More individuals doesn't mean more diversity“The community with more individuals in it is the more diverse one.”
- More mouths split a fixed supply“Each animal still gets its usual share. A bigger population just uses more food in total.”
- Not every learned change is habituation“If an animal acts differently after something happens over and over, that's habituation.”
- Offspring only count if the mother lives that long“To get lifetime offspring, I add up the offspring per female at each age.”
- Per capita means per individual, not per breeder“Only adults breed, so I divide births by the females or the breeders.”
- Read a trend from the low end“If the numbers get bigger as I read down the table, that thing is going up.”
- Respired energy is the part that's gone“If an animal respires 75 percent of its energy, that 75 percent builds its body and feeds whatever eats it.”
- Seed plants can't take hold on bare rock until soil forms“Plants could grow on bare rock right away if enough seeds landed.”
- Short-day plants are really counting the night“A short-day plant flowers once the days get short enough.”
- Taxis steers. Kinesis changes speed or turning.“Animals that gather near a stimulus must have aimed for it, whether by taxis or kinesis.”
- The highest count is not carrying capacity“Carrying capacity is the biggest the population ever got.”
- The stage where most die isn't always the one to save“Most chicks die, so saving chicks has to help the population the most.”
- Top predator isn't a trophic level“The top predator is the tertiary consumer, and any carnivore is a secondary consumer.”
- Trusting what you remember over the data“I already know how this works, so I don't need this experiment's data.”
- Two factors decide it, and you only checked one“The option with the most surface area, or the thinnest barrier, must work best.”
- Weigh a trait's cost and benefit together“A trait that helps should be favored. A trait that costs something should be selected against.”
- Where a species lives isn't all it could use“The places a species lives are the only places it can survive.”