DNA & Proteins
What are the three components of a DNA nucleotide?
A phosphate group, deoxyribose sugar and a nitrogenous base.
Which cell structures are the sites of protein synthesis?
Ribosomes.
What is homeostasis?
Maintaining a relatively stable internal environment within tolerable limits, despite internal or external changes.
What is a gene pool?
All the alleles present in a population at a particular time.
Which RNA molecule carries genetic instructions from DNA to a ribosome?
Messenger RNA (mRNA).
What is the main structural difference between prokaryotic and eukaryotic cells?
Eukaryotic cells have a membrane-bound nucleus and other membrane-bound organelles. Prokaryotic cells lack these; their DNA is located in a nucleoid region.
Which pancreatic hormone lowers blood glucose concentration?
Insulin.
According to the biological species concept, what makes two organisms members of the same species?
They belong to populations that can interbreed naturally and produce viable, fertile offspring, and are reproductively isolated from other such populations.
What does it mean to describe DNA replication as semi-conservative?
Each new DNA molecule contains one original strand and one newly synthesised complementary strand.
Why is the cell membrane described using the fluid mosaic model?
Fluid: many lipids and proteins can move laterally within the membrane. Mosaic: the phospholipid bilayer contains a mixture of components, including different proteins.
What is the difference between a receptor and an effector in a stimulus–response pathway?
A receptor detects a stimulus or change. An effector, such as a muscle or gland, carries out a response.
Distinguish between homologous and analogous structures, giving an example of each
Homologous structures share an evolutionary origin, although their functions may differ; for example, human arms and whale flippers. Analogous structures evolved independently but perform similar functions; for example, bird wings and insect wings as flight structures.
Distinguish between transcription and translation, including where each occurs in a eukaryotic cell.
Transcription: a DNA template is used to produce RNA; for nuclear genes, this occurs in the nucleus. Translation: ribosomes read mRNA codons and assemble amino acids into a polypeptide; this occurs at ribosomes in the cytoplasm or attached to rough ER.
Distinguish between simple diffusion, facilitated diffusion and active transport.
Simple diffusion: movement down a concentration gradient directly through the membrane. Facilitated diffusion: movement down a gradient through membrane proteins. Neither requires direct ATP input. Active transport: movement against a concentration or electrochemical gradient using transport proteins and energy.
Explain why sweating and increased blood flow to the skin help cool the body.
Sweat evaporates, absorbing heat from the skin. Increased skin blood flow brings more heat from the body’s core to its surface, allowing greater heat transfer to cooler surroundings.
What is gene flow, and how does it generally affect genetic differences between populations?
Movement of alleles between populations through migration followed by reproduction, or through gamete movement. It generally reduces genetic differences between populations.
Explain how an enzyme speeds up a reaction, and why a very high temperature can reduce its activity.
An enzyme lowers activation energy, increasing reaction rate. Very high temperatures can disrupt interactions maintaining its shape, causing denaturation. The altered active site may no longer bind the substrate effectively.
A red blood cell is placed in a solution with a lower solute concentration than its cytoplasm. Predict what happens and explain why. Assume the solutes cannot cross the membrane.
Water moves into the cell by osmosis across its selectively permeable membrane. The cell swells and may burst (haemolysis) because it lacks a cell wall.
Compare nervous and endocrine communication in terms of message transmission, speed and duration.
Nervous: electrical impulses along neurons and chemical neurotransmitters at synapses; generally rapid and brief. Endocrine: hormones carried in the bloodstream to cells with appropriate receptors; generally slower and longer-lasting.
A population of insects contains some individuals with an inherited resistance to a pesticide. Explain how repeated pesticide use could increase the frequency of resistance.
Resistance is an existing heritable variation. Pesticide exposure acts as a selection pressure: resistant insects are more likely to survive and reproduce. Their offspring inherit resistance alleles, increasing those alleles’ frequency over generations. The pesticide does not create resistance because insects need it.
A liver cell and a nerve cell contain essentially the same DNA but produce different proteins. Explain why.
The cells express different sets of genes through gene regulation. Different genes are transcribed and translated, producing different proteins and allowing specialised structures and functions.
Trace the pathway of a protein that is produced by a cell and secreted outside it, beginning at the ribosome.
A ribosome attached to the rough ER synthesises the protein, which enters the ER. It is transported in a vesicle to the Golgi apparatus, modified and sorted, then packaged into a secretory vesicle. The vesicle fuses with the cell membrane and releases the protein by exocytosis
Blood glucose rises after a meal. Explain how negative feedback returns it towards its normal range.
Pancreatic beta cells detect increased blood glucose and release insulin. Insulin promotes glucose uptake in responsive tissues and glycogen synthesis, particularly in the liver and muscles. Blood glucose falls towards its normal range, reducing the stimulus for insulin secretion
A storm randomly kills most individuals in a small population, leaving survivors with different allele frequencies from the original population. Identify the process and explain why this is not necessarily natural selection
Genetic drift through a population bottleneck. The survivors are a chance sample, so allele frequencies change randomly. It is not necessarily natural selection because survival may be unrelated to an inherited advantage.
A DNA template sequence changes from 3′–TAC CTT AAA–5′ to 3′–TAC CAT AAA–5′. Write the original and mutated mRNA sequences. Given that GAA codes for glutamate and GUA codes for valine, identify the mutation’s effect on the amino acid sequence.
Original mRNA: 5′–AUG GAA UUU–3′. Mutated mRNA: 5′–AUG GUA UUU–3′. This is a base substitution causing a missense mutation: valine replaces glutamate.
A cell from an organism with 2n = 8 undergoes mitosis or meiosis. How many daughter cells result from each process, how many chromosomes does each contain, and how do the daughter cells compare genetically?
Mitosis: two daughter cells, each with 8 chromosomes, normally genetically identical to each other and the parent cell. Meiosis: four haploid daughter cells, each with 4 chromosomes, usually genetically different from one another and the parent cell.
A person becomes dehydrated, increasing the solute concentration of their blood. Explain how ADH helps restore water balance.
Osmoreceptors in the hypothalamus detect increased blood solute concentration. More ADH is released from the posterior pituitary. ADH increases the water permeability of kidney collecting ducts, increasing water reabsorption. Less water is lost in urine, which becomes more concentrated. Blood solute concentration moves towards normal, reducing ADH release.
A river divides a population of lizards. Explain how the separated populations could eventually become different species
The river reduces or stops gene flow. Mutations, genetic drift and different selection pressures cause the populations to diverge. If reproductive barriers develop so that they can no longer interbreed successfully, allopatric speciation has occurred.
Why does PCR use repeated heating and cooling, primers, and a heat-stable DNA polymerase?
Heating separates the DNA strands; cooling allows primers to bind to complementary sequences; an extension temperature allows polymerase to synthesise DNA. Primers define the target region and provide starting points for synthesis. A heat-stable polymerase survives repeated heating. Repeated cycles amplify the target DNA.
Explain how crossing over and independent assortment generate genetic variation during meiosis.
Crossing over: non-sister chromatids of homologous chromosomes exchange corresponding DNA sections during prophase I, creating new allele combinations. Independent assortment: homologous pairs orient independently at metaphase I, producing different combinations of maternal and paternal chromosomes in daughter cells.
During vigorous exercise, carbon dioxide production increases. Explain how this can affect blood pH and how a change in ventilation helps restore it.
More CO₂ can lead to more carbonic acid and hydrogen ions, lowering blood pH. Chemoreceptors detect changes associated with increased CO₂ and acidity. Increased ventilation removes more CO₂, helping reduce hydrogen-ion concentration and restore pH.
Two plant species release pollen at different times of the year. If crossed artificially, they produce offspring that survive but cannot reproduce. Identify the two reproductive barriers and classify each as prezygotic or postzygotic.
Different pollen-release times: temporal isolation, a prezygotic barrier. Surviving but infertile offspring: hybrid sterility, a postzygotic barrier.
A scientist wants bacteria to produce a human protein using a plasmid. Explain the roles of a restriction enzyme, DNA ligase and the plasmid. Why must the inserted sequence contain no introns?
A restriction enzyme cuts the plasmid and gene-containing DNA to create compatible ends. DNA ligase joins the DNA backbones, forming a recombinant plasmid. The plasmid is a vector that carries the sequence into bacteria. Bacteria generally cannot remove eukaryotic introns, so an intron-free coding sequence, such as one obtained from cDNA, is needed.
A substance makes the inner mitochondrial membrane permeable to hydrogen ions, preventing a hydrogen-ion gradient from being maintained. Predict its effect on ATP production and explain why.
ATP production by oxidative phosphorylation decreases substantially. Hydrogen ions leak across the membrane, so the gradient cannot effectively drive ATP synthase. ATP production does not necessarily stop completely because substrate-level phosphorylation can still occur.
One person with diabetes produces very little insulin. Another produces insulin, but their target cells respond poorly to it. Identify the likely diabetes type in each case and explain why both may develop high blood glucose.
Very little insulin suggests type 1 diabetes, usually caused by autoimmune destruction of pancreatic beta cells. Poor target-cell response suggests type 2 diabetes, involving insulin resistance. Both reduce insulin’s effectiveness in promoting glucose uptake and controlling liver glucose output, allowing blood glucose to remain high.
Under comparable experimental conditions, hybrid DNA from species A and B separates at 88°C, while hybrid DNA from A and C separates at 79°C. Which pair is likely more closely related? Explain the evidence and give one limitation of the conclusion.
A and B are likely more closely related. Their hybrid DNA’s higher melting temperature suggests more complementary sequence matching and greater duplex stability. A limitation is that base composition, especially GC content, also affects melting temperature; the result should be supported by other evidence, such as sequence comparisons.
An enzyme has lost its function after a mutation. Testing shows that its gene is transcribed normally and the protein has the same number of amino acids as before, but one amino acid has changed. Explain how this mutation could prevent the enzyme from functioning, and why another amino acid substitution might have no effect.
The substitution changes the protein’s primary structure. Different amino acid properties can change folding or interactions at the active site, preventing substrate binding or catalysis. Another substitution might preserve function if it involves an amino acid with similar properties or occurs in a region that does not affect folding or activity.
A cell has damaged DNA, but a mutation prevents a cell-cycle checkpoint from stopping division. Explain how this could contribute to cancer, and why avoiding apoptosis would make the situation worse.
The cell divides despite damaged DNA and passes mutations to daughter cells. Further mutations may disrupt growth regulation, contributing to uncontrolled division. Avoiding apoptosis allows damaged cells that would normally be removed to survive, divide and accumulate additional mutations.
A person falls into cold water. Explain how the nervous and endocrine systems coordinate responses that conserve heat and increase heat production. Explain why these responses represent negative feedback.
Thermoreceptors detect cooling and information is integrated by the hypothalamus. Nervous responses include vasoconstriction, reducing heat loss through skin, and shivering, increasing heat production through muscle activity. Hormonal responses can include adrenaline increasing metabolic activity and, over a longer timescale, increased thyroid-hormone activity supporting heat production. These responses oppose the temperature decrease and diminish as temperature returns towards normal.
After a drought, a small bird population has fewer alleles and a higher frequency of an allele associated with deep beaks. Explain how natural selection and genetic drift could both have contributed. What additional evidence would help distinguish their effects?
Natural selection: if deep beaks improve access to food during drought, carriers may survive and reproduce more successfully. Genetic drift: chance survival in a small population can also increase that allele’s frequency and eliminate other alleles. Useful evidence includes survival and reproductive success linked to beak depth, changes in food availability, and comparisons across independent populations. A change in allele frequency alone cannot establish which mechanism caused it.