Cell Structure & Division

Which feature is present in a prokaryotic cell but absent from a typical eukaryotic cell?

• AA plasma membrane made of a lipid bilayer
• BRibosomes for protein synthesis
• CA cell wall containing peptidoglycan (murein)✓
• DCircular DNA

Answer: Prokaryotic cell walls contain peptidoglycan (murein), a polymer unique to bacteria. Eukaryotes have plasma membranes and ribosomes too, and although prokaryotes have circular DNA, mitochondria/chloroplasts in eukaryotes also contain circular DNA, so peptidoglycan is the distinguishing feature.

A protein destined for secretion is synthesised and processed. Which sequence correctly orders the organelles involved in its production and export?

• ASmooth endoplasmic reticulum → Golgi apparatus → ribosome → vesicle → plasma membrane
• BRibosome on rough endoplasmic reticulum → vesicle → Golgi apparatus → vesicle → plasma membrane✓
• CNucleus → free ribosome in the cytoplasm → lysosome → Golgi apparatus → vesicle → plasma membrane
• DNuclear pore → Golgi apparatus → vesicle → rough endoplasmic reticulum → ribosome → plasma membrane

Answer: Secretory proteins are made on ribosomes attached to the rough ER, transported in vesicles to the Golgi for modification (e.g. glycosylation), then packaged into vesicles that fuse with the plasma membrane (exocytosis). The smooth ER synthesises lipids, not proteins.

During which phase of the cell cycle does DNA replication occur, so that each chromosome consists of two sister chromatids?

• AG₁ phase
• BG₂ phase
• CProphase of mitosis
• DS phase✓

Answer: DNA is replicated semi-conservatively during the S (synthesis) phase of interphase, producing two identical sister chromatids per chromosome before mitosis begins. G₁ and G₂ are growth phases, and replication is complete before prophase.

In which stage of mitosis do the centromeres divide and the sister chromatids separate, being pulled to opposite poles of the cell?

• AProphase
• BMetaphase
• CAnaphase✓
• DTelophase

Answer: In anaphase the centromeres split and spindle fibres shorten, pulling the separated sister chromatids (now individual chromosomes) towards opposite poles. In metaphase chromosomes line up on the equator; separation has not yet occurred.

A cell with a diploid number of 2n = 8 undergoes meiosis. How many chromosomes are present in each cell at the end of meiosis I, and what is their structure?

• A4 chromosomes, each a single chromatid
• B8 chromosomes, each two sister chromatids
• C4 chromosomes, each two sister chromatids✓
• D8 chromosomes, each a single chromatid

Answer: Meiosis I separates homologous chromosomes (reductional division), so each daughter cell receives 4 chromosomes (n=4). Sister chromatids do not separate until meiosis II, so each chromosome still consists of two chromatids after meiosis I.

Which two processes in meiosis are the main sources of genetic variation in gametes?

• ASemi-conservative replication and cytokinesis
• BCrossing over in prophase I and independent segregation of homologous chromosomes✓
• CProofreading by DNA repair enzymes and division of the centromeres during anaphase I
• DFormation of the spindle from the centrioles and breakdown of the nuclear envelope in prophase I

Answer: Crossing over (chiasmata) in prophase I recombines alleles between homologous chromosomes, and independent segregation (random assortment) in metaphase I mixes maternal and paternal chromosomes. Together with random fertilisation these generate variation. Replication and cytokinesis do not create new allele combinations.

An object measures 50 µm in real life. A student draws it 30 mm long. What is the magnification of the drawing?

• A×600✓
• B×0.6
• C×60
• D×1500

Answer: Magnification = image size ÷ actual size. Converting to the same units: 30 mm = 30 000 µm. 30 000 ÷ 50 = 600, so the magnification is ×600. Care with units is essential.

Why can a transmission electron microscope resolve much finer detail than a light microscope?

• AElectrons have a much longer wavelength than visible light
• BElectrons pass through glass lenses more easily
• CElectron microscopes use coloured stains to add contrast
• DElectrons have a much shorter wavelength than visible light✓

Answer: Resolution is limited by wavelength: the shorter the wavelength, the greater the resolving power. Electron beams have a far shorter wavelength than visible light, giving resolution of around 0.1 nm versus about 200 nm for a light microscope.

Which statement about the structure of mitochondria is correct?

• AThe inner membrane is folded into cristae, increasing the surface area for the electron transport chain✓
• BThe matrix contains the stacked thylakoids
• CThe outer membrane carries the stalked particles of ATP synthase and is the main site of oxidative phosphorylation
• DThey contain no DNA or ribosomes of their own

Answer: The inner mitochondrial membrane is folded into cristae, increasing surface area for the electron transport chain and ATP synthase. Thylakoids are found in chloroplasts, not mitochondria, and mitochondria contain their own circular DNA and 70S ribosomes.

A scientist separates organelles from a homogenised liver sample by ultracentrifugation at increasing speeds. Which organelle sediments first (at the lowest centrifugal force)?

• ARibosomes
• BMitochondria
• CNuclei✓
• DLysosomes

Answer: In differential centrifugation the densest, largest organelles sediment first at low speeds. Nuclei are the largest and most dense, so they pellet first, followed by mitochondria and lysosomes, then ribosomes at the highest speeds. The sample is kept cold, isotonic and buffered throughout.

What is the function of the rough endoplasmic reticulum?

• AFolding and transporting lipids made on its ribosomes
• BSynthesising ATP for use throughout the whole cell
• CDigesting worn-out organelles using hydrolytic enzymes
• DFolding and transporting proteins made on its ribosomes✓

Answer: Ribosomes stud the rough ER and the proteins they make are folded and moved through its cisternae towards the Golgi. Lipid synthesis is a smooth ER function, ATP production is mitochondrial and hydrolysis is lysosomal.

What occurs during anaphase of mitosis?

• AHomologous chromosomes are pulled towards opposite poles
• BSister chromatids are pulled towards opposite poles✓
• CChromosomes line up singly along the cell equator
• DThe nuclear envelope reforms around each chromosome group

Answer: Spindle fibres shorten and the centromeres divide, so one chromatid of each pair travels to each pole. Homologous chromosomes separating is anaphase I of meiosis; lining up is metaphase and envelope reformation is telophase.

A diploid cell containing 12 chromosomes divides by meiosis. How many chromosomes does each resulting cell contain?

• A24 chromosomes, because the DNA replicates before division
• B12 chromosomes, because the number is always conserved here
• C6 chromosomes, because the homologous pairs are separated✓
• D3 chromosomes, because two divisions each halve the number

Answer: Meiosis halves the chromosome number once, not twice. Homologous pairs separate in meiosis I, and meiosis II then separates sister chromatids without changing the count, giving four haploid cells of 6 chromosomes each.

How does a prokaryotic cell differ from a eukaryotic cell?

• AIt has no cytoplasm and no membrane-bound organelles
• BIt has no nucleus and no membrane-bound organelles✓
• CIt has a nucleus but no membrane-bound organelles
• DIt has membrane-bound organelles but no cell membrane

Answer: Prokaryotic DNA is circular and lies free in the cytoplasm, with no nuclear envelope and no membrane-bound organelles at all. Prokaryotes certainly have cytoplasm and a cell-surface membrane; their ribosomes are the smaller 70S type.

What is the role of the Golgi apparatus?

• AModifying proteins and packaging them into vesicles✓
• BModifying proteins and breaking them into amino acids
• CSynthesising proteins and packaging them into vesicles
• DSynthesising lipids and packaging them into vesicles

Answer: Proteins arrive from the rough ER, are modified — often by adding carbohydrate — and leave in vesicles for secretion or for lysosome formation. Synthesis happens on ribosomes, and breaking proteins down is a lysosomal job.

Why does the cell cycle have checkpoints?

• ATo verify ATP availability before division proceeds
• BTo verify DNA integrity before division proceeds✓
• CTo allow additional ribosomes to be manufactured
• DTo allow the nuclear envelope to be reassembled

Answer: Checkpoints halt the cycle until DNA has replicated correctly and chromosomes are properly attached to the spindle. When those controls fail, damaged cells divide unchecked and a tumour may form — which is why so many cancer drugs target dividing cells.

What do lysosomes contain?

• AHydrolytic enzymes held at a high internal pH
• BRespiratory enzymes held at a low internal pH
• CHydrolytic enzymes held at a low internal pH✓
• DPhotosynthetic pigments held at a low internal pH

Answer: Lysosomes hold hydrolases that work best in acidic conditions, digesting engulfed pathogens and worn-out organelles. Keeping them enclosed and acidic protects the rest of the cell should any leak out.

Why must DNA replicate before mitosis begins?

• ASo each daughter cell receives exactly half the genes
• BSo each daughter cell receives a complete set of genes✓
• CSo the chromosomes become heavy enough to move
• DSo crossing over can occur between the chromatids

Answer: Replication in interphase gives each chromosome two identical sister chromatids, so separating them at anaphase leaves every daughter cell with a full genome. Halving occurs in meiosis, and crossing over does not happen in mitosis.

In cell fractionation, why is the tissue homogenate kept in an ice-cold, isotonic, buffered solution?

• ATo speed up enzyme activity and to lyse the nuclei quickly
• BTo slow enzyme activity, prevent osmotic damage and hold pH steady✓
• CTo dissolve the membranes so the organelles separate freely
• DTo ensure the densest organelles sediment before lighter ones

Answer: Ice-cold slows the hydrolytic enzymes released from damaged lysosomes, isotonic stops water entering or leaving organelles by osmosis and bursting or shrinking them, and the buffer prevents a pH change denaturing proteins. Separation by density happens later, in the centrifuge.

A light micrograph of a diatom (a single-celled alga) has a scale bar 8 mm long labelled 20 µm. The image of the diatom is 36 mm wide.

What is the actual width of the diatom?

• A4.4 µm
• B90 µm✓
• C0.9 µm
• D900 µm

Answer: Magnification = image size ÷ actual size of the scale bar = 8000 µm ÷ 20 µm = ×400. Actual width = 36 mm ÷ 400 = 0.09 mm = 90 µm. The tempting 4.4 µm comes from 20 × 8 ÷ 36, which inverts the scaling; 0.9 µm and 900 µm are slips of a factor of 100 or 10 in converting millimetres to micrometres.

A fixed sample of cultured hamster cells was examined and 400 cells were classified. The cell cycle in this culture takes 24 hours. Interphase: 304 Prophase: 48 Metaphase: 20 Anaphase: 12 Telophase: 16

Assuming the number of cells in each stage is proportional to the time spent in it, how long does metaphase last?

• A1.2 minutes
• B346 minutes
• C72 minutes✓
• D480 minutes

Answer: Fraction of cells in metaphase = 20 ÷ 400 = 0.05. Duration = 0.05 × 24 h = 1.2 h = 72 minutes. The tempting 1.2 minutes forgets to convert hours to minutes; 346 minutes is the length of the whole of mitosis (96 ÷ 400 × 1440 min), and 480 minutes comes from multiplying the count by the cycle time without dividing by the total.

Colchicine, a compound from the autumn crocus, prevents tubulin from assembling into microtubules. Dividing plant cells were treated with colchicine, and the percentage of cells with condensed chromosomes whose chromatids had not separated was recorded. Before treatment: 3% After 6 hours: 28%

Which explanation best accounts for the rise?

• ADNA replication is blocked, so cells build up in interphase before S phase is complete
• BCytokinesis is blocked, so cells that have finished anaphase cannot split into two
• CChromosomes condense faster, so cells pass through prophase much more quickly
• DThe spindle cannot form, so chromatids stay joined and cells stall before anaphase✓

Answer: Spindle fibres are microtubules, so without them chromatids cannot be separated and cells accumulate at the stage just before anaphase, with condensed but unseparated chromosomes. Blocking replication would hold cells in interphase, where chromosomes are not condensed; blocking cytokinesis would give cells whose chromatids HAD separated; and a faster prophase would make cells spend less time with condensed chromosomes, lowering the percentage.

A virus particle has a diameter of 120 nm. What is this diameter in micrometres?

• A0.12 µm✓
• B1.2 µm
• C12 µm
• D0.012 µm

Answer: There are 1000 nm in 1 µm, so 120 nm ÷ 1000 = 0.12 µm. The tempting 1.2 µm divides by 100 instead of 1000; 12 µm and 0.012 µm are further slips of the decimal point.

A bacterium divides by binary fission every 25 minutes under ideal conditions. A culture starts with 400 cells. Assuming no cells die, how many cells are present after 2.5 hours?

• A25 600✓
• B2 400
• C12 800
• D51 200

Answer: 2.5 hours = 150 minutes, which is 150 ÷ 25 = 6 divisions. Each division doubles the number, so 400 × 2⁶ = 400 × 64 = 25 600. The tempting 2 400 adds 400 cells per division (linear growth) instead of doubling; 12 800 and 51 200 use 5 and 7 divisions.