Cell Structure & Division

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

• AA plasma membrane made of a phospholipid 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 → lysosome → Golgi apparatus → plasma membrane
• DGolgi apparatus → 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) toward 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✓
• CDNA repair and centromere division
• DSpindle formation and nuclear envelope breakdown

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 increase 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 is the 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.