STEM Readiness: Biology Readiness - Cell Biology: Cell Cycle - G1, S, G2, and M Phases, Checkpoints, Cyclins, CDKs, Cancer Connection

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• Prokaryotes lack a membrane-bound nucleus; DNA resides in the nucleoid region (e.g., Escherichia coli).
• Eukaryotes have a membrane-bound nucleus containing linear chromosomes (e.g., human liver cell).
• Prokaryotic ribosomes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Staphylococcus aureus); archaea lack peptidoglycan.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin.
• Animal cells lack a cell wall; their outer boundary is the plasma membrane.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membrane systems (e.g., endoplasmic reticulum, Golgi apparatus).
• Prokaryotes lack membrane-bound organelles; metabolic functions occur in cytoplasm or plasma membrane.
• Eukaryotes possess mitochondria for aerobic respiration (e.g., all animal cells).
• Plant cells contain chloroplasts for photosynthesis; animal cells do not.
• The nuclear envelope is a double membrane with nuclear pores regulating transport (e.g., mRNA export).
• Nucleolus within the nucleus synthesizes ribosomal RNA and assembles ribosome subunits.
• Endoplasmic reticulum (ER): Rough ER has ribosomes; smooth ER lacks ribosomes and synthesizes lipids.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery (e.g., insulin processing).
• Lysosomes (in animal cells) contain hydrolytic enzymes for degradation; absent in most plant cells.
• Peroxisomes break down fatty acids and detoxify hydrogen peroxide; present in both plant and animal cells.
• Mycoplasma species are bacteria that lack a cell wall; resistant to penicillin.
• Mature mammalian red blood cells lack a nucleus and mitochondria; rely on anaerobic glycolysis.
• Plasmids are small, circular DNA molecules in prokaryotes; used in genetic engineering (e.g., pUC19).
• Chromosomes in prokaryotes are single, circular DNA molecules; eukaryotes have multiple linear chromosomes.
• Cytoskeleton (microtubules, microfilaments, intermediate filaments) is present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2) and undulate.

Difficulty Level

Intermediate – requires precise differentiation between structural and functional features across domains and kingdoms.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (pseudopeptidoglycan or none) differ.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based cytoskeletal elements.
Trap: The nucleus is the only site of DNA in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA.
Trap: Eukaryotes evolved from prokaryotes directly – Fact: Eukaryotes likely arose from archaeal host cells via endosymbiosis with bacteria.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Presence of peptidoglycan in the cell wall
D) Mitochondria for ATP production
Answer: C
Explanation: Peptidoglycan is a key component of bacterial cell walls, a defining prokaryotic feature.
Why the top distractor is wrong: A is incorrect because prokaryotes lack a membrane-bound nucleus.

Question: A cell is observed to contain mitochondria, a nucleus, and a cell wall made of cellulose. This cell is most likely from a:
A) Fungus
B) Bacterium
C) Plant
D) Animal
Answer: C
Explanation: Cellulose cell walls and mitochondria are present in plant cells; fungi have chitin.
Why the top distractor is wrong: A is incorrect because fungal cell walls are composed of chitin, not cellulose.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Golgi apparatus
B) Nuclear envelope
C) Plasma membrane
D) Lysosome
Answer: C
Explanation: Both cell types have a phospholipid bilayer plasma membrane.
Why the top distractor is wrong: A is incorrect because Golgi apparatus is exclusive to eukaryotes.

Question: What is the sedimentation coefficient of ribosomes in mitochondrial matrix?
A) 50S
B) 70S
C) 80S
D) 30S
Answer: B
Explanation: Mitochondrial ribosomes are 70S, similar to prokaryotes.
Why the top distractor is wrong: C is incorrect because 80S refers to cytoplasmic ribosomes in eukaryotes.

Question: Which of the following is an exception to the typical eukaryotic cell structure?
A) Presence of rough ER
B) Linear chromosomes
C) Mature red blood cell lacking a nucleus
D) 80S ribosomes
Answer: C
Explanation: Mature mammalian red blood cells lack a nucleus, unlike typical eukaryotic cells.
Why the top distractor is wrong: A is incorrect because rough ER is common in eukaryotic cells.

Question: Which evidence best supports the endosymbiotic theory?
A) Mitochondria have double membranes and divide by binary fission
B) Lysosomes contain digestive enzymes
C) The nucleus has a double membrane
D) Eukaryotic ribosomes are larger than prokaryotic ones
Answer: A
Explanation: Mitochondria resemble bacteria in size, division method, and membrane structure.
Why the top distractor is wrong: C is incorrect because the nuclear envelope’s double membrane does not imply endosymbiosis.

Question: Where is DNA located in a prokaryotic cell?
A) Mitochondria
B) Nucleolus
C) Nucleoid
D) Nucleus
Answer: C
Explanation: Prokaryotic DNA is concentrated in the nucleoid, not enclosed by a membrane.
Why the top distractor is wrong: D is incorrect because prokaryotes lack a membrane-bound nucleus.

Last?Minute Revision (20–25 one?liners)

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have 70S ribosomes; eukaryotes have 80S in cytoplasm.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Peptidoglycan is unique to bacterial cell walls.
• Archaea lack peptidoglycan; some have pseudopeptidoglycan.
• Plant cell walls: cellulose; fungal: chitin; animal: no cell wall.
• Both cell types have phospholipid bilayer plasma membranes.
• Only eukaryotes have membrane-bound organelles (e.g., ER, Golgi, lysosomes).
• Nucleoid = prokaryotic DNA region; nucleus = eukaryotic, membrane-bound.
• Eukaryotic chromosomes are linear; prokaryotic are circular.
• Mycoplasma lacks a cell wall – resistant to beta-lactam antibiotics.
• Mature red blood cells lack nucleus and mitochondria.
• Plasmids are small, circular DNA in prokaryotes; used in cloning.
• Nuclear pores regulate transport between nucleus and cytoplasm.
• Nucleolus makes rRNA and assembles ribosomal subunits.
• Rough ER has ribosomes; synthesizes proteins for export.
• Smooth ER synthesizes lipids and detoxifies drugs.
• Golgi apparatus modifies and sorts proteins; forms lysosomes.
• Lysosomes present in animal cells; rare in plants.
• Peroxisomes break down fatty acids and neutralize H?O?.
• Cytoskeleton in eukaryotes: microtubules, microfilaments, intermediate filaments.
• Prokaryotes use FtsZ protein (tubulin homolog) for division.
• Prokaryotic flagella: flagellin, rotates; eukaryotic: 9+2 microtubules, bends.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria; chloroplasts from cyanobacteria.
• Verify from standard textbook: exact lipid composition of archaeal membranes.