STEM Readiness: Biology Readiness - Molecular Biology: Gene Regulation - Operons in Prokaryotes, Enhancers/Silencers, Epigenetics in Eukaryotes

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 include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region lacking a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.
• Ribosomes in prokaryotes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting the endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (absent in archaea, which have pseudopeptidoglycan or other polymers).
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics like penicillin that target peptidoglycan synthesis.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems including the nuclear envelope, ER, and Golgi, which prokaryotes lack.
• The nucleus in eukaryotes contains chromatin (DNA + histones) and a nucleolus where ribosomal RNA is synthesized.
• Prokaryotes have circular DNA as their primary genome; eukaryotes have linear chromosomes associated with histone proteins.
• Plasmids are small, circular DNA molecules found in prokaryotes and some yeasts (eukaryotes), often carrying antibiotic resistance genes.
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide via mitosis and meiosis.
• Flagella in prokaryotes are made of flagellin protein and rotate like a propeller; eukaryotic flagella are composed of microtubules (9+2 arrangement) and move in a whip-like motion.
• Cilia are short, numerous motile structures in some eukaryotes (e.g., Paramecium); prokaryotes lack cilia.
• Endosymbiotic theory posits that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
• Evidence for endosymbiosis includes: mitochondria and chloroplasts have their own DNA (circular), 70S ribosomes, and double membranes.
• Red blood cells (erythrocytes) in mammals lack a nucleus and most organelles, maximizing space for hemoglobin.
• Plant cells contain chloroplasts, large central vacuoles, and plasmodesmata; animal cells have centrioles and lysosomes, which plant cells typically lack.
• Lysosomes (membrane-bound organelles containing hydrolytic enzymes) are common in animal cells but rare in plant cells.
• Peroxisomes are present in both prokaryotes (rare) and eukaryotes, involved in fatty acid oxidation and detoxification of hydrogen peroxide.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, unlike ester-linked lipids in bacteria and eukaryotes.
• Gram-positive bacteria have a thick peptidoglycan layer; Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane with lipopolysaccharides.

Difficulty Level

Intermediate – Covered in introductory biology courses with emphasis on structural and functional comparisons essential for understanding cellular evolution and physiology.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plant cell walls are made of cellulose, fungi use chitin, and archaea lack peptidoglycan.

Trap: Ribosomes are the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S in the cytoplasm, but mitochondria and chloroplasts retain 70S ribosomes.

Trap: The nucleus is the only organelle that contains DNA – Fact: Mitochondria and chloroplasts in eukaryotes contain their own DNA, distinct from nuclear DNA.

Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., photosynthetic bacteria) have thylakoid membranes, but they lack membrane-bound organelles like a nucleus or ER.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Circular DNA
D) Membrane-bound nucleus
Answer: D
Explanation: The membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: Circular DNA is common in prokaryotes but also found in mitochondria and chloroplasts, so it is not exclusive to prokaryotes.

Question: A cell is observed to have a cell wall, chloroplasts, and a large central vacuole. It most likely belongs to which domain?
A) Bacteria
B) Archaea
C) Animalia
D) Plantae
Answer: D
Explanation: Chloroplasts, a large central vacuole, and a cellulose cell wall are characteristic of plant cells.
Why the top distractor is wrong: Bacteria may have cell walls and perform photosynthesis but lack membrane-bound organelles like chloroplasts.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Mitochondria
B) 80S ribosomes
C) Phospholipid bilayer
D) Nucleolus
Answer: C
Explanation: Both cell types have a plasma membrane made of a phospholipid bilayer.
Why the top distractor is wrong: Mitochondria are only found in eukaryotes.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to synthesize proteins
C) Contain 70S ribosomes and circular DNA
D) Divide by binary fission
Answer: C
Explanation: Mitochondria have 70S ribosomes and circular DNA, features shared with prokaryotes.
Why the top distractor is wrong: While a double membrane is supportive, it is not as specific as ribosome type and DNA structure.

Question: Which organism lacks a cell wall and is therefore unaffected by penicillin?
A) Escherichia coli
B) Streptococcus pneumoniae
C) Mycoplasma genitalium
D) Saccharomyces cerevisiae
Answer: C
Explanation: Mycoplasma species lack a cell wall and are naturally resistant to cell wall-targeting antibiotics.
Why the top distractor is wrong: E. coli has a cell wall (Gram-negative), making it a target for some antibiotics, though not all due to outer membrane protection.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria: peptidoglycan cell wall; Archaea: no peptidoglycan, ether-linked lipids.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Mycoplasma has no cell wall – smallest known cells.
• Red blood cells lack a nucleus and mitochondria.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Prokaryotic DNA is circular and naked; eukaryotic DNA is linear and bound to histones.
• Plasmids: common in bacteria, occasionally in yeasts.
• Binary fission = prokaryotic division; mitosis = eukaryotic somatic division.
• Prokaryotic flagella: rotary, made of flagellin; eukaryotic: 9+2 microtubules, whip-like.
• Cilia: only in eukaryotes (e.g., respiratory epithelium).
• Lysosomes: common in animal cells, rare in plants.
• Plant cells: chloroplasts, central vacuole, plasmodesmata.
• Animal cells: centrioles, lysosomes, no cell wall.
• Peroxisomes: contain catalase, break down H?O?; in both domains but more prominent in eukaryotes.
• Nuclear envelope: double membrane with nuclear pores; allows RNA export.
• Nucleolus: site of rRNA synthesis and ribosome assembly.
• Gram-positive bacteria: thick peptidoglycan, no outer membrane.
• Gram-negative bacteria: thin peptidoglycan + outer membrane with LPS.
• Archaea: extremophiles, ether-linked lipids, no peptidoglycan.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria, chloroplasts from cyanobacteria.