STEM Readiness: Biology Readiness - Molecular Biology: DNA Structure - Double Helix, Base Pairing, Antiparallel Strands, Chargaffs Rules

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, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes possess organelles such as mitochondria, lysosomes, and the 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.
• Cell walls are present in most prokaryotes, made of peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cells have cellulose in their cell walls; fungal cells have chitin; animal cells lack a cell wall entirely.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membranes forming organelles (e.g., endoplasmic reticulum, nuclear envelope), absent in prokaryotes.
• The nucleus in eukaryotes contains chromatin (DNA + histone proteins) and a nucleolus where ribosomal RNA is synthesized.
• Nuclear pores regulate transport between nucleus and cytoplasm; prokaryotes lack such structures.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes, used in gene transfer.
• Eukaryotic cells may have centrioles (in animals) involved in spindle formation; prokaryotes lack centrioles.
• Lysosomes (containing hydrolytic enzymes) are found only in animal cells, not in prokaryotes or plant cells.
• Chloroplasts are eukaryotic organelles for photosynthesis, present in plants and algae, absent in prokaryotes (except cyanobacteria, which lack chloroplasts but have thylakoids).
• Cyanobacteria are prokaryotes capable of oxygenic photosynthesis using thylakoid membranes, not chloroplasts.
• Mycoplasma species are bacteria without a cell wall, an exception among prokaryotes.
• Mature mammalian red blood cells lack a nucleus and mitochondria, an exception among eukaryotic cells.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Mitochondria are believed to have originated from aerobic bacteria; chloroplasts from photosynthetic cyanobacteria.
• Eukaryotic cilia and flagella are covered by the plasma membrane; prokaryotic flagella are not.
• Archaea have ether-linked lipids in their membranes (vs ester-linked in bacteria and eukaryotes), a key biochemical distinction.

Difficulty Level

Intermediate – requires distinguishing structural, genetic, and biochemical features across domains and organelles, commonly tested in first-semester biology.

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 (no peptidoglycan) differ.
Trap: Eukaryotes are always larger and more complex than prokaryotes – Fact: While generally true, some prokaryotes (e.g., Thiomargarita namibiensis, ~750 ?m) exceed typical eukaryotic cells in size.
Trap: Ribosome size correlates directly with cell complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, like prokaryotes, due to evolutionary origin.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes (outer and inner).
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have cytoskeletal elements (e.g., FtsZ), ribosomes, and nucleoids, though no membrane-bound organelles.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) Mitochondria
C) Phospholipid bilayer plasma membrane
D) 80S ribosomes
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (80S ribosomes) is only found in eukaryotic cytoplasm; prokaryotes have 70S.

Question: Which structure is found in plant cells but not in animal cells?
A) Lysosome
B) Centriole
C) Cell wall made of cellulose
D) Plasma membrane
Answer: C
Explanation: Plant cells have a cellulose-based cell wall; animal cells lack a cell wall.
Why the top distractor is wrong: A (lysosome) is present in animal cells and some plant vacuoles, but not a defining absence.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Use of oxygen in metabolism
C) Containing 70S ribosomes and circular DNA
D) Ability to divide independently
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic traits, supporting bacterial origin.
Why the top distractor is wrong: A (double membrane) is suggestive but not exclusive to endosymbiosis; it could form via invagination.

Question: A cell is observed to have a nucleus, mitochondria, and a cell wall containing chitin. From which organism is this cell most likely derived?
A) Bacterium
B) Alga
C) Fungus
D) Plant
Answer: C
Explanation: Fungi are eukaryotes with chitin in their cell walls.
Why the top distractor is wrong: B (alga) may have a cell wall but typically contains cellulose, not chitin.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 80S, including those in mitochondria
B) Cytoplasmic ribosomes are 80S; mitochondrial ribosomes are 70S
C) Ribosomes are only found attached to the rough ER
D) Ribosomes are enclosed within membrane-bound organelles
Answer: B
Explanation: Mitochondrial ribosomes are 70S, similar to prokaryotes; cytoplasmic are 80S.
Why the top distractor is wrong: A is false because mitochondrial ribosomes are 70S, not 80S.

Question: Which organism lacks a cell wall and a nucleus?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Human red blood cell
Answer: D
Explanation: Mature human red blood cells lack both a nucleus and a cell wall.
Why the top distractor is wrong: C (Mycoplasma) lacks a cell wall but has DNA in a nucleoid (prokaryotic nucleus equivalent).

Question: What is the primary structural component of the bacterial cell wall?
A) Chitin
B) Cellulose
C) Peptidoglycan
D) Lipopolysaccharide
Answer: C
Explanation: Peptidoglycan is the defining component of bacterial cell walls.
Why the top distractor is wrong: D (lipopolysaccharide) is found in the outer membrane of Gram-negative bacteria but not the primary structural component.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: DNA in nucleoid; eukaryotes: DNA in nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacterial cell wall contains peptidoglycan; archaea lack it.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both cell types have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear envelope has nuclear pores for RNA/protein transport.
• Prokaryotes divide by binary fission; eukaryotes by mitosis/meiosis.
• Prokaryotic flagellum = flagellin, rotates; eukaryotic = microtubules (9+2), bends.
• Plasmids = small circular DNA in prokaryotes.
• Centrioles present in animal cells, absent in prokaryotes and most plants.
• Lysosomes mainly in animal cells; plant vacuoles can have degradative function.
• Cyanobacteria perform photosynthesis without chloroplasts – use thylakoid membranes.
• Mycoplasma – no cell wall, smallest known bacteria.
• Mature human RBCs – no nucleus, no mitochondria.
• Mitochondria likely evolved from aerobic proteobacteria.
• Chloroplasts likely evolved from photosynthetic cyanobacteria.
• Eukaryotic cilia/flagella are membrane-covered; prokaryotic are not.
• Archaeal membranes have ether-linked lipids; bacteria and eukaryotes have ester-linked.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Smooth ER = lipid synthesis, detoxification; rough ER = protein synthesis.
• Golgi apparatus = modifies, sorts, packages proteins for secretion.
• Verify from standard textbook: exact lipid composition in archaea membranes.