STEM Readiness: Biology Readiness - Genetics/Heredity: Population Genetics - Hardy-Weinberg Equilibrium, Calculations and Violations

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 not enclosed by a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, endoplasmic reticulum, and lysosomes.
• Prokaryotic ribosomes 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.
• Most prokaryotes have a cell wall made of peptidoglycan; archaea have cell walls made of pseudopeptidoglycan or other polysaccharides.
• Plant cells have a cell wall of cellulose; fungal cells have a cell wall of chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a plasma membrane composed of a phospholipid bilayer with embedded proteins.
• Eukaryotes have internal membrane systems including the nuclear envelope, endoplasmic reticulum, and Golgi apparatus; prokaryotes do not.
• The nucleus in eukaryotes is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for RNA and protein transport.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and meiosis.
• Prokaryotic DNA is typically a single circular chromosome; eukaryotic DNA is organized into multiple linear chromosomes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; they can carry antibiotic resistance genes.
• Eukaryotic cells may have centrioles involved in spindle formation during cell division; prokaryotes lack centrioles.
• Flagella in prokaryotes are made of flagellin and rotate like a propeller; eukaryotic flagella are made of microtubules (9+2 arrangement) and move in a whip-like motion.
• Mycoplasma species are prokaryotes without a cell wall, making them resistant to antibiotics like penicillin that target peptidoglycan.
• Human red blood cells are eukaryotic cells without a nucleus in their mature form, allowing more space for hemoglobin.
• Chloroplasts are eukaryotic organelles found in plants and algae that perform photosynthesis; they contain thylakoids and chlorophyll.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes for digestion; they are rare or absent in plant cells.
• Peroxisomes contain catalase and break down fatty acids and hydrogen peroxide; present in both plant and animal eukaryotic cells.
• The endosymbiotic theory proposes that mitochondria and chloroplasts originated from engulfed prokaryotes; evidence includes their own DNA, 70S ribosomes, and binary fission.
• Cilia in eukaryotes have the 9+2 microtubule arrangement and are structurally distinct from prokaryotic pili, which are made of pilin and used for attachment or conjugation.
• Gram-positive bacteria have a thick peptidoglycan layer and retain crystal violet stain; Gram-negative bacteria have a thin peptidoglycan layer and outer membrane with lipopolysaccharide.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, unlike the ester-linked lipids in bacteria and eukaryotes.

Difficulty Level

Intermediate – routinely tested in first-year biology courses with emphasis on comparative cell biology and organelle function.

Common Traps (3–5 factual traps)

Trap: All cells with a nucleus are eukaryotic, so red blood cells are prokaryotic – Fact: Mature human red blood cells are eukaryotic cells that lost their nucleus during development; they originate from eukaryotic stem cells.

Trap: Mitochondria are unique to animal cells – Fact: Mitochondria are in nearly all eukaryotic cells, including plants, fungi, and protists; only some anaerobic eukaryotes lack them.

Trap: Prokaryotes have no DNA packaging – Fact: Prokaryotic DNA is supercoiled and associated with nucleoid-associated proteins, though it lacks histones and chromatin structure.

Trap: Eukaryotic ribosomes are always 80S – Fact: Mitochondria and chloroplasts have 70S ribosomes, reflecting their prokaryotic origin.

Trap: Cell walls are exclusive to prokaryotes – Fact: Plants, fungi, and some protists have cell walls; only animal cells lack them among eukaryotes.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Presence of mitochondria
B) Membrane-bound nucleus
C) 80S ribosomes in the cytoplasm
D) DNA located in a nucleoid region
Answer: D
Explanation: Prokaryotes lack a membrane-bound nucleus; their DNA is in the nucleoid.
Why the top distractor is wrong: Distractor B is incorrect because a membrane-bound nucleus is a eukaryotic feature.

Question: Which structure is found in plant cells but not in animal cells?
A) Lysosome
B) Mitochondrion
C) Cellulose cell wall
D) Plasma membrane
Answer: C
Explanation: Plant cells have a rigid cell wall made of cellulose; animal cells do not.
Why the top distractor is wrong: Lysosomes (A) are common in animal cells and sometimes in plant cells, but not exclusive to plants.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Eukaryotic cells have linear chromosomes
B) Mitochondria have their own 70S ribosomes
C) Prokaryotes divide by binary fission
D) The nucleus is surrounded by a double membrane
Answer: B
Explanation: Mitochondria having 70S ribosomes, like prokaryotes, supports their evolutionary origin from bacteria.
Why the top distractor is wrong: While binary fission (C) occurs in mitochondria, it is not unique evidence; 70S ribosomes are more specific.

Question: Which organism lacks a cell wall?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Arabidopsis thaliana
Answer: C
Explanation: Mycoplasma is a bacterium without a cell wall, unlike E. coli (peptidoglycan), yeast (chitin), or plants (cellulose).
Why the top distractor is wrong: E. coli (A) has a peptidoglycan cell wall and is a common model prokaryote.

Question: What is the primary structural component of fungal cell walls?
A) Peptidoglycan
B) Cellulose
C) Chitin
D) Lipopolysaccharide
Answer: C
Explanation: Fungi have cell walls made of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: Peptidoglycan (A) is found in bacterial cell walls, not fungal.

Question: Which of the following is true of both prokaryotes and eukaryotes?
A) Presence of mitochondria
B) 80S ribosomes
C) Phospholipid bilayer plasma membrane
D) Membrane-bound nucleus
Answer: C
Explanation: Both cell types have a plasma membrane made of a phospholipid bilayer.
Why the top distractor is wrong: Mitochondria (A) are only in eukaryotes; prokaryotes generate ATP at the plasma membrane.

Question: Where is DNA located in a eukaryotic cell?
A) Nucleoid and mitochondria
B) Nucleus and lysosomes
C) Nucleus and mitochondria
D) Cytoplasm and Golgi apparatus
Answer: C
Explanation: Eukaryotic DNA is primarily in the nucleus, with additional DNA in mitochondria (and chloroplasts in plants).
Why the top distractor is wrong: The nucleoid (A) is a prokaryotic structure; lysosomes (B) do not contain DNA.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: nucleoid; eukaryotes: nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Peptidoglycan = bacterial cell wall; chitin = fungi; cellulose = plants; no wall = animals.
• Archaea lack peptidoglycan and have ether-linked membrane lipids.
• Mycoplasma – smallest known cells, no cell wall.
• Mature human red blood cells lack a nucleus but are eukaryotic.
• Binary fission = prokaryotic division; mitosis/meiosis = eukaryotic.
• Prokaryotic DNA: single circular chromosome; eukaryotic: multiple linear chromosomes.
• Plasmids are small, circular DNA molecules common in bacteria.
• Eukaryotic flagella: 9+2 microtubule arrangement; prokaryotic: flagellin-based, rotary motion.
• Gram-positive bacteria = thick peptidoglycan; Gram-negative = thin peptidoglycan + outer membrane.
• Nuclear pores regulate transport between nucleus and cytoplasm.
• Endoplasmic reticulum (ER): rough ER has ribosomes; smooth ER lacks them.
• Golgi apparatus modifies, sorts, and packages proteins for secretion.
• Lysosomes contain hydrolytic enzymes; primarily in animal cells.
• Peroxisomes break down fatty acids and detoxify H?O? using catalase.
• Centrioles assist in microtubule organization in animal cell division.
• Chloroplasts perform photosynthesis; contain thylakoids and chlorophyll.
• Cilia and flagella in eukaryotes are made of microtubules (9+2).
• Pili in prokaryotes are made of pilin; used for adhesion or conjugation.
• Mitochondria divide by binary fission, like bacteria.
• Archaea are prokaryotes but differ from bacteria in membrane chemistry and genetics.
• All cells, prokaryotic and eukaryotic, have a phospholipid bilayer plasma membrane.
• Verify from standard textbook: exact protein composition of nuclear pore complex.