STEM Readiness: Biology Readiness - Genetics/Heredity: Mendelian Genetics - Monohybrid/Dihybrid Crosses, Extensions Incomplete, Codominance Linkage

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 contain endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes—evidence for endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan and have different wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems (e.g., nuclear envelope, ER, Golgi) forming compartments; prokaryotes do not.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and cytokinesis.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Eukaryotic chromosomes are linear and associated with histone proteins; prokaryotic chromosomes are circular and lack histones (though archaea have histone-like proteins).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and bend.
• Mycoplasma species are prokaryotes without a cell wall—an exception among bacteria.
• Human red blood cells lack a nucleus and mitochondria—a eukaryotic exception adapted for oxygen transport.
• Endosymbiotic theory proposes that mitochondria originated from aerobic bacteria engulfed by ancestral eukaryotic cells.
• Evidence for endosymbiosis includes mitochondrial circular DNA, 70S ribosomes, and double membranes.
• Chloroplasts are found in plants and algae; they perform photosynthesis and also support endosymbiotic theory.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; plants use vacuoles for similar degradation.
• Eukaryotic nuclei contain nuclear pores for regulated transport; prokaryotes lack such structures.
• Cytoskeleton (microtubules, microfilaments, intermediate filaments) is present in eukaryotes; prokaryotes have homologs (e.g., FtsZ) but no true cytoskeleton.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, unlike bacterial and eukaryotic ester-linked lipids.
• Plant cells contain large central vacuoles for turgor pressure; animal cells have small or temporary vacuoles.

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on comparison and exceptions.

Common Traps (3–5 factual traps)

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

Trap: Ribosome size correlates with cell complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, identical in size to prokaryotes, due to evolutionary origin.

Trap: The nucleus is the only organelle that contains DNA in eukaryotes – Fact: Mitochondria and chloroplasts also contain their own DNA (circular, like prokaryotes).

Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based microcompartments (e.g., carboxysomes), 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) Endoplasmic reticulum
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (endoplasmic reticulum) is only found in eukaryotes.

Question: A cell is observed to contain 80S ribosomes, no cell wall, and a large central vacuole. Which organism is it most likely from?
A) Bacterium
B) Fungus
C) Animal
D) Plant
Answer: D
Explanation: Large central vacuole and 80S ribosomes are eukaryotic; absence of cell wall rules out fungus and bacterium; plants have vacuoles, animals do not.
Why the top distractor is wrong: C (animal) lacks a large central vacuole.

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Linear chromosomes in the nucleus
B) 70S ribosomes in mitochondria
C) 80S ribosomes in the cytoplasm
D) Presence of a nucleoid
Answer: B
Explanation: Mitochondria having 70S ribosomes, like prokaryotes, supports their bacterial origin.
Why the top distractor is wrong: C (80S ribosomes) are typical of eukaryotic cytoplasm and not evidence of endosymbiosis.

Question: Which of the following lacks a nucleus but is classified as a eukaryote?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Human red blood cell
D) Anabaena
Answer: C
Explanation: Mature human red blood cells lose their nucleus but originate from eukaryotic cells.
Why the top distractor is wrong: A (E. coli) is a prokaryote, not a eukaryote.

Question: What is the primary 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: A (peptidoglycan) is found in bacterial, not fungal, cell walls.

Question: Which of the following best describes the DNA in prokaryotes?
A) Linear, multiple chromosomes in a nucleus
B) Circular, single chromosome in the nucleoid
C) Linear, single chromosome in mitochondria
D) Circular, multiple chromosomes in the Golgi
Answer: B
Explanation: Prokaryotes typically have a single circular chromosome located in the nucleoid.
Why the top distractor is wrong: A describes eukaryotic nuclear DNA, not prokaryotic.

Question: Which cell type has a double membrane surrounding its DNA?
A) Prokaryote
B) Mitochondrion
C) Lysosome
D) Ribosome
Answer: B
Explanation: Mitochondria have a double membrane and contain their own DNA.
Why the top distractor is wrong: A (prokaryote) lacks a membrane around its nucleoid.

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

• Prokaryotic 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.
• Bacteria: peptidoglycan cell wall; Archaea: no peptidoglycan.
• Plant cell wall: cellulose; fungal: chitin; animal: no cell wall.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• Plasmids are common in prokaryotes, rare in eukaryotes.
• Eukaryotic chromosomes are linear with histones; prokaryotic are circular without histones (mostly).
• Prokaryotic flagella: flagellin, rotary motion; eukaryotic: microtubules (9+2), bending motion.
• Mycoplasma – no cell wall, still a prokaryote.
• Human red blood cells – no nucleus, no mitochondria, but eukaryotic.
• Mitochondria originated from aerobic bacteria; chloroplasts from cyanobacteria.
• Lysosomes: animal cells; degradation via hydrolytic enzymes.
• Plant vacuoles perform lysosomal functions and maintain turgor pressure.
• Nuclear pores regulate molecular traffic in eukaryotic nuclei.
• Cytoskeleton: eukaryotes only (microtubules, actin filaments).
• FtsZ protein in prokaryotes is a tubulin homolog involved in division.
• Gram-positive: thick peptidoglycan; Gram-negative: thin peptidoglycan + outer membrane.
• Archaeal membranes have ether-linked lipids; bacteria and eukaryotes have ester-linked.
• Chloroplasts found in plants and algae, not in animals or fungi.
• Organelles with own DNA: nucleus, mitochondria, chloroplasts.
• Verify from standard textbook: exact lipid composition in archaea varies widely.