STEM Readiness: Biology Readiness - Genetics/Heredity: Sex-linked Inheritance - X-linked Traits, Carrier Status, Pedigree Analysis

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 organelles such as mitochondria, lysosomes, and the Golgi apparatus.
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting the endosymbiotic theory.
• Cell walls are present in most prokaryotes, composed of peptidoglycan in bacteria; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cells have cellulose-based cell walls; fungal cells have chitin-based walls; animal cells lack a cell wall.
• Mycoplasma, a bacterium, is an exception: it lacks a cell wall and is resistant to antibiotics like penicillin that target peptidoglycan.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport and maintains cell integrity.
• Eukaryotes have internal membranes forming organelles (e.g., nuclear envelope, endoplasmic reticulum), enabling compartmentalization of functions.
• The nucleus in eukaryotes contains chromatin (DNA + histone proteins) and a nucleolus, where ribosomal RNA is synthesized.
• Prokaryotes have circular DNA as their main chromosome; eukaryotes have linear chromosomes organized with histones into chromatin.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; they can carry antibiotic resistance genes and replicate independently.
• Eukaryotic cells undergo mitosis for nuclear division; prokaryotes divide by binary fission.
• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
• Evidence for endosymbiosis includes: mitochondria and chloroplasts have their own circular DNA, 70S ribosomes, and double membranes.
• Red blood cells in mammals are anucleate (lack a nucleus) in maturity, allowing more space for hemoglobin; they cannot divide or repair DNA.
• Plant cells contain chloroplasts for photosynthesis; animal cells lack chloroplasts and central vacuoles.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; they are rare or absent in most plant cells.
• Cilia and flagella in eukaryotes have a 9+2 microtubule arrangement; prokaryotic flagella are composed of flagellin protein and lack microtubules.
• Gram-positive bacteria have a thick peptidoglycan layer and retain crystal violet stain; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, differing from both bacteria and eukaryotes.
• Peroxisomes are single-membrane organelles in eukaryotes that break down fatty acids and detoxify hydrogen peroxide; absent in prokaryotes.
• Centrioles, involved in spindle formation during mitosis, are found in animal cells but not in most plant or fungal cells.

Difficulty Level

Intermediate – requires understanding structural and functional distinctions, with emphasis on comparative features commonly tested in first-year biology.

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 of chitin, and archaea lack peptidoglycan.

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

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

Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have cytoplasm, ribosomes, nucleoid, and sometimes inclusions or protein microcompartments, but no membrane-bound organelles.

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: A membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: Circular DNA is found in prokaryotes and also 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. Which type of cell is it most likely to be?
A) Fungal cell
B) Animal cell
C) Bacterial cell
D) Plant cell
Answer: D
Explanation: Plant cells uniquely have chloroplasts, a large central vacuole, and a cellulose cell wall.
Why the top distractor is wrong: Fungal cells have cell walls but lack chloroplasts and large central vacuoles.

Question: Which structure is present in prokaryotes and eukaryotes?
A) Mitochondria
B) 80S ribosomes
C) Phospholipid bilayer
D) Golgi apparatus
Answer: C
Explanation: Both cell types have a plasma membrane made of a phospholipid bilayer.
Why the top distractor is wrong: 80S ribosomes are only in eukaryotic cytoplasm; prokaryotes have 70S.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Mitochondria are surrounded by a single membrane
B) Mitochondria have 80S ribosomes
C) Mitochondria have circular DNA and 70S ribosomes
D) Mitochondria synthesize all their own proteins
Answer: C
Explanation: Circular DNA and 70S ribosomes in mitochondria resemble those of bacteria, supporting endosymbiosis.
Why the top distractor is wrong: Mitochondria have a double membrane, not single, and do not synthesize all their own proteins.

Question: Which organism lacks a cell wall and is therefore resistant to penicillin?
A) Escherichia coli
B) Bacillus subtilis
C) Mycoplasma pneumoniae
D) Saccharomyces cerevisiae
Answer: C
Explanation: Mycoplasma lacks a cell wall and is naturally resistant to penicillin, which targets peptidoglycan.
Why the top distractor is wrong: E. coli has a cell wall (Gram-negative), so it is not inherently resistant to penicillin.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: DNA in nucleoid; no nuclear membrane.
• Eukaryotes: DNA enclosed in membrane-bound nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria: peptidoglycan cell wall; plants: cellulose; fungi: chitin; animals: no cell wall.
• Mycoplasma has no cell wall – resistant to penicillin.
• Both prokaryotes and eukaryotes have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes do not.
• Nucleolus synthesizes rRNA and assembles ribosomal subunits.
• Prokaryotes have circular chromosome; eukaryotes have linear chromosomes with telomeres.
• Plasmids: small, circular, self-replicating DNA – common in bacteria.
• Binary fission: asexual reproduction in prokaryotes; mitosis in eukaryotes.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria; chloroplasts from cyanobacteria.
• Red blood cells lack a nucleus – cannot divide or repair DNA.
• Plant cells: chloroplasts, central vacuole, plasmodesmata; animal cells: lysosomes, centrioles.
• Lysosomes: contain hydrolytic enzymes; found in animal cells, not typical in plants.
• Prokaryotic flagella made of flagellin; eukaryotic flagella have 9+2 microtubule arrangement.
• Gram-positive: thick peptidoglycan, no outer membrane; Gram-negative: thin peptidoglycan, outer LPS membrane.
• Archaea: no peptidoglycan; ether-linked membrane lipids.
• Peroxisomes: break down fatty acids, detoxify H?O?; absent in prokaryotes.
• Centrioles: present in animal cells, organize spindle fibers; absent in most plants.
• Nuclear envelope: double membrane with nuclear pores; regulates transport.
• Chromatin: DNA wrapped around histone proteins in eukaryotes; prokaryotes lack histones.
• Verify from standard textbook: exact lipid composition in archaeal membranes varies by species.