STEM Readiness: Biology Readiness Genetics Heredity Mitosis vs Meiosis Stages Chromosome Number Genetic Outcomes

Must‑Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 μm in diameter; eukaryotic cells range from 10–100 μm.
• Prokaryotes store DNA in a nucleoid, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• 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, supporting their bacterial origin.
• Most prokaryotes have a cell wall made of peptidoglycan; exceptions include Mycoplasma, which lacks a cell wall entirely.
• Eukaryotic cell walls vary: fungi use chitin, plants use cellulose, and animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Only eukaryotes have membrane-bound organelles, including the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, and nucleus.
• Bacteria and archaea are prokaryotes; archaea lack peptidoglycan and have distinct membrane lipids.
• Animals, plants, fungi, and protists are eukaryotes; all share a common cellular organization.
• The nuclear envelope is a double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
• Chromatin in the eukaryotic nucleus consists of DNA wrapped around histone proteins; prokaryotic DNA is not associated with histones.
• Prokaryotes reproduce by binary fission; eukaryotes use mitosis and meiosis.
• Flagella differ structurally: prokaryotic flagella are made of flagellin and rotate, while eukaryotic flagella are microtubule-based (9+2 arrangement) and undulate.
• Plasmids are small, circular DNA molecules common in prokaryotes; rare in eukaryotes (e.g., 2-micron plasmid in yeast).
• Endosymbiotic theory proposes mitochondria and chloroplasts originated from engulfed prokaryotes; supported by their own circular DNA, 70S ribosomes, and double membranes.
• Red blood cells in mammals lack a nucleus and most organelles, maximizing hemoglobin capacity.
• Plant cells have chloroplasts, a central vacuole, and plasmodesmata; animal cells do not.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; absent in most plant cells.
• The smooth endoplasmic reticulum (SER) synthesizes lipids and detoxifies drugs; the rough ER (RER) has ribosomes and synthesizes proteins.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles.
• Mitochondria generate ATP via oxidative phosphorylation; contain their own genome and replicate independently.
• Peroxisomes break down fatty acids and detoxify hydrogen peroxide; present in both plant and animal eukaryotic cells.
• Cytoskeleton components (microfilaments, intermediate filaments, microtubules) exist only in eukaryotes and maintain cell shape and enable motility.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative bacteria have thin peptidoglycan and an outer membrane with lipopolysaccharide.

Difficulty Level

Intermediate – expected foundational knowledge in first-semester biology but requires precise differentiation of structures and exceptions.

Common Traps (3–5 factual traps)

Trap: All cells with a nucleus are eukaryotic, so any cell without a nucleus must be prokaryotic – Fact: Mature mammalian red blood cells lack a nucleus but are eukaryotic, derived from eukaryotic lineages.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, identical in size to prokaryotes, due to evolutionary origin.
Trap: Cell walls are exclusive to plant cells – Fact: Bacteria (peptidoglycan), fungi (chitin), and plants (cellulose) all have cell walls; animal cells do not.
Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes have intracellular membranes (e.g., thylakoids in cyanobacteria), though not compartmentalized organelles.

Practice MCQs (5–7 questions)

Question: Which structure is found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Peptidoglycan cell wall
D) Plasma membrane
Answer: C
Explanation: Peptidoglycan is a defining component of bacterial cell walls and is absent in eukaryotes.
Why the top distractor is wrong: 70S ribosomes (B) are also in mitochondria and chloroplasts of eukaryotes, so not exclusive to prokaryotes.

Question: Which of the following is a feature shared by mitochondria and prokaryotes?
A) Linear chromosomes
B) 80S ribosomes
C) Double membrane
D) Histone-bound DNA
Answer: C
Explanation: Mitochondria have a double membrane, consistent with endosymbiotic engulfment of a prokaryote.
Why the top distractor is wrong: 80S ribosomes (B) are found in the eukaryotic cytoplasm, not in mitochondria (which have 70S).

Question: A cell is observed to contain a nucleus, mitochondria, and a cell wall made of cellulose. This cell is most likely from a:
A) Fungus
B) Bacterium
C) Plant
D) Animal
Answer: C
Explanation: Cellulose cell walls and the presence of mitochondria and nucleus are characteristic of plant cells.
Why the top distractor is wrong: Fungi (A) have chitin in their cell walls, not cellulose.

Question: Which of the following structures is present in eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Golgi apparatus
D) DNA
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle unique to eukaryotes.
Why the top distractor is wrong: Ribosomes (B) are present in both, though differing in size.

Question: What is the primary function of the nucleolus within the nucleus?
A) DNA replication
B) Ribosome subunit assembly
C) Protein synthesis
D) mRNA splicing
Answer: B
Explanation: The nucleolus is the site of rRNA synthesis and ribosomal subunit assembly.
Why the top distractor is wrong: DNA replication (A) occurs throughout the nucleus, not specifically in the nucleolus.

Question: Which of the following is true regarding the cytoskeleton?
A) It is composed of peptidoglycan in prokaryotes
B) It is absent in plant cells
C) It includes microtubules, microfilaments, and intermediate filaments in eukaryotes
D) It is involved in binary fission only
Answer: C
Explanation: The eukaryotic cytoskeleton consists of microtubules, actin filaments, and intermediate filaments.
Why the top distractor is wrong: Peptidoglycan (A) is a cell wall component, not part of the cytoskeleton.

Question: Which of the following cells lacks mitochondria?
A) Yeast cell
B) Leaf cell
C) Mature human red blood cell
D) Muscle cell
Answer: C
Explanation: Mature human red blood cells lack mitochondria and generate ATP via glycolysis.
Why the top distractor is wrong: Yeast (A) are fungi and have mitochondria for aerobic respiration.

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

• ⚠️ Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Prokaryote size: 0.1–5.0 μm; eukaryote size: 10–100 μm.
• DNA in prokaryotes: nucleoid; in eukaryotes: nucleus.
• Peptidoglycan is found only in bacterial cell walls.
• ⚠️ Mycoplasma has no cell wall – exception among bacteria.
• Archaea lack peptidoglycan and have ether-linked membrane lipids.
• Animal cells: no cell wall, no chloroplasts, have lysosomes.
• Plant cells: cellulose wall, chloroplasts, central vacuole, plasmodesmata.
• Fungal cells: chitin cell wall, no chloroplasts, have lysosomes.
• Nuclear envelope has double membrane and nuclear pores.
• Chromatin = DNA + histone proteins (absent in prokaryotes).
• ⚠️ Mature mammalian RBCs lack a nucleus and organelles.
• Smooth ER: lipid synthesis, detoxification; Rough ER: protein synthesis.
• Golgi apparatus: modifies and packages proteins; has cis and trans faces.
• Lysosomes: contain hydrolytic enzymes, maintain acidic pH.
• Peroxisomes: break down fatty acids, neutralize H₂O₂.
• Cytoskeleton: microfilaments (actin), intermediate filaments, microtubules (tubulin).
• Eukaryotic flagellum: 9+2 microtubule arrangement; prokaryotic: flagellin-based, rotary.
• Plasmids: small circular DNA; common in bacteria, rare in eukaryotes.
• ⚠️ Mitochondria replicate independently of the cell cycle.
• Endosymbiotic theory supported by organelle double membranes, own DNA, and 70S ribosomes.
• ⚠️ Gram-negative bacteria have an outer membrane with lipopolysaccharide (LPS).
• Binary fission: prokaryotic division; no mitosis.
• Verify from standard textbook: exact ribosomal subunit composition (50S/30S in 70S; 60S/40S in 80S).