STEM Readiness: Biology Readiness - Molecular Biology: Mutations - Types, Substitution, Insertion, Deletion, Consequences, Repair Mechanisms

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• DNA in prokaryotes is located in the nucleoid, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes have 70S ribosomes (30S + 50S subunits); eukaryotes have 80S ribosomes (40S + 60S subunits).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Mycoplasma species are bacteria that lack a cell wall, making them resistant to penicillin.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membrane systems (e.g., endoplasmic reticulum, Golgi apparatus); prokaryotes do not.
• Organelles unique to eukaryotes include the nucleus, mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus, and chloroplasts (in plants).
• Bacteria and archaea are prokaryotes; animals, plants, fungi, and protists are eukaryotes.
• Red blood cells in mammals lack a nucleus and most organelles, maximizing hemoglobin capacity.
• Sperm cells are eukaryotic but have highly condensed nuclei and reduced cytoplasm.
• Chloroplasts are found only in plant cells and some protists (e.g., Chlamydomonas); absent in fungi and animals.
• Lysosomes are membrane-bound organelles in animal cells that contain hydrolytic enzymes; plants use vacuoles for similar functions.
• Peroxisomes are present in both plant and animal cells and are involved in fatty acid breakdown and detoxification of hydrogen peroxide.
• The nuclear envelope is a double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
• Nucleolus is located within the nucleus and is the site of ribosomal RNA synthesis and ribosome assembly.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own circular DNA, 70S ribosomes, and ability to replicate independently of the cell.
• Mitochondria are present in nearly all eukaryotic cells (except some anaerobic protists like Giardia); absent in prokaryotes.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Cilia are short, numerous motile structures in some eukaryotic cells (e.g., tracheal epithelium); absent in prokaryotes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes (e.g., 2? plasmid in Saccharomyces cerevisiae).
• Cell division in prokaryotes occurs by binary fission; eukaryotes use mitosis and meiosis.
• Cytoskeleton components (microtubules, microfilaments, intermediate filaments) are present in eukaryotes; prokaryotes have homologs (e.g., FtsZ) but no true cytoskeleton.

Difficulty Level

Intermediate – requires precise differentiation between structural and functional features across domains and organelles.

Common Traps (3–5 factual traps)

• Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; archaea, plants (cellulose), and fungi (chitin) have different wall compositions.
• Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
• Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
• Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., cyanobacteria) have thylakoid membranes for photosynthesis.
• Trap: Eukaryotic cells always have a nucleus – Fact: Mature mammalian red blood cells lack a nucleus.

Practice MCQs (5–7 questions)

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

Question: A cell is observed to have a cell wall made of cellulose, chloroplasts, and a large central vacuole. It is most likely:
A) Fungal cell
B) Animal cell
C) Plant cell
D) Bacterial cell
Answer: C
Explanation: Cellulose cell walls, chloroplasts, and large central vacuoles are characteristic of plant cells.
Why the top distractor is wrong: A is incorrect because fungal cell walls are made of chitin, not cellulose.

Question: Which structure is present in eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Golgi apparatus
D) Circular DNA
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle unique to eukaryotes.
Why the top distractor is wrong: A is incorrect because both cell types have a plasma membrane.

Question: Mitochondria are thought to have evolved from free-living bacteria due to all of the following EXCEPT:
A) They contain circular DNA
B) They have 70S ribosomes
C) They are surrounded by a single membrane
D) They replicate independently by binary fission
Answer: C
Explanation: Mitochondria have a double membrane, consistent with endosymbiotic engulfment.
Why the top distractor is wrong: A is factually correct and supports endosymbiosis; C is false and thus the correct answer.

Question: Which of the following cells lacks a nucleus?
A) Neuron
B) Leukocyte
C) Mature erythrocyte
D) Hepatocyte
Answer: C
Explanation: Mature mammalian red blood cells (erythrocytes) expel their nucleus during development.
Why the top distractor is wrong: A is incorrect because neurons are fully differentiated eukaryotic cells with nuclei.

Question: What is the sedimentation coefficient of cytoplasmic ribosomes in eukaryotic cells?
A) 50S
B) 70S
C) 80S
D) 30S
Answer: C
Explanation: Eukaryotic cytoplasmic ribosomes are 80S (40S + 60S subunits).
Why the top distractor is wrong: B is incorrect because 70S ribosomes are found in prokaryotes and organelles.

Question: Which of the following is found in both prokaryotes and eukaryotes?
A) Nucleolus
B) Peptidoglycan
C) Phospholipid bilayer
D) Endoplasmic reticulum
Answer: C
Explanation: Both domains have a plasma membrane composed of a phospholipid bilayer.
Why the top distractor is wrong: A is incorrect because the nucleolus is found only in eukaryotic nuclei.

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 membrane-bound nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria have peptidoglycan; archaea do not; plants have cellulose; fungi have chitin.
• Animal cells lack cell walls and chloroplasts.
• Mycoplasma has no cell wall – smallest known cells.
• Both prokaryotes and eukaryotes have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes generally do not.
• Lysosomes are in animal cells; plants use vacuoles for degradation.
• Red blood cells (erythrocytes) lack a nucleus and mitochondria.
• Nuclear envelope has double membrane and nuclear pores.
• Nucleolus = site of rRNA synthesis and ribosome subunit assembly.
• Mitochondria generate ATP via oxidative phosphorylation.
• Chloroplasts perform photosynthesis; found in plants and some protists.
• Flagellin = prokaryotic flagella protein; 9+2 microtubule array = eukaryotic flagella.
• Cilia and flagella in eukaryotes use dynein motor proteins for movement.
• Plasmids are common in bacteria; rare in eukaryotes.
• Binary fission = prokaryotic division; mitosis/meiosis = eukaryotic.
• Cytoskeleton (microtubules, actin) is eukaryotic; prokaryotes have FtsZ, MreB, CreS.
• Mitochondria have their own DNA and replicate independently.
• Endosymbiotic theory supported by organelle size, ribosomes, and DNA structure.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Golgi apparatus modifies, sorts, and packages proteins for secretion.
• Verify from standard textbook: Some prokaryotes (e.g., Gemmata) have internal membrane structures.