STEM Readiness: Biology Readiness - Molecular Biology: Transcription and Translation - Central Dogma, mRNA Processing, Codon Table

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 lack a membrane-bound nucleus; DNA is located in the nucleoid region (e.g., Escherichia coli).
• Eukaryotes have a membrane-bound nucleus housing linear DNA (e.g., human hepatocyte).
• Prokaryotic ribosomes are 70S (30S + 50S subunits); eukaryotic cytoplasmic ribosomes are 80S (40S + 60S).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting endosymbiotic origin.
• Prokaryotes have a cell wall made of peptidoglycan (e.g., Staphylococcus aureus), except Mycoplasma, which lacks a cell wall.
• Archaea lack peptidoglycan; their cell walls are made of pseudopeptidoglycan or other polysaccharides (e.g., Methanobacterium).
• Plant cells have a cellulose cell wall; fungal cells have a chitin cell wall; 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., endoplasmic reticulum, Golgi apparatus); prokaryotes do not.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport (e.g., import of transcription factors).
• Nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome assembly (e.g., in human fibroblasts).
• Endoplasmic reticulum (ER): Rough ER has ribosomes and synthesizes proteins (e.g., insulin in beta cells); smooth ER synthesizes lipids and detoxifies (e.g., in liver cells).
• Golgi apparatus modifies, sorts, and packages proteins into vesicles (e.g., glycosylation of lysosomal enzymes).
• Mitochondria are sites of aerobic respiration and have double membranes; inner membrane contains cristae (e.g., abundant in cardiac muscle).
• Chloroplasts (in plants and algae) perform photosynthesis, contain thylakoids, and have chlorophyll (e.g., in Mesophyll cells).
• Lysosomes contain hydrolytic enzymes for intracellular digestion (e.g., breakdown of phagocytosed bacteria in macrophages).
• Peroxisomes break down fatty acids via beta-oxidation and detoxify hydrogen peroxide (e.g., in liver and kidney cells).
• Cytoskeleton components: microtubules (tubulin, 25 nm diameter), microfilaments (actin, 7 nm), intermediate filaments (e.g., keratin, 10 nm).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 array) and undulate.
• Centrioles (composed of microtubules in 9 triplet sets) are present in animal cells and organize spindle fibers; absent in higher plants.
• Plasmodesmata in plants and gap junctions in animals allow intercellular communication; tight junctions seal epithelial cells.
• Endosymbiotic theory is supported by: mitochondria/chloroplasts having circular DNA, 70S ribosomes, double membranes, and independent division.
• Red blood cells (erythrocytes) in mammals lack a nucleus and organelles, maximizing hemoglobin capacity.
• Sieve tube elements in plants lack nuclei at maturity but are supported by companion cells.

Difficulty Level

Intermediate – requires precise differentiation of structures and exceptions commonly tested in first-year biology exams.

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 cell 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 structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based microcompartments (e.g., carboxysomes).
Trap: Eukaryotic DNA is always linear – Fact: Mitochondrial and chloroplast DNA is circular, like prokaryotic DNA.

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) 80S ribosomes
C) Phospholipid bilayer plasma membrane
D) Mitochondria
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D is incorrect because mitochondria are only in eukaryotes.

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

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Presence of 80S ribosomes in the cytoplasm
B) Circular DNA in mitochondria
C) Linear DNA in the nucleus
D) Peptidoglycan in bacterial cell walls
Answer: B
Explanation: Mitochondria have circular DNA similar to bacteria, supporting their prokaryotic origin.
Why the top distractor is wrong: A describes a eukaryotic feature but does not support endosymbiosis.

Question: Which of the following cells lacks a nucleus at maturity?
A) Neuron
B) Leukocyte
C) Erythrocyte
D) Hepatocyte
Answer: C
Explanation: Mature mammalian red blood cells (erythrocytes) expel their nucleus to carry more hemoglobin.
Why the top distractor is wrong: B (leukocyte) is a white blood cell and retains its nucleus for gene expression.

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 refers to prokaryotic ribosomes, not eukaryotic cytoplasmic ones.

Question: Which organelle is responsible for synthesizing lipids and detoxifying drugs in liver cells?
A) Rough endoplasmic reticulum
B) Golgi apparatus
C) Smooth endoplasmic reticulum
D) Lysosome
Answer: C
Explanation: Smooth ER synthesizes lipids and detoxifies substances, especially in hepatocytes.
Why the top distractor is wrong: A synthesizes proteins, not lipids or detox enzymes.

Question: Which of the following is found in animal cells but not in most plant cells?
A) Mitochondria
B) Centrioles
C) Plasma membrane
D) Nucleus
Answer: B
Explanation: Centrioles organize microtubules in animal cells; most plant cells lack them.
Why the top distractor is wrong: A, C, and D are present in both animal and plant cells.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacterial cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Mycoplasma is a bacterium without a cell wall.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes do not.
• Nuclear pores regulate molecule transport in and out of nucleus.
• Nucleolus = site of rRNA synthesis and ribosome subunit assembly.
• Rough ER = protein synthesis (with ribosomes); smooth ER = lipid synthesis, detoxification.
• Golgi apparatus = modifies, sorts, packages proteins into vesicles.
• Mitochondria = ATP production via aerobic respiration; double membrane, cristae.
• Chloroplasts = photosynthesis; contain thylakoids and chlorophyll.
• Lysosomes = contain acid hydrolases for digestion; low internal pH.
• Peroxisomes = break down fatty acids, detoxify H?O?.
• Cytoskeleton: microtubules (25 nm), microfilaments (7 nm), intermediate filaments (10 nm).
• Prokaryotic flagella = flagellin, rotary motion; eukaryotic = 9+2 microtubules, bending motion.
• Centrioles present in animal cells, absent in higher plants.
• Plasmodesmata = plant intercellular channels; gap junctions = animal equivalent.
• Mature mammalian erythrocytes lack nucleus and organelles.
• Sieve tube elements in phloem lack nuclei; depend on companion cells.
• Endosymbiotic theory supported by: circular DNA, 70S ribosomes, double membranes, binary fission in mitochondria/chloroplasts.
• Verify from standard textbook: exact S values for ribosomal subunits in mitochondria across species.