4.7 Cell Biology and Genetics
Key Takeaways
- Eukaryotic cells have a nucleus and membrane-bound organelles; prokaryotic cells do not
- Mitochondria produce ATP; ribosomes synthesize proteins; nucleus contains DNA
- DNA bases pair A-T and G-C; RNA has U instead of T
- Mitosis produces 2 identical diploid cells; meiosis produces 4 different haploid cells
- Dominant alleles are expressed with one copy; recessive require two copies
Last updated: January 2026
Understanding cell structure, function, and genetics is fundamental to the TEAS Science section. This covers cells, DNA, cell division, and heredity.
Cell Structure
Prokaryotic vs. Eukaryotic Cells:
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
| Size | Smaller (1-10 μm) | Larger (10-100 μm) |
| Examples | Bacteria, archaea | Plants, animals, fungi |
| Organelles | Few, no membrane-bound | Many membrane-bound |
Eukaryotic Cell Organelles
| Organelle | Function |
|---|---|
| Nucleus | Contains DNA, controls cell activities |
| Mitochondria | Produces ATP (cellular respiration) |
| Ribosomes | Synthesizes proteins |
| Endoplasmic reticulum (ER) | Protein (rough) and lipid (smooth) synthesis |
| Golgi apparatus | Modifies, packages, and ships proteins |
| Lysosomes | Digests waste and foreign materials |
| Cell membrane | Controls what enters/exits cell |
| Cytoplasm | Gel-like fluid containing organelles |
Plant cell only:
- Cell wall (cellulose)
- Chloroplasts (photosynthesis)
- Central vacuole (storage, structure)
Cell Membrane Structure
The fluid mosaic model describes the cell membrane:
- Phospholipid bilayer (hydrophilic heads, hydrophobic tails)
- Embedded proteins (transport, receptors)
- Cholesterol (membrane fluidity)
- Carbohydrates (cell recognition)
Transport Across Membranes
| Type | Energy | Direction | Examples |
|---|---|---|---|
| Passive diffusion | No | High → Low concentration | O₂, CO₂ |
| Facilitated diffusion | No | High → Low (with proteins) | Glucose |
| Osmosis | No | Water moves toward solute | Water balance |
| Active transport | Yes | Low → High concentration | Na⁺/K⁺ pump |
DNA and RNA
| Feature | DNA | RNA |
|---|---|---|
| Structure | Double helix | Single strand |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Location | Nucleus | Nucleus and cytoplasm |
| Function | Stores genetic info | Protein synthesis |
DNA Replication
Process: DNA makes a copy of itself before cell division Key enzyme: DNA polymerase Result: Two identical DNA molecules
Base pairing rules:
- Adenine (A) pairs with Thymine (T)
- Guanine (G) pairs with Cytosine (C)
Protein Synthesis
| Step | Location | Process |
|---|---|---|
| Transcription | Nucleus | DNA → mRNA |
| Translation | Ribosome | mRNA → Protein |
Key players:
- mRNA: Carries genetic code from DNA
- tRNA: Brings amino acids to ribosome
- rRNA: Part of ribosome structure
Cell Division
Mitosis: Cell division for growth and repair
- Result: 2 identical daughter cells (diploid)
- Phases: Prophase → Metaphase → Anaphase → Telophase
Meiosis: Cell division for gamete (sex cell) production
- Result: 4 genetically different cells (haploid)
- Creates genetic diversity through crossing over
Genetics Terminology
| Term | Definition |
|---|---|
| Gene | Segment of DNA coding for a trait |
| Allele | Different versions of a gene |
| Genotype | Genetic makeup (e.g., Bb) |
| Phenotype | Observable trait (e.g., brown eyes) |
| Dominant | Expressed when one copy present (B) |
| Recessive | Expressed only when two copies present (bb) |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
Punnett Squares
Used to predict offspring genotypes and phenotypes.
Example: Cross Bb × Bb
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Genotype ratio: 1 BB : 2 Bb : 1 bb
- Phenotype ratio: 3 dominant : 1 recessive
Test Your Knowledge
Which organelle is responsible for producing ATP?
A
B
C
D
Test Your Knowledge
What is the result of meiosis?
A
B
C
D
Test Your Knowledge
If a parent has genotype Bb, what percentage of gametes will carry the B allele?
A
B
C
D