4.7 Cell Biology and Genetics
Key Takeaways
- Eukaryotic cells have a membrane-bound nucleus and organelles; prokaryotic cells (bacteria) do not
- Mitochondria make ATP, ribosomes build proteins, the nucleus stores DNA, and the cell membrane controls transport
- Passive transport (diffusion, facilitated diffusion, osmosis) needs no energy; active transport uses ATP to move against the gradient
- Mitosis makes 2 identical diploid cells for growth and repair; meiosis makes 4 genetically different haploid gametes
- DNA bases pair A-T and G-C (RNA uses U for T); a heterozygous parent (Bb) passes each allele to 50% of gametes
The Cell as the Unit of Life
Every living thing is built from cells, and the ATI TEAS 7 Science section expects you to know what each organelle does, how substances cross the membrane, how cells divide, and how traits are inherited. These topics tie directly to nursing: drug action depends on membrane transport, cell division explains both healing and cancer, and genetics underlies inherited disease.
Prokaryotic vs. Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
| Size | Smaller (1–10 micrometers) | Larger (10–100 micrometers) |
| Organelles | Few, none membrane-bound | Many membrane-bound |
| Examples | Bacteria, archaea | Plants, animals, fungi |
The single biggest distinction: prokaryotes have no nucleus, so their DNA floats freely in the cytoplasm. Eukaryotes wall their DNA inside a nucleus.
Eukaryotic Organelles and Their Jobs
| Organelle | Function |
|---|---|
| Nucleus | Stores DNA and directs cell activities |
| Mitochondria | Produce ATP through cellular respiration (the "powerhouse") |
| Ribosomes | Synthesize proteins |
| Endoplasmic reticulum | Rough ER makes proteins; smooth ER makes lipids |
| Golgi apparatus | Modifies, packages, and ships proteins |
| Lysosomes | Digest waste and foreign material |
| Cell membrane | Controls what enters and exits |
| Cytoplasm | Gel-like fluid holding the organelles |
Plant cells add three structures animal cells lack: a cell wall (cellulose), chloroplasts (photosynthesis), and a large central vacuole (storage and structure).
The Cell Membrane and Transport
The fluid mosaic model describes the membrane as a phospholipid bilayer (water-loving heads facing out, water-fearing tails inside) studded with proteins, cholesterol for fluidity, and surface carbohydrates for recognition. The TEAS loves to test how molecules cross it.
| Type | Energy? | Direction | Example |
|---|---|---|---|
| Simple diffusion | No | High to low concentration | O2, CO2 |
| Facilitated diffusion | No | High to low (via proteins) | Glucose |
| Osmosis | No | Water toward higher solute | Water balance |
| Active transport | Yes (ATP) | Low to high concentration | Sodium-potassium pump |
The trick to every transport question: passive transport moves substances down the gradient and costs no energy, while active transport pushes against the gradient and requires ATP. Osmosis is simply the diffusion of water.
DNA, RNA, and Replication
| Feature | DNA | RNA |
|---|---|---|
| Structure | Double helix | Single strand |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Job | Stores genetic information | Carries out protein synthesis |
Base-pairing rules are heavily tested: A pairs with T and G pairs with C in DNA; RNA swaps in uracil (U) for thymine. In DNA replication, the enzyme DNA polymerase unzips the helix and builds a matching strand on each half, producing two identical molecules before division.
Protein Synthesis in Two Steps
- Transcription (in the nucleus) — DNA is copied into messenger RNA (mRNA).
- Translation (at the ribosome) — the mRNA code is read in three-base units to assemble amino acids into a protein.
Cell Division: Mitosis vs. Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells | 2 | 4 |
| Chromosome number | Diploid (full set) | Haploid (half set) |
| Genetically | Identical to parent | Different (genetic variation) |
| Purpose | Growth and repair | Produce gametes |
Mitosis proceeds through prophase, metaphase, anaphase, and telophase (PMAT), producing two identical diploid cells for growth and tissue repair. Meiosis has two rounds of division and produces four genetically different haploid gametes. The variation comes from crossing over (homologous chromosomes swap segments during prophase I) and independent assortment.
Genetics and the Punnett Square
An allele is a version of a gene. A dominant allele (capital letter) shows its trait with just one copy; a recessive allele (lowercase) needs two copies to appear. Genotype is the genetic makeup (e.g., Bb); phenotype is the visible trait. A Punnett square predicts offspring ratios.
Worked Example: Cross two heterozygous brown-eyed parents (Bb x Bb), where B (brown) is dominant over b (blue). Each parent passes B to half its gametes and b to the other half. The square gives:
- 1 BB : 2 Bb : 1 bb
- Genotype ratio 1:2:1; phenotype ratio 3 brown : 1 blue (75% brown, 25% blue). A blue-eyed child (bb) is possible only because each parent carried one recessive b allele — exactly the kind of reasoning the TEAS rewards.
Which organelle produces ATP through cellular respiration?
Which process moves a substance from low concentration to high concentration and requires energy?
What is the result of meiosis?
In DNA, the base adenine (A) always pairs with the base ___.
Type your answer below
Two parents are both heterozygous (Bb) for a trait where B is dominant. What fraction of offspring is expected to show the recessive phenotype?