Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Gel electrophoresis separates DNA fragments by size. The phosphate backbone makes DNA negative, so it moves toward the positive electrode. Short fragments move farthest. A ladder of known sizes lets you estimate band sizes.
- PCR amplifies one target region between two primers, using a heat-stable DNA polymerase. Each cycle: denature (strands separate), anneal (primers bind), extend (polymerase adds nucleotides to the primers).
- Bacterial transformation introduces foreign DNA, usually a plasmid carrying a gene of interest and an antibiotic-resistance marker. Only transformed cells grow on antibiotic plates. Dividing bacteria copy the plasmid: gene cloning.
- DNA sequencing gives the order of nucleotides. Comparing sequences or band patterns gives a DNA fingerprint for forensic identification, kinship, species identification and phylogenetics.
Key relationships
| Equation or rule | Meaning |
|---|---|
| N = N₀ × 2ⁿ | maximum copies after n PCR cycles from N₀ starting copies |
| efficiency = colonies ÷ µg DNA spread | transformation efficiency (transformants per µg) |
| distance falls roughly linearly as log(size) rises | ladder bands crowd together near the wells |
| every band in a child is in the mother or the biological father | basis of a kinship test |
| fewer sequence differences → more closely related | basis of molecular phylogenetics |
Assumptions behind the numbers
- 2ⁿ assumes 100% efficiency in every cycle; real reactions are lower and level off.
- Size estimates from a gel are approximate (2 significant figures).
- A match supports identity; a reliable mismatch excludes.
Mistakes to avoid
- Saying DNA moves toward the negative electrode, or that big fragments go farthest.
- Saying PCR copies the whole genome; only the region between the primers is copied.
- Forgetting why the polymerase must be heat-stable.
- Rounding a number of PCR cycles down.
- Using the total DNA instead of the DNA actually spread when calculating transformation efficiency.
- Saying the antibiotic makes cells resistant; the plasmid gene does.
- Treating a single matching band as proof of identity.
Quick self-check
- What happens at each PCR temperature? (Hot: strands separate; cooler: primers bind; middle: polymerase extends the primers.)
- Three copies, 10 perfect cycles? (3 × 2¹⁰ = 3072 copies)
- A child has a band that neither the mother nor a tested man has. What can you conclude? (If the test is reliable, that man is excluded as the biological father.)
Next: practice questions.