AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/6/6-8)
Unit 6 · Topic 6.8
6.8 Biotechnology
Biotechnology gives scientists tools to copy, sort, read and change DNA. PCR makes millions of copies of a DNA segment, gel electrophoresis separates fragments by size, bacterial transformation puts new genes into bacteria, and DNA sequencing reads the exact base order, with uses in medicine, forensics and evolution research.
Key terms
- PCR
- gel electrophoresis
- bacterial transformation
- DNA sequencing
- genetically modified organism
PCR: copying DNA
The polymerase chain reaction (PCR) makes many copies of one specific stretch of DNA, starting from a tiny sample. Each cycle has three steps:
- Denaturing: heat the mix to about 95 °C so the two DNA strands separate.
- Annealing: cool it so short DNA primers bind to the ends of the target sequence on each strand.
- Extending: a heat-stable DNA polymerase (originally from a bacterium that lives in hot springs) adds nucleotides from each primer, copying the target.
Gel electrophoresis: sorting DNA by size
In gel electrophoresis, DNA samples are loaded into wells at one end of a gel, and an electric current is applied. DNA's phosphate groups give it a negative charge, so it moves toward the positive electrode. The gel acts like a mesh: short fragments slip through faster and travel farther, while long fragments lag behind near the wells. A DNA ladder, a lane of fragments with known sizes, lets you estimate the size of unknown bands.
DNA is often cut first with restriction enzymes, which cut at specific short sequences. Different individuals have different sequences, so the cuts produce different fragment sizes and a unique band pattern.
Bacterial transformation: adding genes
In bacterial transformation, bacteria take in DNA from outside the cell, usually a plasmid. Scientists insert a gene of interest into a plasmid that also carries a selectable marker, such as an antibiotic resistance gene. They treat bacteria (often with calcium chloride and a brief heat shock) so some take up the plasmid. Then they grow the cells on agar containing that antibiotic. Only transformed cells survive, so every colony that grows carries the plasmid.
Because the genetic code is nearly universal, bacteria can then express the new gene. That's how human insulin has been produced in bacteria since the early 1980s. Copying a DNA fragment by putting it into bacteria that multiply is called gene cloning.
DNA sequencing and its uses
DNA sequencing determines the exact order of nucleotides in a DNA molecule. Modern machines can sequence entire genomes quickly. Comparing DNA from different samples, by sequence or by band pattern, gives a DNA fingerprint that can match or exclude individuals.
Uses you should know: identifying organisms and building phylogenetic trees from amplified DNA (7.9); forensic identification from crime-scene samples; diagnosing genetic diseases; and creating genetically modified organisms (GMOs), including transgenic animals that carry a gene from another species, such as goats engineered to make a human protein in their milk. You don't need the technical details of each technique, just what each does and how to interpret its results.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How many copies after PCR?
A PCR reaction starts with 10 copies of a target DNA sequence and runs for 25 cycles. Assuming every cycle doubles the DNA, how many copies are there at the end?
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- Step 1: Each cycle doubles the number of copies, so after n cycles you have (starting copies) × 2ⁿ.
- Step 2: 2²⁵ = 33,554,432.
- Step 3: 10 × 33,554,432 = 335,544,320 ≈ 3.4 × 10⁸.
- Step 4: In practice, copying slows in late cycles as primers and nucleotides run low, so this is an upper limit.
Answer: About 3.4 × 10⁸ copies (335,544,320).
- Example 2
Linear vs. circular DNA on a gel (classic trap)
A restriction enzyme cuts a DNA molecule of 10,000 base pairs (bp) at two sites, located at positions 3,000 and 7,500. Predict the fragment sizes if the molecule is (a) linear and (b) a circular plasmid. Which band in each lane travels farthest?
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- Step 1: (a) Linear: two cuts make three pieces. From 0 to 3,000 = 3,000 bp; from 3,000 to 7,500 = 4,500 bp; from 7,500 to 10,000 = 2,500 bp.
- Step 2: (b) Circular: a circle has no ends, so two cuts make only two pieces. From 3,000 to 7,500 = 4,500 bp; from 7,500 around through 10,000 back to 3,000 = 2,500 + 3,000 = 5,500 bp.
- Step 3: Check: the pieces add up to 10,000 bp both times.
- Step 4: Smaller fragments travel farthest from the wells: 2,500 bp in lane (a), 4,500 bp in lane (b).
Answer: (a) 3 fragments: 2,500, 3,000 and 4,500 bp, with 2,500 bp farthest. (b) 2 fragments: 4,500 and 5,500 bp, with 4,500 bp farthest.
- Example 3
Predicting a transformation experiment
Bacteria are mixed with a plasmid carrying an ampicillin resistance gene and a gene for green fluorescent protein (GFP) that is switched on only when the sugar arabinose is present. Some cells are not given the plasmid. Predict growth and glowing on: (1) no-plasmid cells on plain agar, (2) no-plasmid cells on agar with ampicillin, (3) plasmid-treated cells on agar with ampicillin, (4) plasmid-treated cells on agar with ampicillin and arabinose.
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- Step 1: (1) No antibiotic to kill them, so the cells grow as a lawn; no GFP gene, so no glow.
- Step 2: (2) Without the plasmid, the cells aren't resistant, so ampicillin kills them: no growth.
- Step 3: (3) Only transformed cells survive ampicillin, so you see separate colonies. No arabinose means GFP is off: colonies don't glow.
- Step 4: (4) Transformed colonies grow, and arabinose switches on the GFP gene: colonies glow green under UV light.
- Step 5: Plates 1 and 2 are controls: plate 1 shows the cells were alive, and plate 2 shows ampicillin kills untransformed cells.
Answer: (1) lawn, no glow; (2) no growth; (3) a few colonies, no glow; (4) a few colonies that glow.
Common mistakes
- Saying large DNA fragments travel farther on a gel. Smaller fragments move farther from the wells.
- Saying DNA moves toward the negative electrode. DNA is negatively charged and moves toward the positive end.
- Forgetting that cutting a circular plasmid gives as many fragments as there are cut sites, not one more.
- Thinking every cell on an antibiotic plate was transformed by chance. Selection is the point: only cells with the resistance gene survive.
On the exam
- Gel questions often ask you to match a suspect or parent to a sample, or to estimate fragment sizes from a ladder. Compare band positions lane by lane.
- Transformation questions often ask you to identify the controls and justify them, as in the plate predictions above.
Connected topics
Videos
Check yourself
4 questions on 6.8 Biotechnology. Pick an answer to see if you got it, and why.
| Sample | DNA fragment sizes (base pairs) |
|---|---|
| Crime scene | 1200, 800, 300 |
| Suspect 1 | 1200, 950, 300 |
| Suspect 2 | 1200, 800, 300 |
| Suspect 3 | 1500, 800, 450 |
Experimental data: DNA from each sample was copied by PCR, cut with the same enzyme, and separated by gel electrophoresis. Sizes were found by comparing bands with a DNA ladder.
Which suspect's DNA pattern matches the crime scene sample?
In the gel, the 300 bp band traveled the farthest from the wells. Which of the following best explains this?
Starting with one double-stranded DNA molecule, how many copies of the target sequence are present after 10 cycles of PCR, assuming each cycle doubles the DNA?
Bacteria are mixed with a plasmid carrying an ampicillin-resistance gene and a gene for a green fluorescent protein, then spread on a plate containing ampicillin. Which result is expected?
0 of 4 answered