Biotechnology

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson4 min read

DNA is negative: it runs to the positive pole, smallest pieces farthest. PCR doubles: copies = start × 2ⁿ.

Biotech on the exam runs on four core tools. PCR copies one stretch of DNA, a gel sorts pieces by size, sequencing reads the bases, and transformation puts a gene into living cells.

Key terms

PCR — polymerase chain reaction. It copies one chosen stretch of DNA, doubling the number of copies every cycle.
Primer — a short single strand of DNA that pairs at one end of the target. Polymerase extends it from its 3′ end.
Gel electrophoresis — an electric field pulls DNA through a gel. Short pieces slip through faster, so fragments separate by size.
Plasmid — a small, circular piece of DNA that bacteria copy separately from their chromosome. Labs use it to carry genes into cells.
Transformation — a cell taking up outside DNA, such as a plasmid. Only a small fraction of the cells do.
DNA sequencing — reading the order of bases. Comparing sequences shows how closely related two organisms are.

How it works

Each PCR cycle: heat separates the strands → cooling lets primers pair → a heat-stable polymerase extends each primer from its 3′ end.
Every cycle doubles the target, so copies = start × 2ⁿ after n cycles. Thirty cycles turn one molecule into about a billion.
Run the product on a gel. The phosphate backbone makes DNA negative along its whole length, so every piece moves toward the positive pole. Smaller pieces travel farther.
To get a gene expressed, pair its coding sequence with the right regulatory DNA. The coding sequence decides which protein; the regulatory DNA decides where and when.
Transform bacteria with a plasmid carrying an antibiotic-resistance gene, then plate them on that antibiotic. Only cells that took up the plasmid grow into colonies.

Why it matters

Biotech questions are mostly math and logic, not memory. Expect PCR doubling, a gel to read, or an efficiency to calculate. Sequence tables ask you to compare differences: the reference with the fewest is the closest match, even when it isn't identical. The tools also lean on Unit 6: primers are needed because polymerase can only add to an existing 3′ end.

Exam traps

Trap	Actually
DNA moves toward the negative electrode.	DNA is negative, so it runs toward the positive pole. The smallest pieces end up farthest from the wells.
PCR adds a fixed number of copies each cycle.	It doubles them. Twelve cycles multiply the start by 2¹² = 4,096, not by 12.
Both primers bind the same strand, one at each end.	Each primer binds a different strand. Their 3′ ends point toward each other, so copying converges on the target.
Transformation efficiency is colonies divided by all the DNA you added.	Count only DNA that reached the plate. Plate a tenth of the mix, and only a tenth of the DNA counts.

Worked example

A plant gene has two alleles. Allele S gives a 300-base-pair PCR product. Allele L carries an extra 1,200 base pairs between the same two primer sites. PCR on three plants gives these bands: plant 1, 300 bp only; plant 2, 1,500 bp only; plant 3, both 300 and 1,500 bp. Which plants carry allele L, and which band runs farthest from the wells?

The product runs from one primer to the other, so its size is the DNA between them: S gives 300 bp, and L gives 300 + 1,200 = 1,500 bp.
Each allele a plant carries makes its own band. One band means two copies of one allele; two bands mean one of each.
So plant 1 is SS, plant 2 is LL, and plant 3 is SL.
On the gel, the 300-bp band travels farthest, because shorter pieces move through the gel mesh faster.

Plants 2 and 3 carry allele L: plant 2 has two copies, plant 3 has one. The 300-bp band runs farthest from the wells.

Band size tells you which allele; the number of bands tells you whether a plant carries one allele or two different ones. The near miss is thinking the 1,500-bp band runs farther because it carries more negative charge. It does carry more charge, but its mass rises in step, so the gel sorts by size and the long piece lags.

Now try it on real exam-style questions

Ten questions on Biotechnology, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 6.8 Biotechnology

Along with template DNA, primers and a heat-stable polymerase, what must a PCR mixture contain?

A Free RNA nucleotides
B Free DNA nucleotides
C Free amino acids
D DNA ligase enzyme

Show the answer

B — The polymerase builds each new strand from free DNA nucleotides, matching each one to the template base. RNA nucleotides are used by RNA polymerase in transcription, not to build DNA copies.

Question 2 · 6.8 Biotechnology

Which enzyme forms the covalent bonds that seal a DNA fragment into a cut plasmid?

A DNA polymerase
B Restriction enzyme
C DNA ligase
D Helicase

Show the answer

C — DNA ligase forms the phosphodiester bonds that link the sugar-phosphate backbones of the fragment and the plasmid. DNA polymerase also forms these bonds, but only while adding nucleotides to a growing strand; it does not join two separate pieces.

Question 3 · 6.8 Biotechnology

What does a restriction enzyme do to a DNA molecule?

A It cuts the DNA at one specific base sequence
B It cuts the DNA at random points along its length
C It joins two DNA fragments together into one
D It stops the DNA from being copied by the cell

Show the answer

A — A restriction enzyme recognizes a short, specific base sequence and cuts the DNA there, so the same enzyme always cuts a given molecule at the same places. That predictability is what makes restriction fragments useful; joining fragments is the job of DNA ligase, not a restriction enzyme.