Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA
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Flip to reveal answersWhat does PCR (the polymerase chain reaction) do?
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All 14 Flashcards — Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA
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Question
What does PCR (the polymerase chain reaction) do?
Answer
It **amplifies** DNA — makes **many copies** of a chosen piece of DNA from a tiny sample.
Question
Roughly how much does the DNA increase each PCR cycle?
Answer
It roughly **doubles** every cycle, so the increase is **exponential** (about a billion copies after ~30 cycles).
Question
Name the three steps of one PCR cycle, in order.
Answer
**Denaturation → annealing → extension.**
Question
Why is PCR heated to ~95 °C (denaturation)?
Answer
The high heat **breaks the hydrogen bonds**, separating the double helix into **two single strands**.
Question
What happens at the annealing step (~55 °C)?
Answer
**Primers bind (anneal)** to their matching sequence on each single strand, marking where copying begins.
Question
What happens at the extension step (~72 °C)?
Answer
**Taq polymerase** adds **nucleotides** to each primer to build a new **complementary strand** (72 °C is its optimum).
Question
What is a primer?
Answer
A **short single strand of DNA** that binds to a matching sequence and marks where copying should start.
Question
Why is Taq polymerase used in PCR?
Answer
It is **heat-stable (thermostable)** — it is **not denatured** by the ~95 °C step, so the same enzyme works every cycle.
Question
Where does Taq polymerase come from?
Answer
From **Thermus aquaticus**, a bacterium that lives in **hot springs**, so its enzymes tolerate high temperatures.
Question
What does gel electrophoresis do?
Answer
It **separates DNA fragments by size** so they can be seen and compared as a pattern of **bands**.
Question
In gel electrophoresis, which fragments travel furthest?
Answer
The **smaller** fragments — they slip through the gel sieve more easily. (Small = far.)
Question
Why does DNA move towards the positive electrode in a gel?
Answer
Because DNA is **negatively charged**, so the electric field pulls it towards the **positive electrode**.
Question
On a PCR gel, what does the no-DNA control lane look like, and why?
Answer
It shows **no band** — with no template DNA there is nothing to amplify (it checks for contamination).
Question
Predict the gel result if fewer PCR cycles are run.
Answer
**Fainter bands** — fewer cycles means **less DNA is made** (the amount roughly doubles each cycle).
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DNA replication
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