DNA profiling: PCR & gel electrophoresis
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Flip to reveal answersWhat are the two main stages of DNA profiling?
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Question
What are the two main stages of DNA profiling?
Answer
**PCR** (copies the DNA) then **gel electrophoresis** (separates the copies by size).
Question
What does PCR stand for, and what does it do?
Answer
**Polymerase chain reaction** — it makes **millions of copies** of a chosen piece of DNA (amplification).
Question
Which profiling stage uses the polymerase chain reaction?
Answer
The **amplification (copying)** stage.
Question
What are the three steps of one PCR cycle?
Answer
**Denaturation** (~95 °C), **annealing** of primers (~55 °C) and **extension** by Taq polymerase (~72 °C).
Question
What happens during denaturation in PCR?
Answer
The DNA is heated to ~95 °C, which **separates the double helix into two single strands**.
Question
What happens during annealing in PCR?
Answer
The mixture cools to ~55 °C so that short **primers bind** to each single strand.
Question
What happens during extension in PCR?
Answer
At ~72 °C, **Taq polymerase** adds nucleotides to build a new **complementary strand**.
Question
Why must PCR use Taq polymerase?
Answer
Taq is **heat-stable**, so it survives the ~95 °C step that would destroy a normal enzyme.
Question
What happens to the amount of DNA each PCR cycle?
Answer
It **doubles** — repeated cycling gives millions of copies.
Question
What does gel electrophoresis do?
Answer
It **separates DNA fragments by size** using an electric field.
Question
Why does DNA move toward the anode (+) in a gel?
Answer
Because DNA is **negatively charged**, so it is pulled toward the positive electrode.
Question
On a gel, which fragments travel furthest?
Answer
**Smaller (shorter) fragments** — they slip through the gel more easily.
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