Copying and sorting DNA
Copying and sorting DNA
- To study DNA, we often need millions of copies.
- PCR makes them; gel electrophoresis sorts fragments by length.
- Microarrays and databases handle whole genomes.
PCR
- The polymerase chain reaction (PCR) clones and amplifies DNA — millions of copies.
- It repeats cycles of heating and cooling, using the heat-stable enzyme Taq polymerase.
- Each cycle doubles the DNA.
Practice
The polymerase chain reaction (PCR) is used to:
PCR amplifies DNA through repeated heat/cool cycles using Taq polymerase.
Practice
In each PCR cycle, the amount of DNA:
Each cycle copies every strand, so the DNA roughly doubles — giving exponential growth.
Gel electrophoresis
- Gel electrophoresis separates DNA fragments by length.
- Fragments move through a gel in an electric field, towards the + end.
- Shorter fragments travel further, so the lengths spread into bands.
Practice
In gel electrophoresis, shorter DNA fragments:
Fragments move toward the + end; shorter ones travel further, separating the DNA by length.
Microarrays and databases
- Microarrays study whole genomes and detect which genes are switched on (by picking up their mRNA).
- Databases store DNA and protein sequences, so scientists anywhere can compare them.
Practice
Microarrays are used to:
Microarrays detect gene activity across a genome by picking up the mRNA of expressed genes.
You've got it
Key idea
- PCR amplifies DNA (heat/cool cycles, Taq polymerase); each cycle doubles it
- gel electrophoresis sorts fragments by length — shorter travel further toward the + end
- microarrays show which genes are switched on; databases store sequences for comparison