Copying and sorting DNA
Introduced| English | Meaning |
|---|---|
| amplify/ˈæmplɪfaɪ/ | To make many copies of a selected DNA sequence. |
| gel electrophoresis/dʒel ɪˌlektrəʊfɔːˈriːsɪs/ | A technique that separates charged molecules through a gel using an electric field. |
| microarray/ˈmaɪkrəʊəreɪ/ | A surface carrying many known molecular probes used to test numerous sample targets in parallel. |
| genome/ˈdʒiːnəʊm/ | The complete set of genetic material of an organism or cell, or of a virus. |
| polymerase chain reaction/ˌpɒlɪməˈreɪz tʃeɪn rɪˈækʃn/ | A method that repeatedly copies a chosen DNA region using primers, a DNA polymerase and cycles of temperature change. |
| Taq polymerase/tæk ˌpɒlɪməˈreɪz/ | Taq polymerase |
Amplifying 扩增 the evidence
- To study DNA, we often need millions of copies.
- PCR makes them; gel electrophoresis 凝胶电泳 sorts fragments by length.
- Microarrays 微阵列 and databases handle whole genomes 基因组.
PCR
- The polymerase chain reaction 聚合酶链式反应 (PCR) clones and amplifies DNA — millions of copies.
- It repeats cycles of heating and cooling, using the heat-stable enzyme Taq polymerase Taq聚合酶.
- Each cycle doubles the DNA.
A thermal cycler automates the heat/cool cycles of PCR.
The polymerase chain reaction (PCR)
Step through one cycle. Heat splits the DNA, primers bind, polymerase copies — and the amount doubles every cycle.
The polymerase chain reaction (PCR) is used to:
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PCR amplifies DNA through repeated heat/cool cycles using Taq polymerase.
The technique that makes millions of copies of a DNA sample is the ______ chain reaction.
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PCR (the polymerase chain reaction) uses Taq polymerase to copy DNA over many cycles.
In each PCR cycle, the amount of DNA:
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Each cycle copies every strand, so the DNA roughly doubles — giving exponential growth.
Put one PCR cycle in order.
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Denature → anneal → extend, then repeat. The doubling makes it exponential.
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.
An electric field pulls DNA fragments through the gel; shorter ones travel further, sorting them by length
DNA fragments separated into glowing bands by gel electrophoresis, seen under UV light
In gel electrophoresis, shorter DNA fragments:
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Fragments move toward the + end; shorter ones travel further, separating the DNA by length.
In gel electrophoresis, longer DNA fragments travel further through the gel than shorter ones.
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False. Shorter fragments slip through the gel more easily, so they travel further towards the positive electrode.
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.
Microarrays are used to:
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Microarrays detect gene activity across a genome by picking up the mRNA of expressed genes.
You've got it
- 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
Write from 16 copies after 4 cycles
- PCR starts with one target DNA molecule.
- Four ideal cycles run. Each cycle doubles the DNA.
How many copies are there after the four cycles?
Two, then four, then eight, then sixteen.
How many copies follow four ideal doublings from one?
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2 to the power 4 is 16.