Libraries, Screening, and Functional Selection
From Single Clone to Library
DNA libraries
A library is a collection of cloned DNA fragments that together represent a genome, transcriptome, or targeted set of sequences. Libraries are powerful because they preserve diversity while making it possible to isolate individual clones later.
Library types
Genomic libraries contain fragments of an organism's genome, while cDNA libraries are made from reverse-transcribed mRNA and therefore represent expressed genes from a particular tissue or condition.
Screening approaches
Library screening can use hybridization, PCR, reporter assays, binding assays, or phenotype-based selection. The best method depends on whether the desired clone is identified by sequence, expression, or function.
Design principle
A good screening strategy matches the biological question: sequence-based questions need sequence-based screens, while function-based questions need functional selection.
What is the key difference between a genomic library and a cDNA library?
cDNA is synthesized from mRNA, so it reflects expressed genes and typically lacks introns.
Correct answer: A cDNA library represents expressed RNA-derived sequences
Why might a researcher prefer a cDNA library over a genomic library for protein expression studies?
This makes cDNA more convenient for expression in systems that cannot splice introns efficiently.
Correct answer: Because cDNA represents coding sequences from expressed transcripts and usually lacks introns.
Screening methods by goal
| Goal | Suitable approach |
|---|---|
| Find a known sequence | Hybridization or PCR |
| Find a clone with a phenotype | Functional selection |
| Check expression level | Reporter assay |
| Verify binding partner | Interaction screen |