Complementary DNA
Molecular Biology · Biotechnology
Say it: kom-pluh-MEN-tuh-ree dee-en-AY
Complementary DNA is DNA built from an RNA template, usually messenger RNA, by the enzyme reverse transcriptase, giving a DNA copy of an expressed gene.
Also: cDNA, copy DNA
Definition
What is Complementary DNA?
Complementary DNA (cDNA) is a DNA copy of an RNA molecule. It is made by reverse transcriptase, an enzyme first found in retroviruses, which reads an RNA template and assembles the matching DNA strand. In the lab, messenger RNA is usually primed through its poly-A tail with a short run of T nucleotides, or with random primers, and the single-stranded cDNA that results can be turned into double-stranded DNA by DNA polymerase.
Because it is copied from mature mRNA, cDNA holds only the exons of a gene, with the introns already spliced out. This is what makes it so useful. Bacteria cannot remove introns, so a human gene must be supplied as cDNA for Escherichia coli to make the protein, which is how human growth hormone was first produced in bacteria. A collection of cDNAs made from one tissue, called a cDNA library, represents the genes active in that tissue when it was sampled.
Making cDNA is also the first step of many everyday methods. Reverse transcription PCR and quantitative PCR measure gene expression by amplifying cDNA, and RNA sequencing converts a sample's RNA into cDNA before reading it. Labeled cDNA can serve as a probe for matching sequences.
Word parts
Where does the term “Complementary DNA” come from?
- complement
- that which fills up or completesLatin complementum, from complere (to fill up)
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- Reverse transcriptaseReverse transcriptase is an enzyme that builds DNA from an RNA template, letting retroviruses such as HIV turn their RNA genome into DNA.
- Messenger RNAMessenger RNA is the RNA copy of a gene that carries protein-building instructions from DNA to the ribosomes, where it is read in three-base codons.
- IntronAn intron is a stretch of a gene that is copied into RNA but cut out by splicing before the RNA is used, so it does not code for protein.
- Quantitative PCRQuantitative PCR (qPCR) is a form of PCR that tracks DNA copying by fluorescence after every cycle to measure how much of a target sequence is present.
- RNA sequencingRNA sequencing (RNA-seq) is a method that uses high-throughput sequencing to identify and count the RNA molecules in a sample.
- Molecular cloningMolecular cloning is a set of laboratory methods for inserting a specific DNA fragment into a vector and copying it many times inside host cells.
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