cDNA

Complementary DNA (cDNA) is a DNA molecule synthesized from a single-stranded RNA (typically messenger RNA, or mRNA) template through the action of the enzyme reverse transcriptase. This process yields a DNA copy that is complementary to the original RNA sequence. Because cDNA is derived from mature mRNA, it lacks intronic sequences found in genomic DNA and includes only exonic (protein-coding) regions. This makes cDNA particularly valuable for molecular biology applications requiring the study of expressed genes, as it represents a direct genetic “snapshot” of a cell’s transcriptome at a given time.

Content and Synthesis

The content of cDNA consists exclusively of sequences that were present in the original mRNA molecules. It typically includes the 5' and 3' untranslated regions (UTRs) and the open reading frame (ORF) that codes for proteins. The synthesis of cDNA involves several technical steps:

• RNA Isolation: High-quality mRNA is isolated from cells or tissues.
• Primer Binding: Oligo(dT) primers (which anneal to the poly-A tail of mRNA) or random hexamers bind to the mRNA.
• Reverse Transcription: Using reverse transcriptase, the enzyme extends from the primer, synthesizing a complementary strand of DNA from the RNA template.

Applications of cDNA

cDNA finds extensive application in molecular biology and biotechnology:

• Gene Cloning and Expression: Because it contains only coding regions, cDNA is ideal for cloning eukaryotic genes into expression vectors for protein production, bypassing intron removal that is required when using genomic DNA in prokaryotic systems.
• cDNA Libraries: Collections of cDNA sequences can be stored in libraries, representing the set of genes expressed in a particular cell type or under specific conditions. These libraries are essential resources for gene discovery, comparative genomics, and screening for gene function.
• Quantitative PCR (qPCR) and RT-PCR: cDNA serves as the template for quantitative measurement of gene expression. Reverse transcription polymerase chain reaction (RT-PCR) amplifies specific cDNA segments to quantify mRNA levels, aiding in diagnostics, gene expression profiling, and validation of gene expression differences under various experimental conditions.

By focusing on the expressed regions of the genome, cDNA techniques allow researchers to analyze protein-coding potential and expression dynamics without the confounding presence of non-coding intronic sequences, facilitating a variety of technical applications in genomics, transcriptomics, and biotechnology.