Prof Peter Chantler
Molecular Biology of the Cell
a. Choice of DNA
The first step involves choosing which DNA to clone, genomic DNA or cDNA? If the cloned DNA is to be representative of the entire genome of a particular species (ie. genomic DNA - facilitating genome analysis, the control of gene expression, gene structure and intronic sequences), total DNA must be prepared from somatic cells of that species.
Because genomic DNA is identical throughout all somatic cells of an individual, the exact cellular source of material is not important. However, if the DNA is to be representative only of the coding regions of those genes expressing proteins, complementary DNA (ie. cDNA - representing the mRNA population currently active in the cell and indicative of proteins being actively synthesized) must first be created.
cDNA is so-called because it is complementary to cellular mRNA. Because many genes are switched off in different cells at different times, the identities and amounts of transcribed mRNA are in constant flux within a cell, varying at different stages of development and in response to the changing needs of the cell. Consequently, it is important that the tissue or cell type is carefully chosen with respect to age, function and stage of the cell cycle, prior to cDNA production.
Back to Contents (Recombinant DNA)
Course updated 6 Apr 2004