19.1.3—Sources of genes

The desired gene can be obtained from donor genomic DNA, from donor mRNA as complementary DNA (cDNA), or by artificial synthesis. The best source depends on the sequence needed and whether non-coding introns must be excluded.

Use the source-selection method:

1. Define the required characteristic and the nucleotide sequence that encodes it.
2. Choose a source: extract the sequence from donor DNA; use specialised-cell mRNA as a template for cDNA; or design and synthesise the sequence from the genetic code.
3. Obtain the desired sequence: the donor-DNA route cuts it from the genome, the mRNA route makes single-stranded cDNA and then double-stranded DNA, and the synthesis route joins short DNA pieces into a longer sequence.
4. Check that the product contains the required gene sequence before handing it to the later DNA-transfer step.

Genomic DNA — directly represents a donor chromosome, but may include introns and other non-coding DNA.
cDNA from mRNA — reflects a gene that was expressed in the donor cell and lacks introns present in the processed mRNA.
Artificial synthesis — uses known genetic information to design the sequence, without cutting it from an existing genome; it can avoid introns or unnecessary non-coding sections.

These are alternative ways to obtain the desired gene, not three different gene functions. The matched SME page does not provide a separate DNA-library workflow, sequence-orientation assay, PCR protocol, gel method, or promoter/vector/marker mechanism; those details remain in their later cards and are not invented here.