gDNA = "genomic DNA" and cDNA = "complementary DNA." cDNA is classically associated with being reverse transcribed either from all extracted RNA from a tissue or cell (total RNA) including (in eukaryotes) pre-mRNA, ribosomal RNA, tRNA, snoRNA, miRNA and mRNA, etc.) while cDNA obtained only from reverse transcription of the mRNA (expressed eukaryotic cytosolic mRNA) fraction (e.g., by poly[dT]n and random priming) is complementary DNA (cDNA) made from what is called the "transcriptome." Eukaryotes have introns and exons in the gDNA, while prokaryotes do not. So eukaryotic cDNA reverse transcribed from mRNA lacks introns. Prokaryotic-derived cDNA is always complementary to prokaryotic RNA and gDNA (so is always necessary to have a good DNase treatment prior to gene expression analysis by e.g., qPCR for prokaryotic transcriptome work)...
Electrolysis is more expensive. Electrolysis is when water molecules break down into oxygen and hydrogen gas. Breakdown of methane contains carbon and releases CO2.
