Genomic DNA or gDNA is chromosomal DNA, in contrast to extra-chromosomal DNA such as plasmid DNA. Genomic DNA contains all the biological information to be passed on to the next generation. Most organisms have the same genomic DNA in all their cells, however only certain genes are active in each cell. Genomic DNA contains both so-called coding and non-coding regions, known as introns and exons. Introns are fragments of genes that do not have coding sequences. In general, introns are spliced or "deleted" from the RNA during the transcription process before protein synthesis. This is only possible in eukaryotes, prokaryotes are not able to splice introns. Exons are a necessary part of the coding system and are retained after splicing of introns.