Gene Identification

In contrast to prokaryotes, gene identification in eukaryotes is further complicated because most chromosomal DNA is non-coding. For example, it has been estimated that less than 2% of human nuclear DNA consists of the 20 000–25 000 or so genes that constitute our genomes.
At first sight, this number of genes appears to be too low to underpin the genetic blueprint for sophisticated organisms such as vertebrates. However, it is clear that because of post-transcriptional processing, the number of messenger RNA (mRNA) transcripts vastly exceeds the actual number of genes. Further heterogeneity is introduced by posttranslational modification during protein synthesis and maturation.
Thus a relatively small number of genes appear to be capable of producing several different sequences of mRNA (transcripts) through various mechanisms such as alternative promoter usage, splicing and polyadenylation. The non-genic portion of the genome, i.e. the remainder of the 6 000 000 000 DNA base pairs in the diploid human
genome, is often referred to as ‘junk DNA’ because its function is not yet fully understood.