Noncoding DNA
In genomics and related disciplines, noncoding DNA sequences are components of an organism's DNA that do not encode protein sequences. Some noncoding DNA is transcribed into functional non-coding RNA molecules (e.g. transfer RNA, ribosomal RNA, and regulatory RNAs). Other functions of noncoding DNA include the transcriptional and translational regulation of protein-coding sequences, scaffold attachment regions, origins of DNA replication, centromeres and telomeres.
The amount of noncoding DNA varies greatly among species. For example, it was originally suggested that over 98% of the human genome is noncoding, while 20% of a typical prokaryote genome is noncoding. Work by ENCODE (a part of the Human Genome Project) has shown this is not the case. Where only a small percentage of the genome is responsible for coding proteins, the percentage of the genome performing regulatory functions is growing. As quoted in the September 6th 2012 issue of Time under the title "Junk DNA Not So Useless" ENCODE has revealed that some 80% of the human genome is biochemically active. When there is much non-coding DNA, a large proportion appears to have no biological function for the organism, as theoretically predicted in the 1960s. Since that time, this non-functional portion has often been referred to as "junk DNA", a term that has elicited strong responses over the years.
