There are several types of non-coding or junk DNA. Some of these are described below.
- Noncoding functional RNA – These are functioning RNA sequences that are not translated into protein and examples include Piwi-interacting RNA, ribosomal RNA, transfer RNA and microRNA. Experts have estimated that microRNAs control the translational activity of almost one third of all protein-coding genes among mammals and may play an important role in the progression of certain diseases such as cancer and heart disease, as well as in the immune response to invasion by infective organisms.
- Cis- and Trans-regulatory elements – These elements are sequences that regulate the transcription of a gene. They may be found within introns or in the 5' or 3' untranslated regions. Promoter sequences facilitate the transcription of a particular gene and are usually found upstream of the coding region. Enhancer sequences also influence the degree to which a gene is transcribed.
- Introns – An intron is the noncoding part of a gene, while a codon is the coding part. Introns are transcribed into precursor mRNA, but are subsequently removed before mature messenger RNA is formed. Some of these introns seem to exert biological function. They may help to regulate the activity of tRNA and rRNA, as well as the expression of protein-coding genes.
- Pseudogenes – These are sequences of DNA that are related to known genes but no longer possess the ability to code for a protein. They arise through retrotransposition or functional genes that have been duplicated, but are not functional because they contain mutations that prevent their transcription or alter gene translation.
- Transposons – A transposon is a mobile genetic element that can change its location in the genome, which may reverse a mutation or cause one, therefore altering the size of the cell’s genome.
- Viral sequences – There are several endogenous retrovirus sequences that arise through reverse transcription of retrovirus genomes. Mutation of these sequences can deactivate the viral genome.
- Telomeres – These are segments of repeated DNA that lie at the end of a chromosome, where they prevent deterioration of the chromosome during the process of DNA replication.
Reviewed by Sally Robertson, BSc
Last Updated: Nov 30, 2014