The majority of DNA—approximately 95% of the base sequences in human DNA—does not code for proteins. Noncoding DNA is composed of highly repetitive sequences, some of which include satellites, microsatellites, short interspersed elements (SINEs), and long interspersed elements (LINEs). The 300-base-pair (bp) Alu sequence, named after the restriction enzyme used to identify it, is the repetitive DNA that appears most frequently. Noncoding DNA comprises introns, promoters, and other regions within chromosomes and mitochondria and is involved in regulating gene expression and exon splicing.
RNA transcribed from noncoding DNA may directly influence the transcription of other sequences and participate in normal genome repair and regulation. Some of the repetitive sequences of nontranscribed DNA form telomeric DNA, which is essential for the correct formation and maintenance of chromosomes. Loss of telomeric DNA correlates with cell senescence. Defects in telomeric DNA maintenance have been proposed to be associated with carcinogenesis.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.