By Levi Clancy for Student Reader on
20 - 300
Encode rRNAs, tRNAs, snRNAs and histones. In some cases, the multiple copies allow increased production of identical gene products – rRNA and histones. In these cases, the genes usually exist as tandem arrays. Allows production of millions of copies of the gene product per cell division – needed for ribosome, snRNPs and histones.
1 - 500 bp
Commonly called “satellite DNA” -- Sheared DNA has buoyant density dependent on base content. Total DNA gives rise to a main band of average base content. Certain overrepresented simple sequence repeats give rise to satellite bands due to skewed base content.
Most simple sequence DNA is comprised of 14-500 bp units tandemly repeated in long stretches of 20-100 kb. Most of these very long simple sequence DNAs are either at the centromeres of chromosomes where they may affect chromosome segregation, or serve as telomeres at the chromosome ends.
Unclassified spacer DNA
Also, there is unclassified spacer DNA that accounts for ~25% of the genome.
Microsatellite DNA is defined as having very short repeat units of 1-15 nt, such as CAGCAGCAG etc, repeated 50 or more times. Many human diseases are caused by triplet (especially CAG) repeat expansion mutations. These are thought to accumulate during rare mistakes in DNA synthesis when the nascent daughter strand slips backward along the template strand to insert additional bases into the daughter strand.
Minisatellite DNA has a longer repeat unit length of 15-100 nt or so With tandem array lengths of 500 bp to 20 kb. Differences between individuals in the number of repeats of a minisatellite sequence arise through unequal crossing over between chromosomes during meiosis. Some minisatellite sequences are highly variable in repeat number between individuals. This is the basis of the DNA fingerprinting.