Repetitive Sequences: Selfish DNA

Even before the precise genome sequences are elucidated, one unique feature of the metazoan DNA sequence has been established from a number of studies. A large fraction (perhaps up to 90% or more) of the total genomic sequence in metazoan cells do not encode any information. Some of these sequences are present as noncoding intervening regions in genes, named “introns,” which do not code for proteins. However, the intron sequences are transcribed but are removed during processing (“splicing”) to generate mature mRNA, as discussed later. Many of the other genomic sequences are not even transcribed, and these may often be present as multimeric repeats of shorter units. These repetitive sequences have no known function in the cell, yet are maintained and replicated as an integrated part of the genome; such DNA is referred to as “selfish DNA.”

Metaphase chromosomes are organized in substructures distinguished by their staining with dyes. Euchromatin regions contain transcribed sequences, while heterochromatin regions contain large segments of repetitive sequences. Metaphase chromosomes are also characterized by specific stained sequences (named centromeres) in the middle of the elongated structure, in addition to telomeres at the termini, as discussed earlier. Both centromeres and telomeres have unique repetitive sequences, and in some cases similar sequences have been observed in other regions of chromosomes; these regions are highly condensed and not transcribed.