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Figure 2. Schematic organization of the structures characteristic of one type of repeated sequences found in eukaryotic genomes. Tandem repeats consist of a different variety of sequence units (shown in parentheses) that are found repeated side by side, with the number of copies being very high.

TANDEM REPEATS

Satellites

I ( | 42 bp | )n

II, III ( | ATTCC or other | )n

alpha ( | 171 bp | )n

Mini satellites and

( | 7-100 bp | )n

microsatelittes

( | 1-6 bp | )n

Telomeric and

( | TTAGGG | )n

subtelomeric

repeats

bp = base pairs

n = number of times the unit within parenthesis repeats

endoplasmic reticulum network of membranes within the cell mRNA messenger RNA

promoter DNA

sequence to which RNA polymerase binds to begin transcription introns untranslated portions of genes that interrupt coding regions lost ORF sequences, making them unable to amplify on their own. Instead, they must borrow the factors for amplification from external sources.

LTR retrotransposons are very similar to the genomes of retroviruses. They are flanked by 250 to 600 bp direct repeats called long terminal repeats. In general, not only are these elements defective, but they also appear to have deletions typical of nonautonomous families.

Several different groups of non-LTR retrotransposons can be found throughout most, if not all, eukaryotic genomes. One of these groups, the long interspersed repeated elements (LINEs), constitute about 21 percent of the human genome, with L1 and L2 being the dominant elements. Most of the element copies are incomplete and inactive. Two types of non-autonomous elements are thought to use factors made by LINEs: short interspersed repeated elements (SINEs) and retropseudogenes.

SINEs are derived from two types of genes coding for RNA: 7SL (which aids the movement of new proteins into the endoplasmic reticulum) and transfer RNAs. The most abundant human SINE is Alu, constituting about 13 percent of the human genome.

Retropseudogenes are derived from retrotransposition of mRNA derived from different genes. They can be distinguished from the parental gene by their lack of a functional promoter and by their lack of introns. The human genome is estimated to contain 35,000 copies of different retropseudogenes.

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