RESUMEN
Centromeric region of human chromosome 21 comprises two long alphoid DNA arrays: the well homogenized and CENP-B box-rich alpha21-I and the alpha21-II, containing a set of less homogenized and CENP-B box-poor subfamilies located closer to the short arm of the chromosome. Continuous alphoid fragment of 100 monomers bordering the non-satellite sequences in human chromosome 21 was mapped to the pericentromeric short arm region by fluorescence in situ hybridization (alpha21-II locus). The alphoid sequence contained several rearrangements including five large deletions within monomers and insertions of three truncated L1 elements. No binding sites for centromeric protein CENP-B were found. We analyzed sequences with alphoid/non-alphoid junctions selectively screened from current databases and revealed various rearrangements disrupting the regular tandem alphoid structure, namely, deletions, duplications, inversions, expansions of short oligonucleotide motifs and insertions of different dispersed elements. The detailed analysis of more than 1100 alphoid monomers from junction regions showed that the vast majority of structural alterations and joinings with non-alphoid DNAs occur in alpha satellite families lacking CENP-B boxes. Most analyzed events were found in sequences located toward the edges of the centromeric alphoid arrays. Different dispersed elements were inserted into alphoid DNA at kinkable dinucleotides (TG, CA or TA) situated between pyrimidine/purine tracks. DNA rearrangements resulting from different processes such as recombination and replication occur at kinkable DNA sites alike insertions but irrespectively of the occurrence of pyrimidine/purine tracks. It seems that kinkable dinucleotides TG, CA and TA are part of recognition signals for many proteins involved in recombination, replication, and insertional events. Alphoid DNA is a good model for studying these processes.
Asunto(s)
Autoantígenos , Centrómero/genética , Cromosomas Humanos Par 21/genética , ADN Satélite/genética , Proteínas de Unión al ADN , Mutagénesis Insercional/genética , Conformación de Ácido Nucleico , Recombinación Genética/genética , Elementos Alu/genética , Secuencia de Bases , Sitios de Unión , Centrómero/química , Centrómero/metabolismo , Proteína B del Centrómero , Proteínas Cromosómicas no Histona/metabolismo , Deleción Cromosómica , Inversión Cromosómica , Cromosomas Humanos Par 21/química , Cromosomas Humanos Par 21/metabolismo , Biología Computacional , Intercambio Genético/genética , Replicación del ADN/genética , ADN Satélite/química , ADN Satélite/metabolismo , Bases de Datos como Asunto , Repeticiones de Dinucleótido/genética , Humanos , Hibridación Fluorescente in Situ , Linfocitos , Mutación/genética , Reacción en Cadena de la PolimerasaRESUMEN
It can be invoked from the theory of tandem repeat homogenization that DNA on a satellite/non-satellite border may carry sequence marks of molecular processes basic to satellite evolution. We have sequenced a continuous 17-kb alpha satellite fragment bordering the non-satellite in human chromosome 21, which is devoid of higher-order repeated structure, contains multiple rearrangements, and exhibits higher divergence of monomers towards the border, indicating the lack of efficient homogenization. Remarkably, monomers have been found with mutually supplementary deletions matching each other as reciprocal products of unequal recombination, which provide evidence for unequal cross-over as a mechanism generating deletions in satellite DNA.