The sources of variation in the human genome and genome instability in human cancers.
The human genome is viewed as a stable collection of about 60,000-70,000 genes--a minority of protein--coding DNA sequences--dispersed in a large majority of noncoding DNA sequences--more than 90 per cent of the entire genome sequences. Some of these ubiquitous noncoding DNA sequences, metonymically called "parasitic DNA," "ballast DNA," "selfish DNA" or "extra DNA," especially, the repeated sequences tandemly organized, are not stable but vary with considerable frequency. Recently, the confused or inadequately known origin of native of pathological variations of these DNA sequences appears to be unravelled, with great implications in genome stability. The human chromosomes, the bearer of genome, store and carry it. Their structure is qualified to perform its fastidious functions. The chromosomal conformation, "with variable geometry," exposed to genetoxic action of different damaging factors and to torsional stress after their fast and repeated changes during mitosis. The exaggerate exceeding of the native variation of human genome in disease states, probably, generates genome instability. The chromosome fragility--the cellular phenotypic expression of these molecular instability--reflects the closely relations between the genome and its carrier. The pattern of DNA replication with asynchrony of different domains of "parcelled" genome and the results of replication, susceptible to be corrected by the action of DNA repair genes, render certain limited regions of genome more vulnerable to damaging. These "target" regions focused damaging effects and exhibit an increased susceptibility to breakage and recombination, often with chromosomal expression. The coincidence of these regions, frequently, with locations of many protooncogenes and sometimes, antioncogenes could be subsequently, starting points for a genuine chain of genomic events related to growth cell and cell division. Cancer multistage accumulation of various genomic disorders in a single cell tends to take advantage of discriminating situations of these regions, which themselves can generate other genetic disorders, involving its in carcinogenesis. The gene expression disorders or the genuine mutations of dominant protooncogenes and the recessive behaviour of antioncogenes explain the nature of human cancers--a mixture of inherited and somatically acquired gene disorders. They attest the recessive characteristic of human cell malignancy and emphasize the decisive role of cancer predisposition which operates in interaction with damaging environmental factors. Seemingly, the pivotal causes of genome instability originate from strange behaviour of certain repeated DNA sequences dispersed throughout the human genome. Perhaps they hold the key to the puzzle of cancer processes.