Genetic information in ageing cells

Summary

INTRODUCTION

Fertilization of an egg and the subsequent mixing of maternal and paternal chromosomal complements result in restitution of the diploid state composed of paired allelic sets of genes except, possibly, in the case of the male XY combination. The chromosomal basis of heredity and modality of the parent-to-progeny transfer by Mendelian genetic principles is well established. We also know that the molecular basis of heredity lies in the sequence of trinucleotides in DNA which is, thus, the primary carrier of genetic information. In each species the amount of DNA is maintained at a constant level and the law of DNA constancy is believed to imply that all cells in a lineage contain genetic information which is both qualitatively and quantitatively identical. It is well established that over 90% of the eukaryotic genome is noncoding but the “raisond'etre” for noncoding sequences remains totally obscure. The law of DNA constancy is violated in the case of rDNA amplification during ovogenesis, chromosome elimination from the somatic cell lineage in nematodes, polytenization of chromosomes in diptera larvae, and polyploidization in both animal and plant cells (for details see Davidson, 1976). Cytogenetic maps have been described for a number of species and it is believed that, similar to the germ cells, somatic cells maintain these with fidelity. Since the studies of Hozumi and Tonegawa (1976) on the location of IgG genes, evidence has been accumulating to suggest that the position of one or more genes need not be identical with reference to either each other or, with respect to the total cytogenetic map, even in cells derived from the same lineage.