Physiological ageing of potato tubers: A Review

Numerous theories have been proposed to describe the complex process of ageing in biological systems. Two general groups of ageing theories currently exist: 1) stochastic where the accumulation of random molecular damage leads to loss of information vital to the cell; and, 2) systemic where an organised, genetically based sequence of metabolic activities leads to programmed ageing. Whether these are acceptable models of ageing in potato tubers is unknown although the tuber could provide a useful experimental system for studying ageing. An initial requirement for advancing the concept of ageing in potato tubers must centre on the development of a suitable ageing index. A review of the literature suggests that a modified approach to ‘sprouting capacity’ and ‘incubation period’ may allow tuber ageing to be described in mathematical terms that would, in turn, facilitate the development of a physiological ageing index as well as temperature sensitive predictive models. Although a number of biochemical studies of ageing have been pursued, the development of adequate biomarkers has yet to achieve a coordinated level of development as found in a range of organisms. For example, ageing in other biological systems may be viewed as an outcome of an accumulation of random molecular damage and may be primarily caused by a changing balance between reactive oxygen species and diminishing levels of protective agents such as superoxide dismutase, alpha‐tocopherol or vitamin C. The exploration of these and similar problems in the context of appropriate modelling approaches should allow a better understanding of physiological ageing in potato tubers.     

Studies on the Organisation of the Chicken Genome and its Expression during Myogenesis in vitro

DNA from the chicken genome was analysed both by isopycnic centrifugation in cesium salt density gradients and by reassociation analysis using hydroxyapatite (HAP) chromatography. Centrifugation in neutral CsCl revealed a single non-Gaussian band skewed toward the heavy side, but no discrete satellite components. In heavy metal (Ag⁺ or Hg⁺⁺)-Cs₂SO₄ gradients, 4–8 satellite bands were revealed, comprising 5–9% of the total DNA. Purification of the satellites and recentrifugation in neutral CsCl demonstrated that 80–90% of this DNA would band in the shoulder, with the remainder in the main band. These satellites can account at most for 30% of the heavy shoulder DNA, thus most of the heavy shoulder DNA must be of lower repetition frequencies.
Reassociation analyses of chicken DNA demonstrated that the complexity of the non-repetitive DNA is 9.49 × 10⁸ nucleotide pairs, equivalent to about 90% of the haploid genome. Repetitive DNA comprises only 8–10% of the genome and has the following composition, relative to total DNA: 3.7% intermediate repetitive, 1.9% highly repetitive, and 3.9%"zero-time binding" DNA. This unusually low repetitive DNA content may be related to the small genome size of chickens, relative to other vertebrates, and to the presence of many microchromosomes in the chicken karyotype.
Total cell RNA extracted from prefusion myoblasts, post-fusion myotubes, and myoblasts grown in BrdU was incubated in large excess with ³H-TdR labelled non-repetitive DNA and the resulting hybrids assayed by HAP chromatography. The amount of non-repetitive DNA represented in the RNA was found to increase from 7–8% in the myoblast stage to 10–11% in myotubes. An even smaller proportion, about 5%, is represented in the RNA of myoblasts prevented from differentiating by growth in BrdU.