Somatic doubling

A constant number of chromosomes is present in each cell. The somatic cells (i.e. not sperm or egg) are described as diploid because they contain two complete sets of chromosomes. There are 23 pairs of chromosomes in each cell, 22 pairs of somatic chromosomes (one of each pair derived from each parent) and one pair of sex chromosomes, either two Xs in the female or an X and Y in the male. Together, the 23 chromosomes contain about two metres of linear DNA or about three billion pairs of nucleotides. The linear structure of bases in DNA strands is called the primary structure of the chromosome. The secondary structure is the double heUx, in which the two complementary strands of DNA twist about each other. One turn of the helix is called a pitch and consists of ten nucleotides. 

A small increase in somatic mutation frequency was reported in the eye-colour spot test with Drosophila melanogaster exposed to di(2-ethylhexyl) phthalate in the feed but no effect was observed in two independent wing-spot tests. In a single study, mitotic recombination was not induced by di(2-ethylhexyl) phthalate neither was sex-linked recessive lethal mutation induced in two studies in D. melanogaster treated with di(2-ethylhexyl) phthalate in the feed or by injection. When administered to D. melanogaster, di(2-ethylhexyl) phthalate and A-nitrosodiethylamine induced DNA double-strand breakage and DNA repair, although neither compound was active when administered alone. 

DlPLOID Having the basic chromosome number doubled, that is, having twice the chromosome number of normal gametes, as most somatic cells and as is found in most sporophytic plants. 

Burtis, K. C., and B. X. Baker, Drosophila double sex gene controls somatic sexual differentiation by producing alternatively spliced mRNAs encoding related sex-specific polypeptides. Cell 56 997-1010, 1989. 

It is not difficult to assess the average biomass of a fish stock over many years, using data on the numbers and biomass of samples. In Black Sea anchovy, it is 284 kt in May. By November the figure doubles as a result of replenishment and of somatic increase. The average annual biomass (B) amounts to 426 kt for anchovy, 280 kt for sprat, about 36 kt for horse-mackerel, 6 kt for red mullet and 9 kt for whiting (no data are available for pickerel). Results of the studies on annual specific and absolute production in populations of the fish examined here and Px/B coefficients are given in Table 11. 

The end of a linear chromosome is called a telomere. Telomeres require a special mechanism, because the ends of a linear chromosome can t be replicated by the standard DNA polymerases. Replication requires both a template and a primer at whose 3 end synthesis begins. The primer can t be copied by the polymerase it primes. What copies the DNA complementary to the primer In a circular chromosome, the primer site is to the 3 direction of another polymerase, but in a linear chromosome, no place exists for that polymerase to bind. As a result, unless a special mechanism for copying the ends of chromosomes is used, there will be a progressive loss of information from the end of the linear chromosome. Two characteristics about telomeres help avoid this situation. First, they consist of a short sequence—for example, AGGGTT—repeated many times at the end of each chromosome. Telomeres, therefore, are part of the highly repetitive DNA complement of a eukaryotic cell. Secondly, a specific enzyme, telomerase, carries out the synthesis of this reiterated DNA. Telomerase contains a small RNA subunit that provides the template for the sequence of the telomeric DNA. Eukaryotic somatic cells have a lifespan of only about 50 doublings, unless they are cancerous. One theory holds that a lack of telomerase in cells outside the germ line causes this limitation. 

In a 9-week double-blind, randomized, placebo-con-trolled study, subjects were assigned to either risperidone (n = 14, 13 men and 1 woman mean age 42 years dose titrated upwards up to 2 mg/day) or to placebo (n = 9, 6 men and 3 women mean age 39 years) (43). Adverse effects were reported in seven subjects who took risperidone, including dry mouth, tiredness, weakness, reduced sexual arousal and delayed ejaculation, and a mild dys-tonic reaction. However, five placebo-treated subjects also reported adverse effects, which might have been due to the strong tendency of these patients to become somatically preoccupied. There were no group differences in dropout rates due to adverse effects. 

In a double-blind study nine patients with obsessive-compulsive disorder were given four single-doses of psilocybin 25-300 micrograms/kg (9). One had transient hypertension unrelated to anxiety or somatic symptoms, but there were no other significant adverse effects. 

An inability to replicate telomeres has been linked to cell aging and death. Many somatic cells do not express telomerase when placed in culture they survive a fixed number of population doublings, enter senescence, and then die. Analysis has shown significant telomere shortening in those cells. In contrast, stem cells do express telomerase and appear to have an infinite lifetime in culture. Research is underway to understand the role of telomeres in cell aging, growth, and cancer. 

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