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Biomedical subjects

S A Leslie

Publications and source records attributed to S A Leslie.

7 recordsLinked to original sources
PubMed2001

The effects of changing atmospheric oxygen concentrations and background radiation levels on radiogenic DNA damage rates.

Both background radiation levels and atmospheric oxygen concentrations have changed dramatically over the history of life on earth. Because oxygen has a strong modifying influence on radiogenic mutation rates, these factors must be considered jointly to determine changes in radiogenic mutation rates over time. Using accepted models that describe how both of these parameters have changed through time, we find that radiogenic mutation rates in organisms have fluctuated between about 1.5 to 2.5 times current levels through most of the history of life. The results of this study have interesting implications that may impact our understanding of how modern organisms respond to radiation damage and of models that use molecular clocks to date species divergence times. It is also possible that changing oxygen levels have served to buffer mutation rate changes that result from changes in background radiation levels over time.

Background Radiation↗
PubMed1999

Calculations of background beta-gamma radiation dose through geologic time.

Life on earth is exposed to a background level of ionizing radiation from a number of sources, including beta and gamma radiation from geologic and biologic materials. Radiation dose from geologic emitters has changed because of the chemical evolution of the continental crust, changes in the relative abundances of 235U and 238U, and the radioactive decay of uranium, thorium, and 40K with time. The radiation dose from internal 40K has decreased by a factor of about eight because of changes in the activity concentration of 40K in potassium over the past 4 billion years. Radiation exposure from geologic materials has decreased from about 1.6 mGy y(-1) to 0.66 mGy y(-1) over the past 4 billion years, and radiation exposure to an organism with a potassium concentration of 250 mmol L(-1) has decreased from about 5.5 to about 0.70 mGy y(-1). Accordingly, background radiation exposure from these two sources has dropped from about 7.0 to 1.35 mGy y(-1) during the time life has existed on Earth. The conservative nature of mutation repair mechanisms in modern organisms suggest that these mechanisms may have evolved in the distant past and that organisms may retain some of the capability of efficiently repairing damage from higher radiation levels than exist at present.

Beta Particles↗
PubMed1988

Diagnosis and treatment of hysterical conversion reactions.

Twenty children with hysterical conversion reactions were reviewed. The ways in which they became ill, the attitudes of their families to their illnesses and their underlying personal problems were noted. Treatment comprised avoiding unnecessary investigations and removing symptoms by using graded exercises given by physiotherapists. Exploration of the underlying psychological problems took place at a pace with which the family could cope because many families were reluctant at first to accept that the illness was psychological. Seventeen of the 20 children recovered completely within three months of starting treatment. The results show that early diagnosis and close liaison between paediatricians, physiotherapists, and child psychiatrists are necessary if prolonged handicap is to be avoided.

Adolescent↗