Biomedical subjects

N Lehman

Publications and source records attributed to N Lehman.

22 records · Page 2Linked to original sources

PubMed1987

Evolution of anticodons: variations in the genetic code.

Clues to evolution of the genetic code can be found by comparing usage of anticodons in various organisms and organelles. GC content of DNA varies, as a result of directional mutation pressure (AT/GC pressure), especially in bacteria. Low GC in Mycoplasma is accompanied by use of UGA for tryptophan and, in ciliated protozoa, by use of UAA and UAG for glutamine. These are examples of "stop codon capture," which has been preceded by duplication of tRNA genes followed by nucleotide substitutions in their sequences, including mutational changes in their anticodons. Evolutionary changes in the code may have resulted from disappearance of codons and anticodons resulting from GC pressure and from their reappearance when the direction of the pressure was reversed. In this manner, codon UGA and anticodon UCA for tryptophan could have disappeared under GC pressure and reappeared in Mycoplasma under AT pressure. Stop codon UGA may have been the third of the three stop codons to appear, originating from mutations in UAA. Changes in the code are adaptive and nondeleterious. We propose that the number of anticodons has increased and that evolution continued until three existing forms of the universal code were produced: eukaryotic, eubacterial, and the code for halobacteria and methanococci. These three codes are distinguished from each other by their anticodon pattern. The eukaryotic code contains eight INN (ANN) anticodons that have replaced GNN anticodons as a result of AT pressure. Mitochondrial and chloroplast codes have evolved from the eubacterial code through genomic economization and AT pressure, leading to losses of GNN and CNN anticodons.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗

PubMed1985

Purification of circular DNA using benzoylated naphthoylated DEAE-cellulose.

Un-nicked circular DNA can be separated from protein, RNA, and other DNA in a simple three-step protocol consisting of exonuclease III digestion, extraction with benzoylated naphthoylated DEAE-cellulose (BND cellulose) in 1 M NaCl, and alcohol precipitation of the remaining supercoiled DNA. Exonuclease III treatment introduces single-stranded regions into contaminating linear and nicked circular DNA. This DNA, together with most RNA and protein, is adsorbed onto BND cellulose leaving form I DNA in solution. The protocol can be used to purify analytical as well as preparative amounts of supercoiled DNA. This procedure is a substitute for cesium chloride-ethidium bromide gradient ultracentrifugation and gives a comparable yield of pure form I DNA. Other classes of DNA can be isolated by changing the pretreatment step. Selective digestion of linear DNA with lambda exonuclease permits the isolation of both nicked circular and supercoiled DNA while brief heat-induced or alkali-induced denaturation leads to the recovery of rapidly reannealing DNA. In large-scale purifications, the basic protocol is usually preceded by one or more BND cellulose extractions in 1 M NaCl to remove contaminants absorbing UV or inhibiting exonuclease III.

Cellulose↗

PubMedDate not supplied

Genetic differentiation among Oregon lake populations of the Daphnia pulex species complex.

Gene flow among invertebrate populations inhabiting bodies of nonflowing freshwater such as ponds or lakes must at some stage involve transport across habitat unsuitable for adult stages. Consequently the potential for interpopulational differentiation is high in these species, yet empirical studies of lake populations of Cladocerans such as Daphnia have failed to reveal high levels of genetic distinctiveness among populations and have led to much speculation about how these populations exchange genes and remain cohesive evolutionary units. In this study we surveyed 42 Oregon lake populations of Daphnia from the D. pulex species complex for genetic variation within the mitochondrial DNA control region. We have used this data to test the relative abilities of various ecological factors to explain the observed patterns in genetic differentiation among lakes. Despite limited genetic variation detected among our samples--11 very similar RFLP-defined mtDNA genotypes from 388 individuals--analyses of nucleotide variance using analogs to Wright's F statistics indicate that when multilake populations are defined in terms of the river drainage basin to which they belong, strong and significant amounts of among-population genetic variation can be detected at this locus (F(ST) estimates between 0.5 and 0.6). In contrast, we fail to detect consistent significant among-population variation when populations are defined on the basis of regional physical geography, bird migratory flyways, or lake trophic status. The manner in which the data are compiled, that is, whether RFLPs or nucleotide sequences are used, has little effect on the overall conclusions, yet it is clear that nucleotide sequence data would lower the standard errors of F(ST) estimates. We propose that periodic widescale flooding during the late Pleistocene may be an important mechanism to homogenize genetic differences among lake Daphnia continent-wide south of the southern-most extent of Pleistocene glaciation.

Animals↗

PubMedDate not supplied

Genetic consequences of a severe population bottleneck in the Guadalupe fur seal (Arctocephalus townsendi).

Population bottlenecks may lead to diminished genetic variability and correlative effects on fitness. The Guadalupe fur seal was nearly exterminated by commercial sealers during the late 18th and early 19th centuries. To determine the genetic consequences of this population bottleneck, we compared the variation at a 181 bp section of the mitochondrial DNA (mtDNA) control region from the bones of 26 prebottleneck fur seals versus variation in the extant population. We found 25 different mtDNA genotypes in the prebottleneck fur seals and only 7 genotypes among 32 extant fur seals, including only one of the ancient genotypes. These data demonstrate a substantial loss of genetic variability correlating with the recent population bottleneck. We also found from several genetic measures that the prehistoric population of Guadalupe fur seals was robust and that it had been increasing at some time during the late prehistoric period. Continued recovery of this species may, however, owe more to more immediate nongenetic factors, such as poaching and local availability of food resources during the breeding season and consequent effects on pup survival, than on the reduced genetic variability.

Animals↗