Biomedical subjects

E K Porter

Publications and source records attributed to E K Porter.

7 recordsLinked to original sources

PubMed1984

Nucleic acid metabolism in the nucleus and cytoplasm of angiosperm meiocytes.

Over the course of flowering plant evolution many important stages in the life cycle have become compressed into the period between the premeiotic mitosis and gamete maturation. For this reason the changes in structure or chemistry that take place at this time are not easily ascribed to particular 'developmental stages'. During the meiotic process itself striking changes occur in levels of cytoplasmic and nuclear RNA, while DNA is synthesized in both the chromosomes and organelles. Evidence is presented indicating that a large proportion of the m- and rRNA is purged from the meiocyte cytoplasm by a combination of normal degradation and the activity of specific hydrolases. DNA synthesis in both the plastids and mitochondria precedes the re-differentiation of these organelles, while that in the chromosomes occurs in two major peaks, as has previously been reported (Hotta & Stern, 1971). High-resolution autoradiography did not indicate either peak of DNA synthesis to be localized within the synaptonemal complex, although final confirmation of this conclusion must await further investigation. Examination of these events in organisms where meiosis is removed from other stages of the life cycle suggests that the degradation of RNA and the chromosomal DNA synthesis are features which regularly accompany the meiotic process. The significance of these findings is discussed in terms of the hypothesis that there is at least one stage in the life cycle of all plants at which a purge of information-carrying molecules takes place.

Cell Differentiation↗

PubMed1983

Events in the cytoplasm during male meiosis in Lilium.

An electron microscopic investigation of the events associated with meiosis in Lilium has revealed a number of changes in both the organellar population and the other cytoplasmic components. Ribosome numbers decrease significantly in early prophase and are later replenished in the tetrads, a process most likely involving the newly arising cytoplasmic nucleoloids. The organelles show a cycle of de- and redifferentiation and later in meiosis unusual internal structures can be seen before these organelles enter a division phase resulting in increased numbers. The localization of acid phosphatase during these changes has also been studied using electron microscopic cytochemical methods. In early prophase, considerable amounts of acid phosphatase are found in vesicles scattered through the cytoplasm; activity is also found in association with most membranous surfaces and often markedly associated with condensing mitochondria. Later in prophase the enzyme activity decreases to normal levels. Electron microscopic autoradiography revealed that DNA is synthesized in both plastids and mitochondria during meiotic prophase with activity reaching a peak during zygotene and ceasing by diakinesis and tetrad formation. These changes point to a certain independence of organelles from nuclear control during meiosis. The events are also evaluated in relation to a cytoplasmic clearing mechanism, which may occur in preparation for the changeover from sporophytic to gametophytic control and the development of gametes.

Acid Phosphatase↗

PubMed1983

Changes in poly(a)+ RNA during male meiosis in Lilium.

Levels of poly(A)+ RNA have been investigated at each stage of male meiosis in Lilium (var. Firecracker). Two methods were employed in this work: in one extracts from labelled meiocytes were passed through oligo(dT) columns, while in the other the specific probe [3H]poly(U) was hybridized in situ with resin-embedded sections of pollen mother cells. The label contained in the eluate from the oligo(dT) columns was measured by liquid scintillation, and the quantity of [3H]-poly(U) hybridized was determined by statistical analysis of light microscopic autoradiographs. Both techniques revealed a dramatic decline in detectable poly(A)+ RNA during prophase. Lowest levels are reached in the pachytene stage, following which a gradual restoration of this species of RNA takes place in both nucleus and cytoplasm. The data presented here provide no clear indication as to whether this fall in RNA levels is caused by the action of novel enzymes specific to the meiotic prophase, by a cessation of synthesis and the activity of normal turnover processes, or by a combination of the two. Although there is some evidence from the [3H]poly(U) hybridization study that a small peak of poly(A)+ RNA synthesis may take place in leptotene, both methods indicate that there is a very low of poly(A)+ RNA synthesis throughout prophase. The presence of poly(A)+ RNA was not detected in either the accessory nucleoli or the cytoplasmic nucleoloids that characterize the nucleus and cytoplasm of these cells. These events are considered in terms of the juncture at which they occur in the plant life-cycle.

Meiosis↗

PubMed1982

Nucleic acid synthesis in microsporocytes of Lilium cv. cinnabar: events in the nucleus.

In an electron microscopic autoradiographic study of DNA and RNA synthesis during meiosis isolated Lilium microsporocytes were supplied with [3H]thymidine and [3H]uridine. DNA synthesis occurred in the nucleus during the zygotene and pachytene intervals of meiotic prophase. Most of the activity was associated with the chromatin, but some synthesis early in zygotene was located at the nucleolus. RNA synthesis occurred throughout prophase until diplotene, when all activity ceased until after division. The newly synthesized RNA was found mostly in association with the chromosomal peripheries or in the space between chromosomes. There was also a peak of [3H]uridine incorporation at the nucleolus, which followed shortly after the synthesis of DNA at that site. The localization of DNA and RNA synthesis at the various stages of meiosis is discussed in relation to current concepts of chromosome pairing, crossing-over, ribosomal DNA amplification and cycles of RNA metabolism.

Autoradiography↗

PubMed1981

Origins and genetic nonvariability of the proteins which diffuse from maize pollen.

The major function of pollen is to deliver the sperm nuclei to the embryo sac. It does this by germinating and producing a pollen tube and thus provides a relatively simple developmental system for study. Mutants for many pollen functions are accessible, as it is a haploid cell. Mature pollen was fractionated into diffusible proteins, soluble proteins, and proteins insolubly associated with membrane or wall; these protein fractions have been quantified and cataloged by native and SDS polyacrylamide gel electrophoresis. Diffusible proteins are localized in the pollen grain wall whereas soluble proteins are cytoplasmic. The roles of haploid and diploid genomes in specifying these proteins is discussed. Pollen from maximally divergent maize lines was examined for quantitative and qualitative variation in the diffusible proteins. A surprising conservation was found for these proteins indicating some functional role which is, at present, unknown. Initial experiments on the incorporation of 35S-methionine into germinating pollen indicate that major representatives of the diffusible proteins are made within the pollen grain itself. They are presumably included in the pollen wall during development and diffuse out through the pore region. Studies with pollen mRNA and experiments on incorporation of 35S-methionine into developing anthers are underway and will identify the origin of these proteins. A knowledge of the basic developmental biology of maize pollen is a prerequisite to its judicious use as a monitor of environmental mutagens.

Diffusion↗

PubMed1979

Uptake of selenium-75 by human lymphocytes in vitro.

Selenite uptake by human lymphocytes was studied both in whole blood and in isolated cells. When 75Se-selenite (75Se-SeO3-2--) is supplied to whole blood, it is converted by the erythrocytes to a form which rapidly becomes bound to plasma proteins. Studies with a variety of inhibitors indicated that the process is not energy dependent but that sulfhydryl groups are required. The 75Se bound to plasma proteins is absorbed by lymphocytes in preference to 75SeO3-2-. By the use of selective inhibitors (respiratory, sulfhydryl, protein biosynthetic) it was demonstrated that the uptake of either form of 75Se requires neither energy nor protein synthesis; however, all the sulfhydryl inhibitors cause a decrease of absorption. A scheme which summarizes the pathway of selenite conversion in human blood and uptake by lymphocytes is presented; the data indicate that plasma proteins function as carriers of selenium to lymphocytes.

Blood Proteins↗

PubMed1966

The long shadow.

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Economics, Nursing↗