Examining examinations-6. Multiple-choice items.
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Biomedical subjects
Publications and source records attributed to E Robbins.
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The concentration of the iron-chelating agent, desferrioxamine (Desferal), that just inhibits iron entry into HeLa cells is also the concentration that inhibits DNA synthesis. As a first step in clarification of the mechanism whereby iron may partake in DNA synthesis, we have partially characterized several of the intracellular iron-binding sites. Most cytoplasmic iron appears to be bound to a polysaccharide containing glucose that sediments at about 32 S. Nucleolar iron is bound to a single protein, the mobility of which is independent of the concentration of sodium dodecyl sulfate in an acrylamide gel. In contrast the pattern and mobility of nuclear iron, other than nucleolar, is heterogeneous and markedly affected by the concentration of sodium dodecyl sulfate. The evidence suggests that nuclear iron is bound to protein through one or more intermediate(s).
Measurements of actinomycin-(3)H binding in synchronized HeLa cells reveal that the binding capacity of chromatin decreases progressively during the S phase despite a doubling of nuclear DNA content, reaches a minimal level during G(2) and mitosis, and then increases gradually throughout the subsequent G(1) interval. Since this pattern was evident in experiments with living cells, ethanol-fixed cells, and isolated nuclei, but not with purified DNA, the actinomycin binding profile may reflect changes in the degree of association between DNA and chromosomal proteins at different stages of the cell cycle.
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