Recombination and maternal age-dependent nondisjunction: molecular studies of trisomy 16.

Trisomy 16 is the most common human trisomy, occurring in > or = 1% of all clinically recognized pregnancies. It is thought to be completely dependent on maternal age and thus provides a useful model for studying the association of increasing maternal age and nondisjunction. We have been conducting a study to determine the parent and meiotic stage of origin of trisomy 16 and the possible association of nondisjunction and aberrant recombination. In the present report, we summarize our observations on 62 spontaneous abortions with trisomy 16. All trisomies were maternally derived, and in virtually all the error occurred at meiosis I. In studies of genetic recombination, we observed a highly significant reduction in recombination in the trisomy-generating meioses by comparison with normal female meioses. However, most cases of trisomy 16 had at least one detectable crossover between the nondisjoined chromosomes, indicating that it is reduced--and not absent--recombination that is the important predisposing factor. Additionally, our data indicate an altered distribution of crossing-over in trisomy 16, as we rarely observed crossovers in the proximal long and short arms. Thus, it may be that, at least for trisomy 16, the association between maternal age and trisomy is due to diminished recombination, particularly in the proximal regions of the chromosome.     

Foundations of the use of an enzyme-kinetic analogy in cell-mediated cytotoxicity

A mathematical model of the ⁵¹Cr-release microcytotoxicity assay is utilized to find conditions under which the kinetics of this assay resemble the kinetics of a classical enzyme-substrate reaction. Assuming a steady-state approximation, that “bystander” effector cells do not bind markedly better than the cytotoxic effector cells, and that the programming of the target cells for lysis is irreversible, it is shown that the velocity of label release is v = v_maxT_T/(K_{$\text{1}\text{2}$}+T_T), where both V_max and K_{$\text{1}\text{2}$} are linear functions of the effector-cell population and T_T is the initial target-cell population. Moreover, the expressions for K_{$\text{1}\text{2}$} and V_max are expressed in terms of natural kinetic parameters of the process and attributes of the noncytotoxic bystanders.     

Purification of riboflavin-binding proteins from bovine plasma and discovery of a pregnancy-specific riboflavin-binding protein.

Riboflavin-binding proteins have been purified from bovine plasma using flavinyl agarose beads. At least three major protein bands, migrating in regions assigned to the beta- and gamma-globulins of plasma, are observed by cellulose acetate electrophoresis. These proteins coelute from a calibrated Sephadex G-100 column in the volume corresponding to a molecular weight of approximately 150,000; a small amount of another riboflavin-binding protein (molecular weight approximately 37,000) is also present. Polyacrylamide gel electrophoresis of the proteins, with detection by autoradiography of those having tightly bound [2-14C]riboflavin, reveals one protein band which is present only in preparations from pregnant cows. This protein has been purified to apparent homogeneity by storing the mixture of riboflavin-binding proteins at 8 degrees C for 3 weeks, which precipitates the other, less stable proteins. Hence, bovine plasma, like that of the laying hen, contains a number of riboflavin-binding proteins, one of which correlates with pregnancy.     

Response of yellow birch (Betula alleghaniensis Britton) in acid subsoil to micronutrient additions

Summary Singular additions of Cu, Zn and Mo had little effect on height or diameter of yellow birch growing in acid subsoil. However, dry weight of leaves was increased by Zn and dry weight of stems was increased by both Mo and Zn. In addition, several macronutrient levels in leaves and roots were increased by micronutrients, and Al in roots was decreased (by 50%). Zn was the most effective in causing these favorable changes. Application of Zn is suggested as an alternative or supplement to application of limestone in correcting these problems of yellow birch in acid subsoil.