RESUMEN
Transverse alternating-field electrophoresis (TAFE) refers to a pulsed-field system that uses a vertical gel and a simple electrode geometry. A schematrc of the apparatus is shown in Fig. 1 (1). The electrophoresis tank is a large plexiglass box, in which the gel stands vertically, supported at each side by two thin plexiglass slots and by the buoyancy of the buffer. The electrodes, represented by dots in the figure, are wires stretched across the width of the box (in commercial designs, the electrodes are wired to removable plexiglass inserts). During electrophoresis, the DNA is alternately moved downward and to the left, when the A electrodes are activated, and downward and to the right, when the B electrodes are activated. Note that the angle between the A and B fields is not constant down the length of the gel. At the wells, it is 115°, but is much greater at increasing distances from the wells. As the angle increases, the downward component of the field decreases.
RESUMEN
Human chromosome 21 has been analyzed by pulsed-field gel electrophoresis using somatic cell hybrids containing limited regions of the chromosome and greater than 60 unique sequence probes. Thirty-three independent NotI fragments have been identified, totalling 43 million bp. This must account for essentially the entire long arm, and therefore gaps remaining in the map must be small. The extent of the pulsed-field map has allowed the direct correlation of the physical map with the cytogenetic map: translocation breakpoints can be unambiguously positioned along the long arm and the distances between them measured in base pairs. Three breakpoints have been identified, providing physical confirmation of cytogenetic landmarks. Information on sequence organization has been obtained: (i) 60% of the unique sequence probes are located within 11 physical linkage groups which can be contained in only 20% of the long arm; (ii) 9/21 genes are clustered within 4%; (iii) translocation breakpoints appear to occur within CpG island regions, making their identification difficult by pulsed-field techniques. This analysis contributes to the human genome mapping effort, and provides information to guide the rapid investigation of the biology of chromosome 21.