Ultrastructure of a Burkitt's lymphoma marker chromosome, as investigated by quantitative electron microscopy

Chromosomes of Burkitt's lymphoma cell line AL-1 were examined and measured by whole-mount quantitative electron microscopy after isolation by surface-spreading and critical point drying. The ultrastructure of the chromosomes is based entirely on twisted, irregularly folded fibers about 300 A in diameter. An acrocentric marker chromosome, exceeding all others in length, was seen to be connected to 13-15 chromosomes by common fibers. Dry mass values of 5 marker chromosomes from different metaphases ranged from 20.1 to 25.8 × 1O−13 g. Within each measured metaphase, the dry mass ratio of the marker to 13-15 chromosomes was 2.2 and of the marker to 21-22 chromosomes was 4.3. These ratios identify the marker in terms of dry mass as the missing number 2 chromosome. Average fiber dry mass was 11.2 × 10−16 g per micron. For one marker chromosome, total fiber length per chromatid was 1089 microns. Cross-sectional scans through the chromatids of this marker indicated 83 fiber equivalents in the centromere, 68 to 178 in the long arms, and 155 to 157 in the telomeres. On the assumption that the DNA content of the marker is equal to a chromosome 2, the DNA-packing ratio, i.e., the length of DNA double helix per unit length of chromosomal fiber, appears to be much lower than in metaphase chromosomes derived from normal lymphocytes.