Selective cleaning of the cell debris in human chromosome preparations studied by scanning force microscopy

The chromosome structure is one of most challenging biological structures t o be discovered. Most evidence about the structure comes from optical micro scopy. Scanning force microscopy (SFM) can achieve molecular resolution and allows imaging in liquids. However, little information about the chromosom e structure has been revealed by SFM. In this work, a mild enzymatic treatm ent is applied to the chromosomes to remove selectively the RNA and protein s coming from the cell. The resulting SFM images indicate that a protein fi lm with embedded RNA molecules covers chromosomes in standard cytogenetic p reparations. The thickness of the protein layer is 15-35 nm and the RNA adh eres preferentially to the chromosome surface. The cell material film resul ts in a quite smooth chromosome surface without evidence of any structural detail. After treatment, the chromosome was cleaned from cell residues and individual chromatin fibers at the surface were resolved. Furthermore, insi ghts about the higher order structure of the chromosome can be inferred. (C ) 1999 Academic Press.