Molecular Genetics

Each chromosome contains a single continuous molecule of DNA.

Facts

In regular agarose gel electrophoresis of DNA (far left), fragments greater than about 20 kbp migrate with the same mobility as 20 kbp fragments. This length-independent mobility is thought due to the orientation of DNA during electrophoresis into a wedge-shaped form in which it snakes through the gel (reptation theory). Various forms of alternating field gel electrophoresis overcome length-independent migration. In all versions, the electric field periodically changes direction. The alternation of the electric field causes the DNA molecules to reorient and thus display more of their true length. The techniques allow the separation of very long DNAs (near left). Shearing breaks DNA. To avoid shear in handling DNA, cells are often introduced directly into the agarose gel well together with a lytic solution that releases free DNA. When yeast spheroplast are treated in such a way, alternating field gel electrophoresis reveals 4 to 7 bands (right).

• Similarly, cultured mammalian cells can be placed on top of a layer of a solution that lyses the cells and frees the DNA of bound protein. The layer is atop a gradient of sucrose or other related material. On centrifugation, cells are lysed and the DNA is released.
• From the rate of sedimentation of the DNA, an approximate molecular weight can be calculated. What do you expect the molecular weight to be?

Interpretations

• The number of bands observed is about the same as the number of chromosomes in haploid yeast cells.
• Therefore, each chromosome proabaly consists of a single, large, uninterrupted double-stranded DNA molecule.
• The average molecular weight measured by sedimentation was about the same as the average DNA content per haploid chromosome in this organism.
• Such observations support the concept that there is only a single DNA molecule in each chromosome.

Further information

• PFGE is used to characterize the size and number of chromosomal DNA's in many organisms.
• That one chromosome consists of a single DNA is also concluded from labeling patterns of replicating eucaryotic chromosomes.

This is page 11223 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999, 2000