Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

208 Chapter 4: DNA, Chromosomes, and Genomes
present in the DNA loops are being actively expressed. The majority of the DNA,
however, is not in loops but remains highly condensed on the chromosome axis,
where genes are generally not expressed.
It is thought that the interphase chromosomes of all eukaryotes are similarly
arranged in loops. Although these loops are normally too small and fragile to be
easily observed in a light microscope, other methods can be used to infer their
presence. For example, modern DNA technologies have made it possible to assess
the frequency with which any two loci along an interphase chromosome are held
together, thus revealing likely candidates for the sites on chromatin that form the
bases of loop structures (Figure 4–48). These experiments and others suggest that
the DNA in human chromosomes is likely to be organized into loops of various
lengths. A typical loop might contain between 50,000 and 200,000 nucleotide
pairs of DNA, although loops of a million nucleotide pairs have also been suggested
(Figure 4–49).
Polytene Chromosomes Are Uniquely Useful for Visualizing
Chromatin Structures
Further insight has come from another unusual class of cells—the polytene cells of
flies, such as the fruit fly Drosophila. Some types of cells, in many organisms, grow
abnormally large through multiple cycles of DNA synthesis without cell division.
Such cells, containing increased numbers of standard chromosomes, are said to
be polyploid. In the salivary glands of fly larvae, this process is taken to an extreme
degree, creating huge cells that contain hundreds or thousands of copies of the
(A)
(B)
100 µm
20 µm
Figure 4–47 Lampbrush chromosomes.
(A) A light micrograph of lampbrush
chromosomes in an amphibian oocyte.
Early in oocyte differentiation, each
chromosome replicates to begin
meiosis, and the homologous replicated
chromosomes pair to form this highly
extended structure containing a total of
four replicated DNA double helices, or
chromatids. The lampbrush chromosome
stage persists for months or years, while
the oocyte builds up a supply of materials
required for its ultimate development into
a new individual. (B) An enlarged region
of a similar chromosome, stained with a
fluorescent reagent that makes the loops
active in RNA synthesis clearly visible.
(Courtesy of Joseph G. Gall.)