Figure 2. Chromosomal loops.

Regions of a chromosome can be unravelled, forming a loop, either to allow specialized functions (such as cell division) or in specific cell types or at specific developmental stages. Examples of DNA extrusion from the axis of the chromosome are shown. a, DNA looping was first observed in squash preparations of salamander eggs under the light microscope, by the embryologist Oskar Hertwig in the early 1900s⁹¹. A single egg is shown here. Scale bar, 50 μm. b, Paulson and Laemmli found DNA loops when examining chromosome spreads in isolated mammalian cells²². In the electron micrograph of this cell in metaphase, loops emanate from the protein-rich chromosome scaffold (the darker stained, X-shaped structure towards the bottom of the image); the inset shows a whole metaphase chromosome, highlighting its similarity to the isolated chromosome scaffold. Scale bars, 2 μm. (Image reproduced, with permission, from ref. 22.) c, DNA loops were also found on a single meiotic bivalent chromosome from the oocyte of the newt Triturus viridescens, by using light microscopy²¹. These are sites of intense transcriptional activity. Each lateral loop represents a single DNA duplex. Scale bar, 15 μm. (Image reproduced, with permission, from ref. 21.)d, And loops were observed in DNA undergoing transcription when heat-shock loci in the fly Chironomus tentans were examined by using light microscopy. The depiction of the findings shows chromosomal DNA (red) as a strand that forms loops (centre) but elsewhere is wrapped tightly in bundles (green; proposed by the authors to be nucleosomes)²⁰. e, DNA looping was first proposed to occur at the kinetochore in mammals⁹². Plates of microtubule-binding segments are tandemly repeated and interspersed with linker segments. On microtubule attachment, microtubule-binding segments loop out from the chromosome axis, forming a cluster of repeats on the surface of the chromosome, as depicted for one loop of centromeric DNA in f. f, DNA looping was later proposed to occur in Saccharomyces cerevisiae also¹⁷. The single pericentromeric loop is comparable to multiple loops of linker segments and microtubule-binding segments (e) that form on microtubule attachment in mammalian cells. The kinetochore (not shown here) forms at the junction between microtubules (green rods) and the unique nucleosome (blue) at the apex of the pericentromeric loop.