Nucleic acids and chromatin
Nucleic acids and chromatin

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Nucleic acids and chromatin

8.2 Chromosome scaffolds

Most of the chromosomal DNA chains within the interphase nucleus are believed to be held on a scaffold or backbone structure made from various proteins, with loops of between 20 and 200 kb extruding from attachment sites. This chromosome structure is shown schematically in Figure 40. The scaffold, as well as permitting further compaction, serves to bring the DNA together in organised regions. There are many different protein components of these scaffolds, amongst them DNA topoisomerases.

Figure 40
Figure 40 Loops of 30 nm chromatin fibre held on a scaffold in the eukaryotic nucleus. Loops can undergo decondensation, as a result of histone modification and/or topoisomerase action, when access is required by the cell (lower part of figure).

SAQ 35

How might DNA topoisomerases be involved in regulating the chromatin structure in loops?

Answer

DNA topoisomerases cut the DNA backbone and allow the strands to rotate around the helical axis, resulting in the addition or removal of DNA twists. The action of this enzyme could therefore result in relaxation or compaction of individual loops through the introduction or removal of supercoils.

Compaction is also influenced by the status of the histone tail modifications. As an example, remember that the addition of an acetyl group to lysine residues effectively removes their positive charge, reducing the ability of the histone tails to interact with the DNA backbone and thereby hindering compaction. Thus histone modification and topoisomerase activity can regulate the activity of genes in a looped region and thus regulate genes across longer stretches of DNA. These regions of chromatin are referred to as domains. An additional important implication of this scaffolding arrangement is that the action of topoisomerases and ensymes that modify chromatin could alter the compaction of individual loops without affecting neighbouring domains, as shown in Figure 40.

Finally, if you now look back at Figure 12, you can also see why transcriptionally active regions flanked by attachment sites need topoisomerase activity to release the torsional stresses that build up ahead of the transcription complex.

In most cases, visualising such scaffold and loop structures with the light microscope is impossible, due to the limited resolution and the diffuse nature of the DNA. However, it is possible, even at the light microscope level, to see such arrangements in the so-called lamp brush chromosomes in the developing amphibian oocyte (egg cell). The DNA in these cells is highly transcriptionally active as the oocyte is synthesizing large stores of protein. If you look at the example in Figure 41, you can see large loops extending out from a scaffold-like structure.

Figure 41
Figure 41 Lampbrush chromosomes from the amphibian oocyte.
S377_1

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?
Free Courses

OpenLearn has over 800 free courses across all our subjects designed by our academic experts. From 1 to 100 hours, introductory to advanced, you can start learning at any time and all for free.