A tour of the cell
A tour of the cell

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

A tour of the cell

The arrangement of DNA in the nucleus

You will recall that in eukaryotes, DNA is organised into chromosomes, each of which contains a single, very long double-stranded DNA molecule. These molecules are much longer than the diameter of the nucleus, and are packaged into the nucleus by winding around proteins known as histones, which help to coil the DNA, as shown in Figure 12. This complex of DNA and proteins is known as chromatin. The binding of DNA to histones is very ordered and the degree of packing varies. The DNA is most tightly condensed in mitotic chromosomes (as shown in Figure 12) just before cell division, when the highly condensed chromosomes can be easily seen under the light microscope. The rest of the time, individual chromosomes are not visible by light microscopy because they are far less condensed, allowing easy access to the enzymes that carry out DNA replication and transcription. Constant adjustment of DNA packaging and the spacing between nucleosomes is one way in which gene expression is controlled.

Described image
Figure 12 Schematic diagram illustrating the ordered binding of DNA to histone proteins. The double helix is wound around a complex of histone proteins (shown in purple), forming a series of structures called nucleosomes, which look rather like beads on a string. This compacts the chromatin into a strand which has a width of 11 nm. The complex winds further and then loops and condenses. The greatest degree of chromosome condensation is seen in cells that are about to divide.

If you look at Figure 11a, you see that some areas of chromatin (particularly around the edges of the nucleus) have a dark, electron-dense appearance, while other areas of the nucleus appear paler. The chromatin in the dark areas is known as heterochromatin and is more highly condensed than the DNA in the paler areas which is called euchromatin. In general, heterochromatin contains far fewer genes and is far less 'transcriptionally active' than euchromatin.

S294_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371