Nucleosomes and the beads-on-a-string structure
A nucleosome is DNA wound round a core of histone proteins (proteins that DNA winds around), the first level of DNA packaging and the bead in beads on a string.
Remember
Beads on a string means nucleosomes, DNA wrapped round a core of eight histone proteins (an octamer), joined by short stretches of linker DNA; the more compact forms of chromatin are built on this unit.
Each cell holds around 2 metres of DNA, so it has to be packaged to fit inside the nucleus. The first level of packaging is the nucleosome. About 147 base pairs of DNA wrap around a histone octamer, a core of eight histone proteins made of two copies each of H2A, H2B, H3 and H4. Picture thread wound on a spool: the histones are the spool and the DNA is the thread.
Between neighbouring nucleosomes lies a short stretch of linker DNA. Under a transmission electron microscope (one that passes a beam of electrons through a thin specimen), the nucleosomes look like beads and the linker DNA like the string joining them. That is the classic beads on a string picture of chromatin, the complex of DNA and its packaging proteins.
This level of packaging shortens the DNA a great deal, yet genes can still be read (transcribed, meaning copied into RNA) during interphase, the stage between cell divisions. Loosely packed euchromatin has DNA that is generally more accessible for transcription, whereas densely packed heterochromatin has DNA that is generally less accessible. Both are built from nucleosomes.
Nucleosomes can fold into higher-order structures, which are levels above the individual beads. In the solenoid model they coil further into a thicker 30 nm fibre, but this is not a universal arrangement in living cells. The most condensed form is the metaphase chromosome, seen during cell division, and it hides the bead pattern.
Key facts
- Nucleosome
- about 147 base pairs of DNA wrapped around a histone octamer, the basic unit of chromatin.
- Histone octamer
- a core of eight histone proteins, two copies each of H2A, H2B, H3 and H4.
- Linker DNA
- the short DNA between neighbouring nucleosomes, which forms the string.
- Beads on a string
- the nucleosomes are the beads, seen with a transmission electron microscope at a low level of compaction.
- Order of compaction
- DNA, then nucleosomes (beads on a string), then higher-order folding, then the mitotic chromosome.
- Solenoid model
- the 30 nm fibre is a higher-order fold that is not a universal arrangement in living cells.
Exam traps
- The solenoid model has nucleosomes coiling further into a thicker 30 nm fibre, a level above the individual beads, so it does not account for the beads themselves, and it is not a universal arrangement of chromatin.
- Metaphase chromosomes do not show the beads, because chromatin is packed so tightly in dividing cells that the repeating bead pattern is hidden.
- Linker DNA is the string, not the bead, because it lies between histone cores and the beads themselves are the histone-wrapped nucleosomes.
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