The big idea: In eukaryotes, DNA is wound around balls of protein called histones. One ball with its DNA is a nucleosome: the basic unit of packing in a chromosome.
The points to remember
- A nucleosome is a length of DNA wrapped around a core of eight histone proteins.
- The core is a ball of eight histones, called an octamer: four types of histone, two of each.
- The DNA double helix wraps around the outside of the core almost twice.
- A ninth histone (H1) binds where the DNA enters and leaves, holding the DNA on the core.
- Linker DNA joins one nucleosome to the next, like the string between beads.
- Nucleosomes help pack long DNA molecules into the nucleus. Only eukaryotes have them: bacterial DNA is naked.
Remember it as: Eight in the bead, one on the string.
Real example: the nucleosome in atomic detail: In 1997 Karolin Luger and her colleagues used X-ray crystallography to work out the nucleosome core atom by atom: 146 base pairs of DNA wrapped about 1.65 times around the eight histones.
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The big idea: Computer models let you turn a nucleosome round, zoom in and colour its parts. Exam questions show such a model and ask what each region is, or what the software is good and bad at.
The points to remember
- Molecular visualization software builds a 3-D model from data on the position of every atom.
- The DNA shows as a double helix on the outside, wrapped round the core.
- The core shows as proteins: you can see their alpha helices, and colouring by chain shows the eight histones.
- Histones are positively charged and the DNA's phosphates are negatively charged, so the two attract and hold together.
Benefits of the software
- Shows a structure too small to see directly
- Can be rotated to see it in 3-D from any angle
- Can zoom in on one part, or colour parts to tell them apart
Drawbacks
- It is a model: simplified, so some detail is left out
- It rests on assumptions and may not match the real structure exactly
- It can be costly to make or run
Try it yourself: On the RCSB Protein Data Bank website, search for the structure 1KX5 (a nucleosome core) and open the 3-D view. Rotate it to find the DNA outside and the histones inside.
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How it is asked: Paper 1A: what a nucleosome is, or labels on a diagram. Paper 1B, on a computer model: identify a region with a reason, describe the core or the whole nucleosome, or give a benefit and a drawback of the software.
The image of a nucleosome was made using molecular visualization software. Describe the structure of the nucleosome.
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The skill question that goes with the model:
Suggest one benefit and one disadvantage of using molecular visualization to study a nucleosome.
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See the answer broken into clear points, with guidance on how each point earns marks.