(16)
12. Beskrive kortfattet hvordan DNA er organiseret i nukleosomer og kromatosomer ved hjælp af histon proteiner.
Stryer, s.875-7, fig. 31.16
Devlin, s.68-9, fig. 2.47
Devlin, s.348-9, fig. 8.18

Histones - small basic proteins that are used for packing DNA. They have a high content of the basic amino acids lysine and arginine, and are bound to the negatively charged DNA backbone. A histone is built up of:

• central non-polar region - forms a globular structure

• a -N and a -C terminal regions - contain most of the basic groups

Five major histones are present in DNA: H1, H2A, H2B, H3 and H4.
- H1 is larger then the other histones, has more basic amino acids and is species and tissue specific.
- an additiional histone, called H5 can also be used for DNA-packing and has a similar function to H1.
- H3 and H4 are very similar in stricture and are the most susteined ones in evolution.  

Nucleosome - disk-shaped structure consisting of:

• a core of eight proteins, octameric histone cluster:

- 2 x H3
- 2 x H4
- 2 x (H2A-H2B)

•  negatively supercoiled DNA segment, wrapped around the histone octamer 1.75 times

Because the DNA makes a negative supercoil around the histones, when the DNA is straightened out, it will be unwinded. This unwinding is exactly what is needed to separate the two DNA strands during replication and transcription. 

Nucleosomes are not radnomly (periodically) created in the chromatine, it all depends on the DNAs willingness to bend. Long A-tracts and regions rish in C-G resist bending, while short A-tracts are willing to bend.

Chromatosome

• a core of eight proteins - octameric histone cluster:

- 2 x H3
- 2 x H4
- 2 x (H2A-H2B)

• negatively supercoiled DNA segment, wrapped around the histone octamer 2 times.
   

• protein H1, which has a different structure from the other histones and is species specific, seals off the nucleosome at the location at which a linker DNA enters and leaves the nucleosome.

The linker DNA joins the nucleosomes into a polynucleosome.

Some interesting numbers:

• About 200 bp of DNA make up a single nucleosomal unit, with about 130-160 bp in direct contact with the octamer core (average 146bp)

• Some of the remaining DNA binds histones H1 and H5 and the rest is linker DNA between nucleosomes. Linker-DNA is 20-90 bp long.   

In order for the DNA to interact with transcription factors, two conditions need to be met:

• The DNAs major groove needs to be facing away from the histone. The major groove is the one contacted by the transcription factors. A pattern has been established that in A-T rich regions the minor groove is in the inside of the nucleosome core, while the major groove is left free for protein interactions (and v.v. for C-G rich region). However, there is still no clear way to predict which groove will face the outside of the nucleosome.
   

• The interaction between the DNA and the histone proteins need to be week. Acetylation of ξ – amino group near the N-terminus of the histones reduces the positive charge carried by the histone and thus weakens the electrostatic interaction between the histones and the DNA. In general, the acetylation (euchromatin) leads to activation of gene expression, while deacetylation (heterochromatin) reverses the effect.
  (Devlin, s.348 - fig.8.18)

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