DNA supercoiling

DNA is a big molecule

Ruptured viral particle (bacteriophage) X95 000

DNA about 1.7µm

© 2010 Paul Billiet ODWS

Some DNA moleculesOrganism Molecular

mass / 106 Daltons

Base pairs

x 106

Length

Virus ФX174

32 0.000533 1.7µm

E. coli 2800 4.2 1.2mm

Human 1 800 000 3000 2m (46 molecules)

© 2010 Paul Billiet ODWS

THE NUCLEOSOME

Uncoiled the DNA of a human would stretch 2m

The average diameter of a nucleus is 10µm

The problem: To pack the DNA into the nucleus and yet have access to the genetic information.

© 2010 Paul Billiet ODWS

THE NUCLEOSOME

Histones are made of a lot of basic amino acids

Combined with acidic DNA they make a stable nucleoprotein called chromatin

Histone core = an octomer (8 histone molecules)

Histone molecules

© 2010 Paul Billiet ODWS

DNA wraps twice round the core

DNA strand

© 2010 Paul Billiet ODWS

DNA held in place by another histone (H1) = the nucleosome

H1

© 2010 Paul Billiet ODWS

Nucleosomes are joined by linker DNA

Linker DNA

© 2010 Paul Billiet ODWS

DNA super coiling

DNA double helix

Nucleosome beads on a string

30-nm chromatin fibre

© 2010 Paul Billiet ODWS

DNA super coiling30-nm chromatin fibre

Chromosome in extended form

Condensed section of chromosome in mitosis

© 2010 Paul Billiet ODWS

DNA super coiling

Condensed section of chromosome in mitosis

Condensed chromosome

© 2010 Paul Billiet ODWS

Nucleosome functions

Nucleosomes help to supercoil the DNA molecule

Nucleosomes help to regulate gene expression (transcription)

© 2010 Paul Billiet ODWS