1. DNA supercoiling

2. DNA is a big molecule Ruptured viral particle (bacteriophage) X95 000 DNA about 1.7µm © 2010 Paul Billiet ODWSODWS

3. Some DNA molecules OrganismMolecular mass / 10 6 Daltons Base pairs x 10 6 Length Virus ФX174 320.0005331.7µm E. coli28004.21.2mm Human1 800 00030002m (46 molecules) © 2010 Paul Billiet ODWSODWS

4. 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 ODWSODWS

5. 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 ODWSODWS

6. DNA wraps twice round the core DNA strand © 2010 Paul Billiet ODWSODWS

7. DNA held in place by another histone (H1) = the nucleosome H1 © 2010 Paul Billiet ODWSODWS

8. Nucleosomes are joined by linker DNA Linker DNA © 2010 Paul Billiet ODWSODWS

9. DNA super coiling DNA double helix Nucleosome beads on a string 30-nm chromatin fibre © 2010 Paul Billiet ODWSODWS

10. DNA super coiling 30-nm chromatin fibre Chromosome in extended form Condensed section of chromosome in mitosis © 2010 Paul Billiet ODWSODWS

11. DNA super coiling Condensed section of chromosome in mitosis Condensed chromosome © 2010 Paul Billiet ODWSODWS

12. Nucleosome functions Nucleosomes help to supercoil the DNA molecule Nucleosomes help to regulate gene expression (transcription) © 2010 Paul Billiet ODWSODWS