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DNA supercoiling DNA is a big molecule Ruptured viral particle (bacteriophage) X95 000 DNA about 1.7µm © 2010 Paul Billiet ODWS Some DNA molecules Organism Virus ФX174 Molecular Base pairs mass x 106 / 106 Daltons 32 0.000533 Length 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 molecules © 2010 Paul Billiet ODWS Histone core = an octomer (8 histone molecules) 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 coiling 30-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