• Total DNA = 6x10⁹ nt
• 1.3x10⁸ nt per chromosome
• average chrom length ~5cm long
• nucleus diameter ~5mm
• thus DNA must be compacted 10,000 fold
• achieved by folding DNA into chromatin
• 50% weight of chromatin is DNA, other 50% is protein (histones)

• H1, H2A, H2B, H3, H4
• Very basic, highly charged
• Highly conserved throughout evolution
• H2A, H2B, H3, H4 Þ 2 every 200bp DNA
• H1 Þ 1 every 200bp DNA

• Regulatory proteins at extremely low levels
• activate or repress certain genes
• proteins which locally modify DNA and/or chromatin
• effect expression of genes at that location
• enzymes that modify histones

Nucleosomes (beads on string)

• Basic repeating unit of chromatin
• 2 molec H2A, H2B, H3, H4 with 146 bp DNA Þ nucleosome core particle
• 1 molec H1 with 50 bp DNA Þ spacer DNA
• thus 200 bp per nucleosome
• DNA wrapped in 1.8 helical turns around the histones
• Compaction ratio Þ 6-7 fold

• solenoid model (superhelical coil of nucleosomes)
• nucleosomes oriented radially around the helix
• ~6 nucleosome strans per helical turn
• H1 is required for the compaction of the nucleosomes
• Compaction ratio » 35 fold

• Nuclear matrix = nuclear components remaining after nuclei extraction
• structural scaffold to which chromatin is attached
• loop domains of chromatin between the attachment sites
• susceptible to torsional strain due to twisting or supercoiling

• Heterochromatin
• dense staining
• highly compacted inactive genes
• Euchromatin
• lightly stained
• less compacted transcriptionally active genes
• correlation btwn less dense and more active
• decondensation probably precedes gene activation