Relaxing Supercoiling in Chromatin Loops √When histones are The uired for reproduction or display (a) 30-nm fiber removed,30-nm fibers and nucleosomes disappear Remove histones (b √Leaves supercoiled DNA double helix 600500000000 DNA duplex Nick DNA √Helical turns are DNA double helix superhelices,not ordinary double helix √DNA is nicked to relax 13-11
13-11 Relaxing Supercoiling in Chromatin Loops ✓ When histones are removed, 30-nm fibers and nucleosomes disappear ✓ Leaves supercoiled DNA duplex ✓ Helical turns are superhelices, not ordinary double helix ✓ DNA is nicked to relax
13.3 Chromatin Structure and Gene Activity Histones,especially H1,have a repressive effect on gene activity in vitro Two families of 5S rRNA genes studied are oocyte(卵母细胞)and somatic(体细胞)genes -Oocyte genes are expressed only in oocytes -Somatic genes are expressed both in oocytes and somatic cells -Somatic genes form more stable complexes with transcription factors 13-12
13-12 13.3 Chromatin Structure and Gene Activity ✓ Histones, especially H1, have a repressive effect on gene activity in vitro ✓ Two families of 5S rRNA genes studied are oocyte (卵母细胞) and somatic (体细胞) genes – Oocyte genes are expressed only in oocytes – Somatic genes are expressed both in oocytes and somatic cells – Somatic genes form more stable complexes with transcription factors