Basics of chromosome structure Noyau Chromosome Paires de bases (A.T.G.C) Telomere Centromere Telomere Eukaryotic chromosomes • Usually linear • Variable in number • DNA interacts with proteins to form chromatin • Centromeres ensure segregation • Telomeres cap ends • Must be compacted to fit in nucleus chromatin (DNA <& proteins) • highly coiled DNA • histories • non-histone chromosomal proteins (DNA & RNA polymerase, v transcription factors, topoisomerases, histone modifying proteins) Chromosome packing short region of DNA double helix nucleosome "beads-on-a-string" form of chromatin 30-nrn chromatin fiber of packed nucleosomes section of chromosome in an extended form (loop domains) condensed section of chromosome OWWWWX/ 2 nm entire mitotic chromosome NET RESULT: EACH DNA MOLECULE HAS BEEN PACKAGED INTO A MITOTIC CHROMOSOME THAT IS 50,000x SHORTER THAN ITS EXTENDED LENGTH Chromatin structure and gene expression Chromatin compaction limits or enhances access to transcription factors Accessible chromatin is referred to as euchromatin and is active Inaccessible chromatin is called heterochromatin and is generally inactive heterochromatic bands Scheme Of plant Chromosome (after Haslop-Harrison) centromere " : 11 Intercalary tandem repeats Centromere associated tandem repeat Telomeric and sub-Tel omeric repeats mi um Sil LlHrT I i I li Dispersed tandem repeats Dispersed Ty-1-copia-like reiroelements LTR and microsatellites Single and low-copy sequences Including genes Arabidopsis chromosomes centromere i Chr. 3 23.2 Mb 1 ,„.1111 III 1 _ .11111 IIIIIILIIIIIUIII IJ______lUIIIIIIIIIII 1 1 , ' MT/gp HNAs.....y.......' m ii m 'i in m1 i i\n ii ii ii i \ míní The frequency of features was given pseudo-colour assignments, from red (high density) to deep blue (low density). Gene density ("Genes') ranged from 38 per 100 kb to 1 gene per 100 kb; Transposable element densities C TEs') ranged from 33 per 100 kb to 1 per 100 kb. Arabidopsis Genome Initiative, Nature 408, 2000 Mitotic and meiotic chromosomes mitotic chromosomes of Pinus meiotic (pachytene) chromosomes of Antirrhinum Cell cycle, chromosomes and chromatids y cell division {rmtosis) cycle begins cell prepare to divide cell grows replication of DNA cell decides whether to continue Chromosomes and chromatids during mitosis and meiosis 1 chromatid 2 chromatids 1 chromatid Mitosis f \ V^Ja^x y_BL j ~\ r 1 chromatid Meiosis 2 chromatids CenťoBorre ------- Spirdlů microtubule A\ \ ß\