Chromosome Wilhelm Gottfried Waldeyer (1836 - 1921) +.> Basics of chromosome structure 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/ entire mitotic chromosome 2*nm T 1 nm i 30 nm 1 T 300 nm 1 í 700 nm I T 1400 nm 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 l Chr. 3 23.2 Mb ^^^^^^^^^^^^H p7h^_iii min : n mini [%■ 1 Mill HUM J ■nn mm 1^31 TE IIIIIIIIIIIII11BH ■■minim MT/CP I I ........Mill II III____'l III li' I "in ii i J ii i l\ wM 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 s> cell division {rmtosis) cycle begins cell prepare to divide cell grows replication of DNA ceil 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\ ?-. ^ *> ., b * - J ■ Wk * vr WaSA*" W t*2 f^^TB ■ ^^ ^J iL /*^i ^ \ *Ěa j S Birchler et al. 2003, Genetics 164 Meiotic drive The ability of one homolog to enhance its probability of transmission at the expense of its partner (e.g. in Aa heterozygote, A-bearing gametes are produced more frequently than a-bearing gametes). preferential transmission of the Ab 10 chromosome the 1:1 segregation (normal chromosome 10) Meiotic drive in maize Preferential transmission of the knob-bearing chromosomes during female meiosis. But only if the Ab 10 chromosome is present. heterozygote for Ab 10 \ crossing-over located between the knob and centromere \ cross-over products that carry the knob on only one of its two chromatids (heteromorphic dyad) \ pseudokinetochore activity of the knob direct the knob-bearing Chromatide to one of the four products of meiosis II, which is one of the two "outside" nuclei, and the only one that is available for fertilization Birchler et al. 2003, Genetics 164 Megasporoge nes is and meiotic drive in maize Female meiosis (megasporogenesis) is asymmetric: -out of 4 haploid products only one will become the egg; other three degenerate - the outermost (basal) megaspore differentiates into the megagametophyte via a few mitoses to produce the egg, polar nuclei, and associated cells Knob-bearing chromatids are pulled towards the outermost megaspores during meiosis II ahead of the centromeres. Consequently, instead of a 50% expected ratio of transmission in a heterozygote, knob transmission in female meiosis varies from 59 to 82%. Birchler et al. 2003, Genetics 164 Het knobs in rice AAAA AAA* het knobs rice chromosome 4 Jiao et al. 2005, Plant Cell M Heterochromatic segment 1 found in Brachycome dichromosomatica (Asteraceae) The terminal knob contains the Bdsl tandem repeat. Houben etal. 2000, Chromosoms 109 YňCs BňCs Het knob hk4S in Arabidopsis The hk4S originated by an inversion event that relocated pericentromeric sequence to an interstitial position. chromosome 4 1 uin centromere region pALI 5S i DNA knob hk4S mi306 mi2)3 gal CICĚH11 CIC11H10 I I 73ZA17 I I I I CIC7C3 | CICSB1 ' I I d FZIIi C1ÍL7 TUl TíĚrtZB F9H3 • *- JL ] G Fransz et al. 2000, Cell 100 "Just get in the lab and start to work; you can't help but find something." Marcus Rhoades