Chromosomal structures and transposable elements Package DNA in
Chromosomal structures and transposable elements -Package DNA in limited space: supercoiling
Bacterial chromosome -Unique DNA molecule -Naked DNA no histones
Eukaryotic chromosome -Highly compacted -Euchromatin and heterochromatin
CHROMATIN
Nucleosomes are DNA wrapped around a protein octamer made by dimers of histones H 2 A, H 2 B, H 3 e H 4
HISTONES H 1 are highly conserved, small, basic proteins Linker histone H 2 A H 2 B helix Core histones variable H 3 H 4 conserved N Histone acetylation is a reversible modification of lysines in the N-termini of the core histones. Result: • reduced binding to DNA • destabilization of chromatin
Histone octamer assembly H 3 -H 4 tetramer Histone octamer H 2 A-H 2 B dimer
< 11 nm > Histone octamer organizes 145 bp of DNA < 6 nm > • Each core histone dimer has 6 DNA binding surfaces that organize 3 DNA turns; • The histone octamer organizes 145 bp of DNA in 1 3/4 helical turn of DNA: 48 nm of DNA packaged in a disc of 6 x 11 nm
30 nm fiber Chromatin decondensed (beads on a string)
Nuclear - chromosome compaction + 2 M Na. Cl histones chromatid 1 mm Mitosis DNA loops mitotic chromosome 10 mm radial loop chrosome model + 2 M Na. Cl histones chromatid Compaction by chromosome scaffold / nuclear matrix
Variation in the chromatin structure -Polytenic chromosomes in Drosophila, DNA replication non associated to cell division. -Sensitivity to DNAse I correlates with gene activity
Polytenic chromosomes
DNAse I sensitivity
In chicken erythoblasts, DNAse. I sensitivity correlates transcription of globin genes
Chromatin fibers 11 nm 30 nm chromatin fiber + charged N termini (bind DNA on neighboring nucleosomes) (beads) highly acetylated core histones (especially H 3 and H 4) • High level of histone H 1 • Reduced level of histone H 1 • NO gene transcription • Gene transcription possible
Denaturation and renaturation of DNA
The structure of the centromer -Chromosomes without centromers are lost in mitosis Centromers are made of repetitive sequences
Telomeric structure
Transposable elements -Mobile DNA elements found in the genomes of all eukaryotes -They can cause mutations
2. NON REPLICATIVE transposition RNA intermediates: retrotrasposons
Mutagenic effects
Bacterial transposable elements Insertion sequences: - sono semplici elementi trasponibili che portano unicamente le informazioni necessarie per la trasposizione
Composite transposons: Intervening DNA
Non composite transposons: -no intervening DNA, repetitive sequences -some phages use transposition to insert their genome into the bacterial DNA.
Transposable elements in eukaryotes Yeast
Barbara Mc. Clintock discovered transposable elements studying variegated maize
Ac e Ds transposable elements in maize
Drosophila transposable elements
Human transposable elements Distribution of genes and other sequences in the genome • The majority of transposons are inactive – Very few are currently active • The transposons may have played an active role in genome evolution
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