Download - Curs - material nuclear
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Time for replication,
transcriptionTime for cell division: no
gene expression
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Chromosomes contain the genetic material Genes are physically located within the
chromosomes
Chromosomes are composed ofDNA andproteins
Primary function of genetic material is tostore needed information
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Typical chromosome contains a single, linear,double-stranded DNA molecule
DNA must be folded and packaged
Chromatin is the DNA-protein complexmaking up chromosomes
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Chromatin is the combination ofDNA andproteins found in eukaryotic chromosomes.
Proteins in chromatin are: histones: small, positively charged
non-histone proteins: transcription factors,enzymes
Packaging ofDNA with proteins leads tocompacted structure, which facilitates fittingthe DNA into the nucleus
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Stains darkly (highly condensed)
Repetitive sequences
Replicates later in the cell cycle
Little or no recombination Transcriptionally repressive:silences gene expression
Stains lightly (decondensed)
Single copy sequences (genes)
Replicates early in the cell cycle
Recombines
Transcriptionally active:permissive for gene expression
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How do you fit approximately 2 meters (human diploidnucleus) into a space that averages maybe 5 millionthsof a meter wide? How do you replicate, repair and transcribe tightlypackaged DNA?
Diameter of nucleus = 5-10 mmDNA must be packaged to protect it, but must still be accessibleto allow gene expression and cellular responsiveness
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1. Nucleosome (11 nm)
2. Solenoid (30 nm)
3. Loops (300 nm)
4. Coiled loops (700 nm)5. Metaphase chromosome (1400 nm)
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Level 1:
nucleosome formation
Level 2:
30 nm fiber
Level 3:
Nuclear scaffolding
Level 4:
Mitot ic (metaphase)
chromosome
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Nucleosomes are formed by winding about200 base pairs of the DNA duplex around acore of histone proteins.
The histone core is made of: 2 molecules histone H2A
2 molecules histone H2B
2 molecules histone H3
2 molecules histone H4 Another histone, H1, binds outside the core.
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string on a bead for obvious reasons
Linker can vary (8-114bp or more)
Compaction ratio is approx 7 fold
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30 nm fiber is coil of nucleosomes with
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A 30 nm diameter chromatin fiber
The zig-zag model, H1 histone has a role in contraction
of nucleosomes Compaction ratio approx 100 fold
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The 30nm fiber is
organized to loops
that can beopened
up individually
300 nm coiled chromatin
fibersradial loops
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Form a structural scaffolding to which loops of
chromatin are attached
nuclear matrix (or chromosome scaffold)
2 scaffold proteins are found (Non histones proteins)
30-nm fibers interact with the nuclear matrix
Compacts 30-nm fiber by participating in theformation of radial loop domains
Anchored to nuclear matrix
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1. Total lenght of
mitotic
chromosome
is:180 mm
Haploid genom: 2 m
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