curs - material nuclear

7/29/2019 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

6/turn


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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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curs - material nuclear

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