Lecture 26: DNA Packing & Chromatin Structure

Nucleosome

-10 nm in diameter -A nucleosome is DNA wrapped 2X around a core of histones (proteins) -Histone core is 2 copies each of H2A, H2B, H3, and H4 -Histone H1 promotes condensation into the 30 nm fiber. --30 nM Fiber is a Solenoid with 6 nucleosomes per turn

Cell Cycle

-Chromatin undergoes changes in packing during the cell cycle --Cells that are going to divide use a cell cycle program, which can be divided into two main phases, interphase (90%) and mitosis (10%). -In interphase cells, some chromatin is organized into a 10-nm fiber, but much is compacted into a 30-nm fiber and looped. -During mitosis, chromatin condenses further into the chromosomes that are easily visible in a light microscope

More chromatin

-Most chromatin is loosely packed during interphase and condenses prior to mitosis; loosely packed chromatin is called euchromatin. -During interphase a few regions of chromatin (centromeres and telomeres) are always highly condensed, called heterochromatin. --Heterochromatin is strongly transcriptionally repressed (ie, it is transcriptionally silenced) -Histones can undergo chemical modifications that result in changes in chromatin organization, including: Acetylation - looser, methylation- tighter, and phosphorylation

Promoters of active genes are often deficient in nucleosomes

-Nucleosomes do not seem to interfere with RNA or DNA synthesis once it starts (ie, the elongation and termination phases), but can interfere with the initiation phase. -Can also be shown for cellular genes by DNase I digestion of chromatin - promoter regions are hypersensitive to DNase I. -look at ppt: SV40 virus minichromosomes with a nucleosome-free zone at its twin promoters.

Chromatin

-The bacterial chromosome is associated with a small amount of protein that promotes packing of the DNA into a nucleoid. -However, eukaryotic chromosomes are associated with a large amount of protein, and together form a complex called chromatin. -Chromosomes fit into the nucleus through an elaborate, multilevel system of packing. Human chromosomes contain between 200 and 2000 genes.

Learning Outcomes

1. You should be able to describe the different levels of structure of eukaryotic chromatin. Nucleosome 10 nm fiber --> 30 nm fiber from Histone H1 --> looped domains 300 nm fiber --> chromatid 700nm --> replicated/ metaphase chromosome 1400 nm 2. You should know the basic phases of the cell cycle, and how it relates to chromatin structure. Mitotic M phase - mitosis and cytokinesis Interphase - growth & copying of chromosomes before mitosis. 1. G1 phase - first gap 2. S - DNA synthesis 3. G2 phase - second gap 3. You should know that chromosomes occupy "fields" in the interphase nucleus. ??? read about 4. You should what the difference is between hetero- and eu-chromatin. hetero - highly condensed regions of chromatin during interphase; transcriptionally repressed eu - loosely packed chromatin during interphase 5. You should know the composition and structure of nucleosomes, and where they are typically not found on chromatin. - a DNA wrapped 2x around a core of histones (proteins) - often not found on active genes' promoters - because they do not interfere with RNA/DNA synth once it starts (elongation/ termination) but can interfere with initiation phase

Phases of cell cycle

The cell cycle consists of -Mitotic (M) phase (mitosis and cytokinesis) -Interphase (growth and copying of chromosomes before mitosis) Interphase can be divided into 3 subphases: 1. G1 phase ("first gap") 2. S phase ("synthesis") 3. G2 phase ("second gap") The cell grows during all three subphases, but chromosomes are duplicated only during S phase. -Interphase chromosomes occupy specific restricted regions in the nucleus and the fibers of different chromosomes do not become entangled