Lecture 2 - DNA Packaging Chromatin DNA is packaged in the...

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DNA Packaging Chromatin DNA is packaged in the nucleus as chromatin, a DNA-protein complex. Chromatin that is transcribed is referred to as euchromatin, while heterochromatin is not transcribed Individual Chromosome In resting cells, chromatin is amorphous and dispersed within the nucleus. Just prior to cell division, chromatin becomes organized into chromosomes. The chromosome number, size and shape is referred to as the cell karyotype and is species specific. Each contains three required functional elements: autonomously replicating sequences (ARS); the centromere (spindle attachment site during nuclear division); and the telomeres (ends), which are involved in completion of DNA replication.
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Histones are small basic proteins that participate in packaging DNA by the formation of nucleosomes. A nucleosome core contains two each of histones 2A, 2B, 3 and 4 with DNA wrapped around the histones. Histone 1 occupies the linker region between nucleosomes. Histones are highly conserved proteins, which attach to DNA by highly specific salt bridges. Histones and Nucleosomes.
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The nucleosome consists of approximately 146 bp of DNA corresponding to 1 3/4 superhelical turns wound around a histone octamer. The H1 histone is associated with the linker DNA
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Changes in DNA packing and gene expression occur in part by covalent modification of core histones (H2A, H2B, H3, H4). Histones undergo reversible acetylation of lysine, catalyzed by the enzymes histone acetylase and histone deacetylase. Histome tail lysines can also be modified by ubiquitination and methylation. Histone Code
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Nucleosomes can form higher order structures such as 300 angstrom fibers resulting in further compacting of DNA
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Basic Elements of a Gene The most current molecular definition is: a gene consists of all the DNA sequences necessary to produce a single polypeptide or RNA product.
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The human haploid genome contains about 3.5 x 109 basepairs. Although the genome contains enough DNA to encode nearly 3 x 106 genes, no more than 25,000 genes are encoded. A gene also contains DNA sequence that dictates the manner and to some extent the degree that the protein will be expressed or synthesized. Only one DNA strand codes for the protein, this is called the sense strand. The other strand does not code for the protein and is referred to as the anti-sense strand. A gene consists of both a sense and antisense strand. A pseudogene is a DNA sequence that resembles a known gene, but exists at a different locus, and either lacks important DNA coding or regulatory sequence required for gene expression or may contain an inserted stop signal.
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Defining regions of a gene: Promoter : Sites at which an RNA polymerase and other regulators proteins bind to DNA. Eukaryotic genes utilize a variety of promoters. Cis
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This note was uploaded on 10/13/2011 for the course NS 3200 at Cornell University (Engineering School).

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Lecture 2 - DNA Packaging Chromatin DNA is packaged in the...

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