Chapter 11 and 12 - Nucleus and Control of Gene Expresion

The human cell contains 46 chromosomes each

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Unformatted text preview: s a single, continuous DNA molecule The larger the chromosome, the longer the DNA but stretched out all the DNA would be 2 m long, and the nucleus is 10^-5 m wide therefore chromosomes need to stay separate from one another Chromosomes and associated proteins are called chromatin Histones orderly package DNA they are small proteins rich in arginine and lysine Histones are divided into 5 classes, depending on argininine/lysine ratio and they are highly conserved Histones interact with the backbone of DNA and often other histones In early 1970s, chromatin was treated with nucleases DNA was chopped into 200 base pair fragments but naked DNA, without proteins, produced random sized fragments which means that the proteins must be protecting DNA from digestion In 1974, Kornbers proposed these chromatin structures called nucleosomes Nucleosomes o o Each nucleosome contains a nucleosome core particle 146 base paris of supercoiled DNA wrapped almost twice around a disk shaped complex of histone molecules The histone core consists of 2 copies of each of H2A, H2B, H3 and H4 assembled into an octamer 7 o o o o o Histone H1 is a linker histone binds to link DNA that connects one nucleosome to the next When H1 molecules are removed, DNA appears like beads on a string DNA and histones are held together by noncovalent bonds Assembly of nucleosomes is the first step in compaction of DNA Nucleosome is 10nm if the 200 base pairs of the nucleosome were stretched out they would be 70nm packing ratio of 7:1 Chromatin Structure o o o o o o A 30nm filament is another level of chromatin packaging, maintained by histone H1 Formation of 30nm filament increases packing ratio to 40:1 30nm chromatin fiber is gathered into a series of large, supercoiled loops (domains) May form even thicker (80-100nm) fibers DNA loops are tethered to proteins that are part of nuclear matrix The ultimate compaction occurs in mitotic chromosome where the packing ration is 10 000:1 Heterochromatin and Euchromatin o o o o o o o o o o o o After mitosis, highly compacted mitotic chromosomes either return to a dispersed state or remain condensed Euchromatin returns to a dispersed state after mitosis Heterochromatin is condewnsed during interphase Heterochromatin has little transcriptional activity Whether a region of genome is in heterochromatin or euchromatin is inherited from one cell to the next Heterochromatin is divided into 2 classes: constitutive heterochromatin and facultative heterochromatin Constitutive heterochromatin remains condensed all the time Facultative heterochromatin is inactivated during certain phases of the organism s life or in certain types of cells Constitutive heterochromatin: found mostly around centromeres and telomeres, consists of highly repeated sequences and few genes DNA that is silenced Facultative heterochromatin: one example is Xinactivation in XX females, one X chromosome remains condensed in heterochromatic clump (Barr Body) X inactivation is a random process, making adult females genetic mosaics in some cells one X is inactive, in other cells the other X is inactive Example: Calico cats males are brown or black but females are calico (brown and black patches) because of random X inactivation Chromatin Activity o o o The histone code hypothesis states that the activity of chromatin region depends on the degree of chemical modifitcation of histone tails Histone tail modifications influence chromatin in 2 ways: serve as docking sites to recruit nonhisto...
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This document was uploaded on 04/10/2014.

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