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chromosome dynamics.docx

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McMaster University
Bhagwati Gupta

October 10 , 2013 Biology 2C03: Genetics Chromosome Dynamics Chromosome Dynamics - Banding pattern allowed geneticist to look for certain characteristics - Drosophila polytene chromosome of the third instar salivary gland If Genes are on Chromosomes, What Exactly are Chromosomes? - DNA is associated with proteins to form chromatin or chromosomes - Chromosomes are compacted and organized within the nucleus. The extent of compaction changes during the cell cycle and when genes are transcribed into RNA - Change is dependent on which greenness are going to be transcribed DNA and Proteins Combine to make Chromosomes - Nucleosome core of eight histone molecules Heterochromatin and Euchromatin - Heterochromatin:  Highly compact throughout cell cycle  Low gene expression  Low gene density  Found at the centromere and telomeres  Difficulty getting additional factors and occurrence of transcription leads to low gene expression and low gene density - Constitutive heterochromatin = always heterochromatic and transcriptionally inactive - Facultative heterochromatin = converts between heterochromatin and euchromatin – during cell cycle - Euchromatin:  Most of chromosome  Gene rich  Transcriptionally active - Euchromatin structure changes as genes are transcribed or turned off Chromosome Structure - EM image of chromatin: beads on a string – histone optimers - Histones Histone Tails - Modifications to the tails of the proteins can affect how accessible DNA will be - Histone tails are modified to alter chromatin structure  Chromatin remodelling  Changes the ability of transcription factors to access DNA  May provide binding sites for other proteins - Combinations of modifications may serve as codes (histone code) to carry out unique function - What genes are transcribed is regulated in part by the histone code depending on the cell type - What modifications may occur on histone tails:  Methylation  Acetylation  Phosphorylation - Chromosomes are dynamic and may change structures Dynamic Chromosomes - Within interphase, chromatin structure changes to allow transcription - Evidence: 1) Cytology:  Chromosomal puff – within this puff chromatin seems less tightly bound, is it – puff is more accessible than the other regions  Several helices that aren’t compacted together 2) Accessibility to DNase:  The enzyme, DNAse, can cut DNA into small pieces, however, it can only access the DNA if the chromatin is in an open conformation (as there is more room/accessibility)  There are 3 functional globin genes in the chicken genome, but they are transcribed at different times in development – is there different sensitivity at different times and in different cell types  None of the globin genes are transcribed in erythroblasts in the first 24 hours (no genes were being transcribed)  Ery
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