Chapter 11_post

Genetics: A Conceptual Approach

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Chapter 11 Chromosome Structure Dr. Ed Otto George Mason University
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Key objectives Discuss DNA packing: how it’s achieved and issues it creates Understand the molecular structure of bacterial and eukaryotic chromosomes Note: we will not cover transposable elements (pp 297-309)
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DNA Packing
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The Ultimate Storage Problem The chromosome in E. coli is 4.6 million base pairs long. Stretched out straight, this is ~1000X the length of the bacterial cell Human cells contain more than 6 billion base pairs, which would measure 1.8 meters if chromosomes were stretched end to end - Even DNA in the smallest chromosome would stretch 14,000X the length of the nucleus How can so much DNA be packed into such small spaces?
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Supercoiling The structure of DNA consists of three hierarchical levels: 1.Primary structure: nucleotide sequence 2.Secondary structure: double-stranded helix 3.Tertiary structure: higher-order folding that allows DNA to pack into the confined space of the cell One type of DNA tertiary structure is ______________ , which takes place when the double helix is subjected to strain by being overwound or underwound
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Supercoiling The lowest energy state for B-DNA is when it has 10 bp per 360 o turn; this is the relaxed state If energy is used to add or remove turns, strain is placed on the molecule, causing the molecule to supercoil, or twist on itself. Molecules that are overrotated exhibit positive supercoiling; underrotated molecules have negative supercoiling Supercoiling is a partial solution to the packing problem because supercoiled DNA occupies less space than relaxed DNA
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Supercoiling
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Supercoiling Supercoiling takes place when the strain of over or underrotation cannot be compensated by the turning of the ends of the double helix - In a linear molecule, extra rotations can be alleviated if the end is free to rotate - In a circular molecule, this is not the case because a circular molecule has no free ends Both bacterial and eukaryotic chromosomes usually fold into loops stabilized by proteins (which prevents the free rotation of ends) and supercoiling takes place within the loops
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Supercoiling Supercoiling is achieved by topoisomerases, which are enzymes that add or remove rotations by temporarily breaking the nucleotide strands, rotating the ends around each other, and then rejoining the broken ends. Most DNA found is cells is negatively supercoiled. Negative supercoiled DNA is underrotated, so separation of the two strands during replication and transcription is easier (requires less energy)
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Bacterial and Eukaryotic Chromosomes
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Bacterial Chromosomes Most bacterial genomes consist of a single ________________ DNA in the relaxed state is too large to fit in the cell, so DNA exists as a series of twisted loops held in
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This note was uploaded on 05/06/2010 for the course BIO 311 taught by Professor Otto during the Spring '10 term at George Mason.

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Chapter 11_post - Chapter 11 Chromosome Structure Dr. Ed...

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