6e Chapter28 DNA replication 2007

6e Chapter28 DNA replication 2007 - 2002-2007 by Changwon...

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© 2002-2007 by Changwon Kang Chapter 28 1 Chapter 28: DNA Replication, Repair, and Recombination A. Faithful copying of genetic information is essential. (1) Two strands are separated locally for each to serve as a template for replication. (2) Error rate of ~10 10 is achieved by 10 4 during replication and 10 7 during proofreading. (3) DNA can be damaged in various ways and must be repaired by repair system. (4) DNA recombination repairs when both strands are broken, generates genetic diversity in meiosis and in immune system. B. Various forms of DNA (1) B-DNA: Watson and Crick deduced the double helix structure in 1953. 1. A right-handed helix with two chains running in opposite directions. 2. An acid (located in nucleus) with phosphate-deoxyribose units outside of the helix and various bases inside. 3. Bases are paired specifically through hydrogen bonds, A=T and C G. (2) A-DNA in dehydrated DNA 1. H 2 O neutralizes the repulsion between phosphate chains. When H 2 O molecules are too few, the two chains are pushed away to form A type. 2. Base pairs are tilted rather than perpendicular to the helix axis with C-3'-endo puckering. (B-DNA is in C-2'-endo puckering.) Right-handed A-DNA is wider and shorter than B-DNA. 3. RNA duplexes and some RNA-DNA hybrids adopt A-form, because the 2'-O crashes with other atoms in B-form but projects outward away from other atoms in A-form helix. (3) Major and minor grooves are formed in A- and B-from duplexes. 1. Two grooves are formed in a double helix, because two glycosidic bonds point to one direction rather than in two opposite directions. 2. The minor groove is formed on the glycosidic bond side of base pairs, and the major groove is formed on the opposite side. 3. Major and minor grooves should not be defined by width and depth. Distance between phosphate backbone chains (width): A minor > B major > A major B minor Distance between base pairs and front surface (depth): A major > B major > B minor > A minor 4. Each groove provides potential hydrogen-bond donor and acceptor atoms. 5. The major groove displays more features for distinguishing base pairs than the minor groove. 6. B-major and A-minor grooves are wide enough for protein access to base pairs for specific interactions. (4) Local DNA structures are not uniform. 1 The rotation angles range from 28 ° (loosely wound) to 42 ° (tightly wound) [average is 36 ° ]. 2 Two bases are not always coplanar. This propeller twist enhances base stacking along a strand. 3 Local variations depend on base sequences, water content and salts. Sequence-specifically binding proteins may sense the precise shapes. 4 A left-handed Z-helix can be formed at purine-pyrimidine-alternating sequences under high salt concentrations. The phosphate backbone zigzags along the helix axis. C. Supercoiled structures
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This note was uploaded on 05/05/2008 for the course BIOL 4094 taught by Professor Bartlett during the Spring '03 term at LSU.

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6e Chapter28 DNA replication 2007 - 2002-2007 by Changwon...

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