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DNA notes 9/19/10 6:00 PM Outline: Key points on DNA Structure and Replication A) DNA is a double helix molecule • The two strands are held together by hydrogen bonds between the base pairs A=T (2 bonds) and G=C (3 bonds) • The base pairs, like the steps on a spiral staircase, extend in to the center of the molecule • The "frame" of the double helix comes from the phosphatedeoxyribose linkages that connect nucleotides together in each strand • The strands run in opposite directions, that is, the end with a free 5' phosphate on one strand is matched with the free 3'OH of the complementary strand B) DNA is replicated in a semi-conservative fashion • Each strand serves as the template for making a new complementary strand • Nucleotides, in the form of dATP, dCTP, dGTP and dTTP align to the old strand s by base pairing • DNA polymerases connect nucleotides in a 5' to 3' direction only, and only to a pre-existing string of nucleotides (primer) *Replication is continuous in one strand (leading strand) as the old double helix unwinds *Replication in the lagging strand is discontinuous, resulting in a series of "Okazki fragments" that must be connected together by DNA ligase *The primers for both strands are first made of RNA by an RNA polymerase (primase), and then extended using DNA bases by DNA polymerase III ( E. coli) *In E. coli, the RNA primers are removed and replaced with DNA bases by DNA Polymerase I *Both pol I and pol III can also back up and remove a misspaired base (exonuclease activity) • Unwinding of a double helix requires a helicase and also a topoisomerase to relieve supercoiling • Single stranded regions of DNA formed during replication are protected by SSBs, (single strand binding proteins) C) Replication begins at structures called "origins" or just "ori" http://www.tamu.edu/classes/plan/magill/gene603/Lecture%20outlines/Molecular/DNA_notes.html Page 1 of 2 DNA notes 9/19/10 6:00 PM • Origins have repeated sequence motifs, including an AT rich region • Circular DNAs are replicated under several models *E. coli uses bidirectional replication from a single origin (Theta) *T2 replicates using a rolling circle (Sigma) *Mitochondrial DNA starts replicating one strand and the other starts when its origin is opened (D-loop) • Linear chromosmes replicate from many origins D) The ends of linear DNA molecules (telomeres) can't replicate using DNA polymerase alone •The telomeres are 50 -100 repeats of simple sequences, eg. (TTAGGG) •Telomerase, an enzyme that includes complementary RNA 'rebuilds' the ends http://www.tamu.edu/classes/plan/magill/gene603/Lecture%20outlines/Molecular/DNA_notes.html Page 2 of 2