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Lecture 6

Lecture 6 IS Genomic Replication word.doc

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Western University
Biology 1001A

Lecture 6 - IS 1. Purine and Pyrimidine base-pairing in DNA/RNA - DNA has four nucleotides each with a five-carbon sugar deoxyribose, a phosphate group (phosphodiester bond) and four nitrogenous bases. Nucleo- tides join to form a polynucleotide chain with a sugar-phosphate backbone - Purines: adenine and guanine (built from a pair of fused rings of car- bon and nitrogen); three hydrogen bonds - Pyrimidines: thymine and cytosine (built from a single carbon ring); two hydrogen bonds - Purine always pairs with Pyrimidine: adenine with thymine, guanine with cytosine - DNA is in a double helix model and has complementary base pairing. The polynucleotide chain twists around each other in a right handed way - The two strands of a double helix fit together stably only if they are antipar- allel (run in opposite directions) - nucleotides in groups of 3 (codon) make enough to make a protein 2. Outcome of the classic Meselson and Stahl experiment - They demonstrated that DNA replication is indeed semiconservative - They distinguished parental DNA molecules from new ones by tagging a heavy nonradioactive nitrogen isotope to the parental DNA 3. Direction of movement of DNA polymerase on the template strand - DNA polymerase can only add nucleotides only to the 3’ end, therefore moves from the 5’ to the 3’ end - DNA polymerase catalyses Deoxyribonucleoside triphosphates, which are the substrates for the polymerization reactions - At the Origin of Replication, DNA helicase unwinds the two parent strands of DNA - This unwinding creates a Y shaped structure called a replication fork - Single Stranded Binding Proteins (SSBP) coats the singlestranded DNA seg- ments which keeps the two strands from pairing back together - Topoisomerase cuts the DNA ahead of the replication fork, turns the DNA on one side of the break in the opposite direction of the twisting force and re- joins the two strands - Primase synthesizes a primer made of RNA so that the DNA polymerase can start adding nucleotides. Primer eventually removed and is replaced by DNA - In leading strand DNA polymerase III just synthesizes straight - Lagging strand, there is discontinuous replication where the polymerase synthesizes in the direction opposite to the DNA unwinding in short lengths called Okazaki fragments. Polymerase III adds nucleotides and DNA poly- merase I uses 5’ to 3’ exonuclease activity and removes the RNA primer at the 5’ end of the previously newly synthesized Okazaki fragment and its 5’ to 3’ polymerization activity to replace the RNA nucleotide
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