MCB 121 - Lecture 3

MCB 121 - Lecture 3 - MCB 121 Lecture 3 DNA replication I...

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MCB 121 - Lecture 3 DNA replication I. Basic rules II. Enzymology III. Replication Fork IV. Topology ------------------------------------------------------------------------------------------------------------------------- I. Basic Rules - initiates from origin of replication ( bidirectional ) - DNA polymerase always require a primer , cannot create DNA sequence de novo. * primer is created by primase * primase lays down RNA sequence as the template 3' hydroxyl of RNA makes the nucleophilic attack onto alpha phosphate of the nucleotide ** notes that RNA has the ribose sugar (2' and 3' OH) - elongation occurs in 5' to 3' direction of growing chain . - elongation is semidiscontinuous = one strand is not synthesized continuously (lagging) and another strand (leading) is. * lagging strand is elongated in segments, made up by Okazaki fragments * Okazaki fragments = combination of RNA primer and DNA fragment - process of DNA replication is semiconservative = the production of replication, one strand is from parental (conversed), another one is newly formed.
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- when chromosomes replicate, the synthesized DNA is called replicon . = replicon is the DNA sequence synthesized from one origin - Diagram of bidirectional fork. MAKE SURE YOU KNOW THIS FOR EXAM!!!!!! * bidirectional fork = two forks in opposite direction * leading strand has only 1 primer associated with it. * lagging strand associates with more than 1 primer. Basic Mechanism: 3’ OH of priming nucleotide makes nucleophillic attack ( sn2 reaction ) on alpha phosphate on incoming complementary nucleotide (dXMP) n + dXTP → (dXMP) n+1 + p-p ( pyrophospate ) p-p   2 p (carried out by  pyrophosphatase ), this step makes polymerization step  irreversible List of factors. 1. Primase: make RNA primer (template) - this is the enzyme that can lay down both RNA and DNA 2. DNA polymerase: synthesis; nucleotide addition - in eukaryote, it's called DNA Pol alpha - for leading strand : DNA pol э(epsilon) - for lagging strand : DNA pol λ (delta) 3. Helicase: unwind duplex DNA at fork 4. Clamp: keep DNA polymerase from falling off the template DNA 5. Clamp loader: travel along with polymerase 6. single-strand binding protein (ssb) or Replication protein A (RPA): keeps ssDNA from folding
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MCB 121 - Lecture 3 - MCB 121 Lecture 3 DNA replication I...

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