LSCI1002 - Tracing the hereditary pattern-Part I - Tracing...

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Discovery of the nature of the genetic material by Frederick Griffith, 1928 Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Mouse dies Mouse healthy Mouse healthy Living S cells RESULTS The hereditary material of harmful Streptococcus pneumonia cells transformed harmless cells into killers.
Discovery of the nature of the genetic material Hershey and Chase Experiment, 1952
3.4 nm 1 nm C C C C C C C G G G G G G G T T T A A A 0.34 nm Structural model of DNA provides insights to the basis of DNA replicationFranklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical. Rosalind Franklin Franklin’s X-ray diffraction photograph of DNA Watson and Crick’s semiconservative model of DNA replication
DNA Replication: How does it get started? Replication begins at origins of replication, where the two DNA strands are separated. Replication proceeds in both directions from each origin, until the entire molecule is copied. In an eukaryotic cell Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules 5
Topoisomerase: corrects “overwinding” ahead of replication forks Primase: start an RNA chain from scratch using the parental DNA as a template RNA primer Helicase: untwist the double helix at the replication forks Single-strand binding proteins: bind to and stabilize single-stranded DNA 5ʹ′3ʹ′5ʹ′5ʹ′3ʹ′3ʹ′Various enzymes and accessory proteins participate in the initiation process
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How are the new DNA strands synthesized?
Overview of DNA replication DNA polymerase IIIsynthesizes leading strand continuously Parental DNA Lagging strandis synthesized in short Okazaki fragments by DNA polymerase IIIOrigin of replication Helicase Primase synthesizes a short RNA primer DNA polymerase Ireplaces the RNA primer with DNA nucleotides 3ʹ′3ʹ′3ʹ′5ʹ′5ʹ′5ʹ′5ʹ′Okazaki fragments are joined by DNA ligase Okazaki fragments• DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork. • For the lagging strand, DNA polymerase works in the direction away from the replication fork.
Proofreading and Repairing DNA DNA replication is very accurate (an average error rate: < 1 mistake per billion bases). DNA damage: spontaneous changes of DNA caused by harmful chemical and physical agents In most cases, DNA repair mechanisms fix DNA by enzymatically excising and replacing any damaged bases. Nuclease DNA

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