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1 DNA Replication - Weebly · PDF file DNA Replication is Semi-Conservative In semi-conservative replication DNA one old strand is always conserved in the new molecule. Separating

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  • DNA Replication Life Science: Molecular

  • - James Watson and Francis Crick

    “It has not escaped our notice that the pairing we have postulated immediately suggests a possible

    copying mechanism for the genetic material.”

  • DNA Replication

    n Replication is the process of copying the DNA.

    n Before a cell can divide, the DNA must be copied so that each cell receives its own set of instructions.

  • Chargaff’s Rules

    n Erwin Chargaff discovered... - DNA varied from one species to another in

    particular in the relative amounts of A,C,T,G. - In any DNA the amount of A was the same as the

    amount of T, and the amount of C was the same as the amount of G.

  • n Two complete DNA strands can be created from a single DNA molecule by matching complementary bases.

    Complementary Base Pairing

  • 3'

    5' 3'

    3' 5'

    Replication Enzymes

    n Replication requires enzymes n Topoisomerase unwinds the helix n Helicase separates bases n DNA Polymerase copies each strand n Ligase joins the lagging strand

  • n The two strands are antiparallel, running in opposite directions.

    n DNA is read and copied from the 5’ end to the 3’ end.

    Complementary Base Pairing

  • DNA Replication is Semi-Conservative

    In semi-conservative replication DNA one old strand is always conserved in the new molecule.

  • Separating the Strands

    n Strong covalent bonds hold the phosphate-sugar backbone together.

    n Bases are held together with relatively weak hydrogen bonds.

    n Helicase separates the two strands at the bases, so replication can occur.

    Sugar-phosphate backbone

    Weak hydrogen bonds

  • Copying the Strands

    n DNA polymerase assembles new strands by matching unpaired complementary bases.

    n Adenine always pairs with Thymine 
 Cytosine always pairs with Guanine

  • Okazaki Fragments

    n Okazaki fragments are short fragments that occur on the lagging strand during DNA replication.

  • DNA Replication

  • Okazaki Fragments

  • Okazaki Fragments

    Which strand is leading? Which is lagging?

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