Ch6

  • Helicase: Unwinds DNA to begin replication.

  • Gyrase: Relieves tension during unwinding.

  • Primase: Adds RNA primers at the start of the new strand (5’ end).

  • Pol III Adds remaining nucleotides to the unwinding double helix.

Strands:

  • Leading Strand: Moves towards the replication fork; DNA synthesis is continuous.

  • Lagging Strand: Moves away from the replication fork; DNA synthesis is fragmented (discontinuous).

Processing:

  • RNA primers removed by DNA Polymerase I, leaving little fragments on the lagging strand called Okazaki fragments.

  • DNA Ligase fills in the gaps to link the fragments together.

  • Both DNA Poly I and III proofread.

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Strand Synthesis

  • Leading strand is continuous

  • Lagging strand is discontinuous, forming Okazaki fragments.

Processing of Replicated DNA

  • RNA primers on the lagging strand are removed by Pol I.

  • DNA Ligase joins Okazaki fragments.

  • DNA Polymerase I and III proofread for accuracy.

Nucleotide Composition and Structure

  • Nucleotide consists of phosphate group, five-carbon sugar, and nitrogen-containing base.

  • Phosphate group binds to sugar of the next nucleotide.

  • 5’ end contains a free phosphate, while 3’ end contains a free sugar.

Historical Perspectives on DNA

  • Initially, proteins were believed to contain the genetic blueprint.

  • Watson and Crick’s discovery elucidated DNA’s double helix structure.

Base Pairing and Chromosomal Composition

  • Purines (two rings) pair with pyrimidines (one ring).

  • Chargaff's rule states A=T, C=G.

  • Chromosomes consist of proteins and DNA.

Semiconservative Replication

  • Meselson experiment confirmed one old strand remains.

  • Conservative (separate), Dispersive (mixed), Semi-Conservative (alt)

Key Enzymes in DNA Replication

  • Helicase unwinds and unzips the parent strand.

  • Single-strand binding proteins (SSBP) prevent strand reunion.

  • Primase lays down RNA primers.

  • DNA Polymerase III makes new DNA.

  • DNA Ligase joins okazaki fragments.

  • Polymerase I and III proofread and correct errors.

Milestones in DNA Discovery

  • Griffith: Bacterial transformation.

  • Hershey and Chase: Confirmed DNA as hereditary material.

  • Franklin: Discovered DNA’s helical shape.

  • Watson and Crick: Revealed DNA’s double helix structure.

Additional Insights

  • Gyrase relieves tension during unwinding.

  • Okazaki fragments are formed on the lagging strand.

  • Hydrogen bonds stabilize the DNA helix.