Dr. Attya Bhatti

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DNA Replication

• Dr. Attya Bhatti

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DNA Replication

• Replication is a complex process in which dozens
  of proteins, enzymes, and DNA structures take
  part a single defective component, such as DNA
  helicase, can disrupt the whole process
• Cell doubles its DNA before division

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Proposed Models of Replication
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Semi-conservative Replication

• Each DNA serves as template for the synthesis of
  a new DNA template molecule

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Meselson and Stahls Experiment
To determine which of the three models of
replication applied to E. coli cells, Matthew
Meselson and Franklinv Stahl needed a way to
distinguish old and new DNA Two isotopes of
nitrogen, 14N (the common form) and 15N (a rare,
heavy form) were used
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Meselson and Stahl Demonstrated that DNA
Replication is Semiconservative
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Synthesis Phase

• S phase during interphase of the cell cycle
• Nucleus of eukaryotes

DNA replication is take place is S phase
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Modes of Replication

• Individual units of replication are called
  replicons, each of which contains a replication
  origin
• Theta model
• Rolling circle model
• Linear eukaryotic relpication

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Theta replication

• A common type of replication that takes place in
  circular DNA.
• Generates a structure that resembles the Greek
  letter theta.
• DNA unwinds at one particular location, the
  origin, and a replication bubble is formed.

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• Replication bubble The unwinding of the double
  helix generates a loop
• Replication fork The point of unwinding where
  the two single nucleotide strands separate from
  the double-stranded DNA helix
• Bidirectional replication If there are two
  replication forks, one at each end of the
  replication bubble, the forks proceed outward in
  both directions

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Rolling-circle replication

• Takes place in some viruses and in the F factor
  (a small circle of extrachromosomal DNA that
  controls mating.
• Initiated by a break in one of the nucleotide
  strands that creates a 3-OH group and a
  5-phosphate group.
• New nucleotides are added to the 3 end of the
  broken strand

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Rolling-circle replication takes place in some
viruses and in the F factor of E. coli
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Linear Eukaryotic Replication

• Large linear chromosomes in eukaryotic cells,
  contain far too much DNA to be replicated
  speedily from a single origin.
• Eukaryotic replication proceeds at a rate ranging
  from 500 to 5000 nucleotides per minute at each
  replication fork.
• At each replication origin the DNA unwinds and
  produces a replication bubble.
• Replication and fusion of all the replicons leads
  to two identical DNA molecules.

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Requirements of Replication

• Template consisting of single-stranded DNA
• Raw materials (substrates) to be assembled into a
  new nucleotide strand
• Enzymes and other proteins that read the
  template and assemble the substrates into a DNA
  molecule

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In replication, new DNA is synthesized
from deoxyribonucleoside triphosphates (dNTPs)
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Direction of Replication

• All DNA synthesis is 5?3
• New nucleotides are always added to the 3 end of
  the growing nucleotide strand.
• At each replication fork, synthesis of the
  leading strand proceeds continuously and that of
  the lagging strand proceeds discontinuously

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The Mechanism of Replication

• Replication takes place in four stages
• Initiation
• Unwinding
• Elongation
• Termination