Date Topic Page 5/4/09 DNA Replication??? Admit

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• Admit Slip Complete the handout-MCAS
• Todays Agenda
• Announcements lab report are due, warning
  notices, posted grades
• Discuss the AS
• Introduce DNA Replication
• Complete the DNA Replication Recall
• Closure- 3 2 1

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What does DNA look like?

• DNA has..
• A sugar/phosphate backbone
• 4 Chemical bases

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The bases

• It is composed of 4 chemical bases Adenosine
  (A), Thymine (T), Cytosine (C) and Guanine (G).
• A always pairs with T
• C always pairs with G
• The pairs are held together by hydrogen bonds
  (WNCs).

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What does DNA do?

• DNA is the genetic code.
• It determines our physical characteristics from
  our hair color to what we are allergic to.
• Our DNA codes for 20 amino acids which are the
  building blocks of life.

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What Is DNA Replication

• DNA Replication is the process in which the DNA
  within a cell makes an exact copy of itself.
• Why does DNA replicate?
• During which phase of the cell cycle does DNA
  replicate?

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DNA Replication models
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The Three Possible DNA Replication Models

• Conservative- would leave the original strand
  intact and copy it.
• Dispersive-would produce two DNA molecule with
  sections of both old and new along each strand.
• Semiconservative would produce DNA molecule with
  both one old strand and one new strand.

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DNA Replication
Replication occurs during Interphase
Replication fork
Replication fork
Replication bubble
Hydrogen bond
DNA replication is the process where an entire
double-stranded DNA is copied to produce a
second, identical DNA double helix.
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DNA Replication
DNA helicase

• Helicase unwinds the double helix starting at a
  replication bubble.
• The two strands separate as the hydrogen bonds
  between base pairs are broken.
• Two replication forks form and the DNA is
  unwound in opposite directions.

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DNA Replication

• Helicase has completed unwinding the DNA strand.
• Single strand Binding Proteins (SSB) keep the two
  strands from re-annealing (coming back together).

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DNA Replication
Leading Strand
Primase
RNA Primer
Lagging Strand

• Primase is an RNA polymerase that makes the RNA
  primer.
• These primers tell the DNA polymerase where to
  start copying the DNA.

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DNA Replication
Leading Strand
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Direction of Replication
DNA Polymerase
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Lagging Strand
Direction of Replication

• The DNA polymerase starts at the 3 end of the
  RNA primer of the leading stand CONTINUOUSLY.
• DNA is copied in 5 to 3 direction.
• DNA polymerase copies the lagging strand DIS-
  continuously.

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DNA Replication

• The dis-continuous pieces of DNA copied on the
  lagging strand are known as Okazaki fragments.

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DNA Replication
Another DNA Polymerase removes the RNA primers
and replaces them with DNA.
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DNA Replication
ligase
Finally the gaps in the sugar phosphate backbone
are sealed by DNA ligase There are now 2
identical double helices of DNA.
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DNA Replication ANIMATION

• http//www.wiley.com/college/pratt/0471393878/stud
  ent/animations/dna_replication/index.html