DNA Replication

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DNA Replication
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DNA is very long!... but it is highly folded
packed tightly to fit into the cell!For example,
a human cell contains more than 1 meter of DNA
made of more than 3 X 109 base pairs!
Chromosome
E. coli bacterium
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Bases on the chromosome
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DNA Structure

• Double helix made of nucleotides instructions
  for life
• Each nucleotide is made of three part
  phosphate, a 5-carbon sugar and a nitrogen base

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Nucleotide Base Pairing

• Four bases A (adenine), C (cytosine), G
  (guanine), T (Thymine)
• A will always bond with T
• C will always bond with G

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Eukaryotic Chromosomes

• Contain DNA and proteins called histones
• Tightly packed DNA and proteins form chromatin
• During mitosis, the chromatin condenses to form
  tightly packed chromosomes

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Figure 12-10 Chromosome Structure of Eukaryotes
Section 12-2
Nucleosome
Chromosome
DNA double helix
Coils
Supercoils
Histones
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Watson Crick again

• Earlier Discovered double helix of DNA
• Then DNA can be copied or replicated, because
  each strand of the DNA double helix has all the
  information needed to reconstruct the other half
  by way of base pairing
• The strands are complementary!

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DNA Replication

• The process of making a copy of the DNA
• Occurs inside the nucleus of the cell
• Occurs when the cell is going to divide so each
  resulting cell will have a complete set of DNA
• During DNA replication, the DNA separates into
  two strands, then produces two new complementary
  strands following the rules of base pairing.
• Each strand serves as a template, or model, for
  the new strand.
• Replication occurs in both directions
• The site where separation occurs is called the
  replication fork

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DNA Replication

• The two strands of DNA unwind or unzip breaking
  the hydrogen bonds and separating. Then each
  strand becomes the guide or template for the
  making of a new strand.
• A protein called an enzyme called DNA polymerase
  breaks the nitrogen base bonds and the two
  strands of DNA separate, polymerizes individual
  nucleotides to produce DNA and proof reads the
  new DNA.
• The bases on each strand pair up with new bases
  found in the cytoplasm
• Then the sugar and phosphate groups form the
  sides of each new DNA strand
• Each new DNA molecule contains an original DNA
  strand and a new DNA strand (semi conservative
  model)

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Figure 1211 DNA Replication
Section 12-2
Original strand
DNA polymerase
New strand
Growth
DNA polymerase
Growth
Replication fork
Replication fork
Nitrogenous bases
New strand
Original strand
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STEP 1

• Two original strands of DNA separates by
  unwinding
• This occurs through the action of an enzyme that
  breaks the hydrogen bonds between the strands
• The two areas on either end where the DNA
  separates is the REPLICATION FORK

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STEP 2

• At the replication fork, DNA polymerase moves
  along the DNA Strands adding nucleotides
• As DNA polymerase moves along two strands of DNA
  form

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Step 3

• DNA Polymerase continues until all the
  nucleotides have been added
• Two new identical molecules of DNA are formed
• Another enzyme connects all the pieces of DNA
  together

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Proofreading

• At the end of Replication, DNA polymerase goes
  through the DNA to make sure there are no errors
• This prevents mistakes in the DNA sequences
• Mutations- Change in the sequence of the DNA

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Like a Zipper.
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Figure 1211 DNA Replication
Section 12-2
Original strand
DNA polymerase
New strand
Growth
DNA polymerase
Growth
Replication fork
Replication fork
Nitrogenous bases
New strand
Original strand
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