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Published byBuddy Cunningham Modified over 6 years ago
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There’s one last thing you need to know about DNA structure…
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The DNA molecule is a double helix.
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Double Helix Two strands of nucleotides twisted around each other. T N
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Single helix Double helix
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DNA is twisted because the covalent bonds in the backbone are bent.
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Bent bonds = spiral shape
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Now that we know the shape of DNA, we can think of replication in a new way:
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DNA Replication Process that separates the double helix and then copies each strand. S D H C S
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DNA replication occurs before a cell divides to ensure that both daughter cells can keep making the same protein. B D D C P Insulin
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DNA replication is performed by enzymes.
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Replication occurs in 3 basic steps.
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1. First, the cell un-twists the double helix.
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Why is the enzyme called “helicase?”
2. Next, an enzyme called helicase breaks the hydrogen bonds; this allows the two strands to separate. E H H S Why is the enzyme called “helicase?” Helicase separates the double helix Helicase
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DNA Polymerase polymerizes nucleotides
3. Finally, another enzyme called DNA polymerase uses the old strand as a template to build a complementary new strand. E D P S C Guide DNA Polymerase DNA Polymerase polymerizes nucleotides Why is the enzyme called “polymerase?” DNA Polymerase
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The final result is two identical DNA molecules…
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…each with one old strand and one new strand.
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REPLICATION 2. DNA polymerase 4. Template strand Free nucleotides 3. 5. New strand Helicase New 6. strand Thymine 9. 8. 7. DNA polymerase Template strand
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DNA replication is a nearly perfect process for 2 reasons.
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1. DNA polymerase “knows” which nucleotides to add because it follows base-pairing rules.
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2. DNA polymerase also “proofreads” each strand, making sure there are no errors.
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Since bonds change during DNA replication, it is considered a chemical reaction.
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However, the reaction has high activation energy…
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…to prevent the DNA molecule from changing accidentally.
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DNA replication will only occur if enzymes are present to lower the A
DNA replication will only occur if enzymes are present to lower the A.E. of the reaction. R E L
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