DNA Structure and Function

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DNA Structure and Function

• Chapter 12

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Structure of the Hereditary Material

• Experiments in the 1950s showed that DNA is the
  hereditary material
• Scientists raced to determine the structure of
  DNA
• 1953 - Watson and Crick proposed that DNA is a
  double helix

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Structure of Nucleotides in DNA

• Each nucleotide consists of
• Deoxyribose (5-carbon sugar)
• Phosphate group
• A nitrogen-containing base
• Four bases
• Adenine, Guanine, Thymine, Cytosine

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Nucleotide Bases
ADENINE (A)
GUANINE (G)
phosphate group
deoxyribose
THYMINE (T)
CYTOSINE (C)
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Composition of DNA

• Chargaff showed
• Amount of adenine relative to guanine differs
  among species
• Amount of adenine always equals amount of thymine
  and amount of guanine always equals amount of
  cytosine
• AT and GC

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Watson-Crick Model

• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen bonds
  between bases
• A binds with T and C with G
• Molecule is a double helix

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Watson-Crick Model
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DNA Structure Helps Explain How it Duplicates

• DNA is two nucleotide strands held together by
  hydrogen bonds
• Hydrogen bonds between two strands are easily
  broken
• Each single strand then serves as template for
  new strand

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DNA Replication

• Each parent strand remains intact
• Every DNA molecule is half old and half new

new
new
old
old
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Base Pairing During Replication

• Each old strand serves as the template for
  complementary new strand

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Enzymes in Replication

• Enzymes unwind the two strands
• DNA polymerase attaches complementary nucleotides
  
• DNA ligase fills in gaps
• Enzymes wind two strands together

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Continuous and Discontinuous Assembly
Strands can only be assembled in the 5 to 3
direction
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DNA Repair

• Mistakes can occur during replication
• DNA polymerase can read correct sequence from
  complementary strand and, together with DNA
  ligase, can repair mistakes in incorrect strand

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Cloning

• Making a genetically identical copy of an
  individual
• Researchers have been creating clones for decades
• These clones were created by embryo splitting

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Dolly Cloned from an Adult Cell

• Showed that differentiated cells could be used to
  create clones
• Sheep udder cell was combined with enucleated egg
  cell
• Dolly is genetically identical to the sheep that
  donated the udder cell

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More Clones

• Mice
• Cows
• Pigs
• Goats
• Guar (endangered species)

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Fig. 12.2, p. 191
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phosphate group
ADENINE (A) base with a double-ring structure
GUANINE (G) base with a double-ring structure
sugar (deoxyribose)
THYMINE (T) base with a single-ring structure
CYTOSINE (C) base with a single-ring structure
Fig. 12.6, p. 194
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2-nanometer diameter, overall
distance between each pair of bases 0.34
nanometer
each full twist of the DNA double helix 3.4
nanometers
Fig. 12.7, p. 195
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Click to view animation.
animation
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or
or
one base pair
in-text, p. 195
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new
new
old
old
Fig. 12.9, p. 196
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Click to view animation.
animation
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continuous assembly on one strand
discontinuous assembly on other strand
one parent DNA strand
newly forming DNA strand
Fig. 12.10, p. 197
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Click to view animation.
animation