Chapter 12: DNA

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Chapter 12 DNA RNA

• Georgia Performance Standards
• Compare and contrast the structure and function
  of
• DNA and RNA.
• Explain how DNA stores and transmits cellular
• information.

• Essential Questions
• EQ Why do you need DNA RNA?
• EQ How does the cell make protein?
• EQ How do chromosomes carry traits?

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Hook

• Link cell organelles (nucleus-chp. 7) and
  macromolecules (nucleic acids- chp. 2) to DNA
  RNA (chp. 12).
• Probing Questions
• Where in the cell is DNA located for eukaryotes
  and prokaryotes?
• DNA is what type of macromolecule? (choose one)
• Carbohydrate
• Protein
• Lipid
• Nucleic Acid

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Warm-up Review
Section 12-1

• Macromolecules
• Carbohydrates (sugars)
• Lipids
• Proteins
• Nucleic Acids (DNA RNA)

Go to Section
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Nucleic Acids

• Function
• Nucleic acids store and transmit hereditary, or
  genetic, information.
• Types
• Ribonucleic acid (RNA)
• Deoxyribonucleic acid (DNA).

• Structure
• Contain hydrogen, oxygen, nitrogen, carbon, and
  phosphorus.
• Monomers of nucleotides
• 5-carbon sugar
• Phosphate group
• Base

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A Nucleotide (Monomers of Nucleic Acids)
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• DNA Nucleotides
• DNA is made up of a series of monomers called
  nucleotides.
• Each nucleotide has three parts a deoxyribose
  molecule, a phosphate group, and a nitrogenous
  base.
• There are four different bases in DNA adenine,
  guanine, cytosine, and thymine.

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The backbone of a DNA chain

• Formed by alternating sugar and phosphate groups
  of each nucleotide.
• The nitrogenous bases stick out sideways from the
  chain and are linked by hydrogen bonds.
• The nucleotides can be joined together in any
  order, meaning that any sequence of bases is
  possible.

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• Chargaffs Rules
• Erwin Chargaff showed that the percentages of
  guanine and cytosine in DNA and adenine and
  thymine are almost equal.
• AT (2 hydrogen bonds hold them together)
• G C (3 hydrogen bonds hold them together)

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What Shape Does DNA Have?

• X-Ray Evidence 
• In the early 1950s, a British scientist named
  Rosalind Franklin began to study DNA.
• She used a technique called X-ray diffraction to
  get information about the structure of the DNA
  molecule.
• The angle of the X suggests that there are two
  strands in the structure.
• Other clues suggest that the nitrogenous bases
  are near the center of the molecule.

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What Shape Does DNA Have?

• The Double Helix Shape of DNA
• Francis Crick and James Watson were trying to
  understand the structure of DNA by building
  three-dimensional models of the molecule.
• Then, early in 1953, Watson was shown a copy of
  Franklins remarkable X-ray pattern.
• Within weeks, Watson and Crick had figured out
  the structure of DNA.
• Watson and Cricks model of DNA was a double
  helix, in which two strands were wound around
  each other.

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What is a Double Helix?

• A double helix looks like a twisted ladder or a
  spiral staircase.
• The double helix accounted for many of the
  features in Franklins X-ray pattern but did not
  explain what forces held the two strands
  together.
• Watson and Crick found the answer. They
  discovered that hydrogen bonds could form between
  certain nitrogenous bases and provide just enough
  force to hold the two strands together.
• The principle of base pairing (A-T and G-C),
  explained Chargaffs rules.

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• DNA is a double helix in which two strands are
  wound around each other.
• Each strand is made up of a chain of
  nucleotides.
• The two strands are held together by hydrogen
  bonds between adenine and thymine and between
  guanine and cytosine.

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Group Activity

• Use plastic models to build a DNA structure.
• Nitrogenous Bases
• A orange
• G Yellow
• T Green
• C Blue
• Black Pieces Sugar (Deoxyribose)
• White tube phosphate group
• Toothpicks hydrogen bonds

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What Carries Genetic Material (DNA or Protein)?

• Many Experiments were carried out to verify that
  genetic material is carried by DNA and not
  protein.
• 1. Griffith and Transformation DNA can be
  transformed (Video Clip)
• 2. Avery and other scientists discovered that DNA
  is the nucleic acid that stores and transmits the
  genetic information from one generation of an
  organism to the next.
• 3. Hershey and Chase concluded that the genetic
  material of the bacteriophage was DNA, not
  protein.

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Avery Repeats Griffiths work.

• He treated the extract with enzymes that
  destroyed proteins, lipids, carbohydrates, and
  other molecules, including the nucleic acid RNA.
• Transformation still occurred.
• Obviously these molecules were not responsible
  for the transformation.
• If they had been, transformation would not have
  occurred, because the molecules would have been
  destroyed by the enzymes.
• He repeated the experiment, this time using
  enzymes that would break down DNA.
• Transformation did not occur.
• Therefore, DNA was the transforming factor.

• Why? to determine which molecule in the
  heat-killed bacteria was most important for
  transformation.
• If transformation required just one particular
  molecule, that might well be the molecule of the
  gene.
• Avery and his colleagues made an extract, or
  juice, from the heat-killed bacteria.

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Checkpoint Questions

• Describe Watson and Cricks model of the DNA
  molecule. What was Rosalind Franklins
  contribution to the discovery of the shape of
  DNA?
• What are the four kinds of bases found in DNA?
  Which ones are purines and which are pyrimadines?
• How did Watson and Cricks model explain why
  there are equal amounts of thymine and adenine in
  DNA? Hint Chargaff's Rule
• List the conclusions Griffith, Avery, Hershey,
  and Chase drew from their experiments.
• Why did Hershey and Chase grow viruses in
  cultures that contained both radioactive
  phosphorus and radioactive sulfur? What might
  have happened if they had used only one
  radioactive substance?

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122 Chromosomes and DNA Replication 

• Prokaryotic cells
• No nuclei and many of the organelles found in
  eukaryotes.
• DNA in the cytoplasm.
• Most prokaryotes have a single circular DNA
  molecule called a plasmid.
• The plasmid is considered the chromosome

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Prokaryotic Chromosome Structure
Chromosome
E. coli bacterium
Bases on the chromosome
Go to Section
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DNA and Chromosomes

• Eukaryotic cells
• DNA is in the nucleus
• Chromosome form (cell division)
• of chromosomes varies from one species to the
  next.
• Humans have 46
• Fruit flies have 8
• Chromosomes contain both DNA and protein, which
  is tightly packed together to form chromatin.
• Protein is called histone
• The DNA and histone collectively are called
  nucleosomes
• fold long lengths of DNA into the tiny nucleus.
• The nucleosomes are tightly packed to form
  supercoils

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Chromosome Structure of Eukaryotes
Section 12-2
Nucleosome
Chromosome
DNA double helix
Coils
Supercoils
Histones
Go to Section
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12-2 Warm-up

• A Perfect Copy
• When a cell divides, each daughter cell receives
  a complete set of chromosomes.
• This means that each new cell has a complete set
  of the DNA code.
• Before a cell can divide, the DNA must be copied
  so that there are two sets ready to be
  distributed to the new cells.

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12-2 Warm-up

• 1. On a sheet of paper, draw a curving or
  zig-zagging line that divides the paper into two
  halves. Vary the bends in the line as you draw
  it. Without tracing, copy the line on a second
  sheet of paper.
• 2. Hold the papers side by side, and compare the
  lines. Do they look the same?
• 3. Now, stack the papers, one on top of the
  other, and hold the papers up to the light. Are
  the lines the same?
• 4. How could you use the original paper to draw
  exact copies of the line without tracing it?
• 5. Why is it important that the copies of DNA
  that are given to new daughter cells be exact
  copies of the original?

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DNA Replication

• The double helical shape of DNA allows for
  strands to be copied.
• Strands are considered to be complimentary
• DNA must be replicated (copied) before a cell
  divides.
• The DNA molecule separates into 2 strands
• 2 new complimentary strands are produced
• Each strand serves as a template for the new
  strand.

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 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
Go to Section
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DNA Replication

• Prokaryotes
• DNA replication begins at a single point in the
  chromosomes and proceeds in two directions until
  the entire chromosome is replicated.
• Takes about 20 minutes

• Eukaryotes
• DNA replication occurs in hundreds of places and
  proceeds in both directions until each
  chromosomes is completely copied
• The sites where replication and separation occur
  are call replication forks.

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How Does Replication Occur?

• Many enzymes are used
• Some unzip the strands, some hold the strands
  open, some glue the strands together.
• DNA Polymerase
• enzyme that adds the complimentary bases to each
  strand.
• Proofreads each strand
• See video

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Checkpoint Questions

• Explain how DNA is replicated.
• Where and in what form is eukaryotic DNA found?
• 3. How are the long DNA molecules found in
  eukaryotes packed into short chromosomes?
• 4. How are histones related to nucleosomes?
• 5. What is the role of DNA polymerase in DNA
  replication?
• 6.  How is the structure of chromosomes in
  eukaryotes different from the structure of
  chromosomes in prokaryotes?

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Homework

• Compare and contrast the process of DNA
  replication in prokaryotes and eukaryotes.
• Compare the location, steps, and end products in
  each kind of cell.
• You may use a Venn diagram, a chart or table, a
  drawing, or write an essay.