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Lesson Overview
12.2 The Structure of DNA
Lesson Overview         The Structure of DNA


  THINK ABOUT IT
       The DNA molecule must somehow specify how to assemble proteins,
       which are needed to regulate the various functions of each cell.

       What kind of structure could serve this purpose without varying from cell
       to cell?

       Understanding the structure of DNA has been the key to understanding
       how genes work.
Lesson Overview        The Structure of DNA


  The Components of DNA
     What are the chemical components of DNA?
Lesson Overview          The Structure of DNA


  The Components of DNA
     What are the chemical components of DNA?

     DNA is a nucleic acid made up of nucleotides joined into long strands or
     chains by covalent bonds.
Lesson Overview          The Structure of DNA


  Nucleic Acids and Nucleotides
       Nucleic acids are long, slightly acidic molecules originally identified in
       cell nuclei.

       Nucleic acids are made up of nucleotides, linked together to form long
       chains.

       The nucleotides that make up DNA are shown.
Lesson Overview        The Structure of DNA


  Nucleic Acids and Nucleotides
       DNA’s nucleotides are made up of three basic components: a 5-carbon
       sugar called deoxyribose, a phosphate group, and a nitrogenous base.
Lesson Overview         The Structure of DNA


  Nitrogenous Bases and Covalent Bonds
       The nucleotides in a strand of DNA are joined by covalent bonds formed
       between their sugar and phosphate groups.
Lesson Overview         The Structure of DNA


  Nitrogenous Bases and Covalent Bonds
       DNA has four kinds of nitrogenous bases: adenine (A), guanine (G),
       cytosine (C), and thymine (T).

       The nitrogenous bases stick out sideways from the nucleotide chain.
Lesson Overview         The Structure of DNA


  Nitrogenous Bases and Covalent Bonds
       The nucleotides can be joined together in any order, meaning that any
       sequence of bases is possible.
Lesson Overview          The Structure of DNA


  Solving the Structure of DNA
     What clues helped scientists solve the structure of DNA?
Lesson Overview         The Structure of DNA


  Solving the Structure of DNA
     What clues helped scientists solve the structure of DNA?

     The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a
     model that explained the specific structure and properties of DNA.
Lesson Overview         The Structure of DNA


  Chargaff’s Rules
       Erwin Chargaff discovered that the percentages of adenine [A] and
       thymine [T] bases are almost equal in any sample of DNA.

       The same thing is true for the other two nucleotides, guanine [G] and
       cytosine [C].

       The observation that [A] = [T] and [G] = [C] became known as one of
       “Chargaff’s rules.”
Lesson Overview         The Structure of DNA


  Franklin’s X-Rays
       In the 1950s, British scientist Rosalind Franklin used a technique called
       X-ray diffraction to get information about the structure of the DNA
       molecule.
Lesson Overview         The Structure of DNA


  Franklin’s X-Rays
       X-ray diffraction revealed an X-shaped pattern showing that the strands
       in DNA are twisted around each other like the coils of a spring.

       The angle of the X-shaped pattern suggested that there are two strands
       in the structure.

       Other clues suggest that the nitrogenous bases are near the center of
       the DNA molecule.
Lesson Overview          The Structure of DNA


  The Work of Watson and Crick
       At the same time, James Watson, an American biologist, and Francis
       Crick, a British physicist, were also trying to understand the structure of
       DNA.

       They built three-dimensional models of the molecule.
Lesson Overview         The Structure of DNA


  The Work of Watson and Crick
       Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern.

       The clues in Franklin’s X-ray pattern enabled Watson and Crick to build
       a model that explained the specific structure and properties of DNA.
Lesson Overview        The Structure of DNA


  The Work of Watson and Crick
       Watson and Crick’s breakthrough model of DNA was a double helix, in
       which two strands were wound around each other.
Lesson Overview        The Structure of DNA


  The Double-Helix Model
     What does the double-helix model tell us about DNA?
Lesson Overview         The Structure of DNA


  The Double-Helix Model
      What does the double-helix model tell us about DNA?

      The double-helix model explains Chargaff’s rule of base pairing and how
      the two strands of DNA are held together.
Lesson Overview          The Structure of DNA


  The Double-Helix Model
       A double helix looks like a twisted ladder.

       In the double-helix model of DNA, the two strands twist around each
       other like spiral staircases.

       The double helix accounted for Franklin’s X-ray pattern and explains
       Chargaff’s rule of base pairing and how the two strands of DNA are held
       together.
Lesson Overview          The Structure of DNA


  Antiparallel Strands
       In the double-helix model, the two strands of DNA are “antiparallel”—
       they run in opposite directions.

       This arrangement enables the nitrogenous bases on both strands to
       come into contact at the center of the molecule.

       It also allows each strand of the double helix to carry a sequence of
       nucleotides, arranged almost like letters in a four-letter alphabet.
Lesson Overview          The Structure of DNA


  Hydrogen Bonding
       Watson and Crick discovered
       that hydrogen bonds could
       form between certain
       nitrogenous bases, providing
       just enough force to hold the
       two DNA strands together.

       Hydrogen bonds are relatively
       weak chemical forces that
       allow the two strands of the
       helix to separate.

       The ability of the two strands
       to separate is critical to DNA’s
       functions.
Lesson Overview          The Structure of DNA


  Base Pairing
       Watson and Crick’s model
       showed that hydrogen bonds
       could create a nearly perfect fit
       between nitrogenous bases
       along the center of the molecule.

       These bonds would form only
       between certain base pairs—
       adenine with thymine, and
       guanine with cytosine.

       This nearly perfect fit between
       A–T and G–C nucleotides is
       known as base pairing, and is
       illustrated in the figure.
Lesson Overview         The Structure of DNA


  Base Pairing
       Watson and Crick realized that
       base pairing explained Chargaff’s
       rule. It gave a reason why [A] =
       [T] and [G] = [C].

       For every adenine in a double-
       stranded DNA molecule, there
       had to be exactly one thymine.
       For each cytosine, there was one
       guanine.

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Powerpoint12.2

  • 1. Lesson Overview 12.2 The Structure of DNA
  • 2. Lesson Overview The Structure of DNA THINK ABOUT IT The DNA molecule must somehow specify how to assemble proteins, which are needed to regulate the various functions of each cell. What kind of structure could serve this purpose without varying from cell to cell? Understanding the structure of DNA has been the key to understanding how genes work.
  • 3. Lesson Overview The Structure of DNA The Components of DNA What are the chemical components of DNA?
  • 4. Lesson Overview The Structure of DNA The Components of DNA What are the chemical components of DNA? DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.
  • 5. Lesson Overview The Structure of DNA Nucleic Acids and Nucleotides Nucleic acids are long, slightly acidic molecules originally identified in cell nuclei. Nucleic acids are made up of nucleotides, linked together to form long chains. The nucleotides that make up DNA are shown.
  • 6. Lesson Overview The Structure of DNA Nucleic Acids and Nucleotides DNA’s nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.
  • 7. Lesson Overview The Structure of DNA Nitrogenous Bases and Covalent Bonds The nucleotides in a strand of DNA are joined by covalent bonds formed between their sugar and phosphate groups.
  • 8. Lesson Overview The Structure of DNA Nitrogenous Bases and Covalent Bonds DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The nitrogenous bases stick out sideways from the nucleotide chain.
  • 9. Lesson Overview The Structure of DNA Nitrogenous Bases and Covalent Bonds The nucleotides can be joined together in any order, meaning that any sequence of bases is possible.
  • 10. Lesson Overview The Structure of DNA Solving the Structure of DNA What clues helped scientists solve the structure of DNA?
  • 11. Lesson Overview The Structure of DNA Solving the Structure of DNA What clues helped scientists solve the structure of DNA? The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA.
  • 12. Lesson Overview The Structure of DNA Chargaff’s Rules Erwin Chargaff discovered that the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C]. The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules.”
  • 13. Lesson Overview The Structure of DNA Franklin’s X-Rays In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.
  • 14. Lesson Overview The Structure of DNA Franklin’s X-Rays X-ray diffraction revealed an X-shaped pattern showing that the strands in DNA are twisted around each other like the coils of a spring. The angle of the X-shaped pattern suggested that there are two strands in the structure. Other clues suggest that the nitrogenous bases are near the center of the DNA molecule.
  • 15. Lesson Overview The Structure of DNA The Work of Watson and Crick At the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA. They built three-dimensional models of the molecule.
  • 16. Lesson Overview The Structure of DNA The Work of Watson and Crick Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern. The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA.
  • 17. Lesson Overview The Structure of DNA The Work of Watson and Crick Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other.
  • 18. Lesson Overview The Structure of DNA The Double-Helix Model What does the double-helix model tell us about DNA?
  • 19. Lesson Overview The Structure of DNA The Double-Helix Model What does the double-helix model tell us about DNA? The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.
  • 20. Lesson Overview The Structure of DNA The Double-Helix Model A double helix looks like a twisted ladder. In the double-helix model of DNA, the two strands twist around each other like spiral staircases. The double helix accounted for Franklin’s X-ray pattern and explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.
  • 21. Lesson Overview The Structure of DNA Antiparallel Strands In the double-helix model, the two strands of DNA are “antiparallel”— they run in opposite directions. This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule. It also allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet.
  • 22. Lesson Overview The Structure of DNA Hydrogen Bonding Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases, providing just enough force to hold the two DNA strands together. Hydrogen bonds are relatively weak chemical forces that allow the two strands of the helix to separate. The ability of the two strands to separate is critical to DNA’s functions.
  • 23. Lesson Overview The Structure of DNA Base Pairing Watson and Crick’s model showed that hydrogen bonds could create a nearly perfect fit between nitrogenous bases along the center of the molecule. These bonds would form only between certain base pairs— adenine with thymine, and guanine with cytosine. This nearly perfect fit between A–T and G–C nucleotides is known as base pairing, and is illustrated in the figure.
  • 24. Lesson Overview The Structure of DNA Base Pairing Watson and Crick realized that base pairing explained Chargaff’s rule. It gave a reason why [A] = [T] and [G] = [C]. For every adenine in a double- stranded DNA molecule, there had to be exactly one thymine. For each cytosine, there was one guanine.