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13.1: The Structure of DNA. Objectives. What is genetic material composed of? What experiments helped identify the role of DNA? What is the shape of a DNA molecule? How is information organized in a DNA molecule? What scientific investigation led to the discovery of DNA’s structure?.

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Objectives

  • What is genetic material composed of?

  • What experiments helped identify the role of DNA?

  • What is the shape of a DNA molecule?

  • How is information organized in a DNA molecule?

  • What scientific investigation led to the discovery of DNA’s structure?


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DNA: The Genetic Material

  • The instructions for inherited traits are called genes. A gene is a small segment of deoxyribonucleic acid, or DNA, that is located in a chromosome.

  • DNA is the primary material that causes inheritable characteristics in related groups of organisms.

  • DNA is a simple molecule, composed of only four different subunits.


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Searching for the Genetic Material

  • Three major experiments led to the conclusion that DNA is the genetic material in cells. These experiments were performed by Griffith, Avery, and Hershey and Chase.

    Griffith

  • worked with two related strains of

    bacteria which cause pneumonia

    in mice.

    • Smooth bacteria caused pneumonia

    • Rough bacteria did not cause pneumonia

  • Griffith discovered that when harmless live bacteria were mixed with heat-killed disease-causing bacteria and then injected into mice, the mice died.



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Searching for the Genetic Material

  • These results led Griffith to discover transformation.

    • Transformation is a change in genotype that is caused when cells take up foreign genetic material.

  • Griffith’s experiments led to the conclusion that genetic material could be transferred between cells.


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Searching for the Genetic Material

Avery

  • He wanted to determine whether the transforming agent in Griffith’s experiments was protein, RNA, or DNA.

  • Avery used enzymes to destroy each of these molecules in heat-killed bacteria.

  • Avery’s experiments led to the conclusion that DNA is responsible for transformation in bacteria.


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Searching for the Genetic Material

  • Hershey and Chase studied bacteriophages.

    • viruses that infect bacterial cells and cause the cells to produce viruses.

  • By using radioactive isotopes,

    • phosphorus-32

      • labeled the DNA because it contains phosphorous and proteins don’t

    • sulfur-35

      • labeled the proteins because DNA doesn’t contain sulfur

  • Hershey and Chase showed that DNA, not protein, is the genetic material in viruses.


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Checkpoint

  • What did Griffith study in his experiment?

    • Which strain caused the disease?

    • Which strain was harmless?

    • What was the conclusion of his experiments?

  • Summarize what Avery did in his experiment?

    • What was his conclusion?

  • What 2 things did Hershey and Chase use in their experiment?

    • What was their conclusion?


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The Shape of DNA

  • A DNA molecule is shaped like a spiral staircase and is composed of two parallel strands of linked subunits.

  • The spiral shape of DNA is known as a double helix.

  • Each strand of DNA is made up of linked subunits called nucleotides.


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The Shape of DNA

  • A nucleotide is made up of three parts:

    • a phosphate group

    • a five-carbon sugar group

    • a nitrogen-containing base

  • The phosphate groups and the sugar molecules of nucleotides link together to form a “backbone” for the DNA strand.

  • The five-carbon sugar in DNA is called deoxyribose, from which DNA gets its full name, deoxyribonucleic acid.



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The Information in DNA

  • The information in DNA is contained in the order of the bases, while the base-pairing structure allows the information to be copied.

  • In DNA, each nucleotide has the same sugar group and phosphate group, but each nucleotide can have one of four nitrogenous bases.

  • The four kinds of bases are adenine (A), guanine (G), thymine (T), and cytosine (C).

  • Bases A and G have a double-ring structure and are classified as purines.


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The Information in DNA

  • Bases T and C have a single-ring structure and are classified as pyrimidines.

  • A purine on one strand of a DNA molecule is always paired with a pyrimidine on the other strand.

    • adenine always pairs with thymine

    • guanine always pairs with cytosine.

  • Base-pairing rules are dictated by the chemical structure of the bases.

  • The hydrogen bonds between bases keep the two strands of DNA together.


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The Information in DNA

  • Paired bases are said to be complementary because they fit together like puzzle pieces.

  • Because of base-pairing rules, if the sequence of bases is known for one strand of DNA, then the sequence of bases for the complementary strand can be quickly identified.


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Discovering DNA’s Structure

  • Watson and Crick used information from experiments by Chargaff, Wilkins, and Franklin to determine the three-dimensional structure of DNA.

    Chargaff

  • showed that the amount of adenine always equaled the amount of thymine, and the amount of guanine always equaled the amount of cytosine.

    Franklin and Wilkins

  • developed X-ray diffraction images

    of strands of DNA suggested the

    DNA molecule resembled a tightly

    coiled helix.


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Discovering DNA’s Structure

Watson and Crick

  • used both Chargaff’s data and the X-ray diffraction studies to create a complete three-dimensional model of DNA.

  • Their model showed a “spiral staircase” in which two strands of nucleotides twisted around a central axis.


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Checkpoint

  • What is the name given to the shape of DNA?

  • What are the 3 parts that make up a nucleotide?

  • What makes up the backbone of DNA?

  • What is the name of the sugar in DNA?

  • What determines the information coded on the DNA?

    • How many different types are there and what are their names?

  • What is the difference between purines and pyrimidines?

  • What are the base pairing rules?

  • What holds to the two strands of DNA together?

  • Name the 3 scientists that played a role in the discover of the structure of DNA and their contribution.


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Summary

  • DNA is the primary material that causes inheritable characteristics in related groups of organisms.

  • Three major experiments led to the conclusion that DNA is the genetic material in cells. These experiments were performed by Griffith, Avery, and Hershey and Chase.

  • A DNA molecule is shaped like a spiral staircase and is composed of two parallel strands of linked subunits.


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Summary

  • The information in DNA is contained in the order of the bases, while the base-pairing structure allows the information to be copied.

  • Watson and Crick used information from experiments by Chargaff, Wilkins, and Franklin to determine the three-dimensional structure of DNA.


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