DNA Structure and Function

Identifying the Substance of the Gene

Bacterial Transformation (Griffith's Experiment)

Principle of Bacterial Transformation

Bacterial transformation is a process where bacteria uptake foreign genetic material (naked DNA) from their environment, leading to a change in their genetic makeup.

Griffith's Experiment

  • Experiment Setup: Griffith conducted experiments using two strains of bacteria: a disease-causing S strain and a harmless R strain.

  • Observations:

    • Mice injected with the S strain died.

    • Mice injected with the R strain lived.

    • Mice injected with heat-killed S strain lived.

    • Mice injected with a mixture of heat-killed S strain and live R strain died.

  • Conclusion: The heat-killed S-type bacteria passed their disease-causing ability to the harmless R-type bacteria, transforming them. The transferred chemical compound had to be a gene.

Avery's Experiment

Identifying the Transformation Molecule

  • Who: Oswald Avery

  • When: 1944

  • Why: To identify the substance causing transformation.

  • What: Avery and his team treated mixtures from heat-killed S-type bacteria with enzymes that break down proteins, lipids, carbohydrates, and RNA. Transformation still occurred. However, when they used an enzyme to break down DNA, transformation did not occur.

  • Conclusion: DNA is the molecule responsible for transformation.

Experimental Results

  • S strain extract treated with enzymes that destroyed polysaccharides, lipids, RNA, or proteins still resulted in the death of mice and recovery of live S strain.

  • S strain extract treated with enzymes that destroyed DNA resulted in the mouse living and no live S strain being recovered.

Hershey and Chase Experiment

Role of Bacteriophages

  • Who: Alfred Hershey and Martha Chase

  • When: 1952

  • Why: To confirm whether DNA or protein is the genetic material.

  • What: They used bacteriophages (viruses that infect bacteria) and labeled the virus protein coat with radioactive isotope 35S^{35}S and the DNA with radioactive isotope 32P^{32}P.

  • Observations: Radioactive 32P^{32}P entered the bacteria, while 35S^{35}S remained outside.

  • Conclusion: DNA, not protein, is the transforming substance and carries genetic information.

Role of DNA in Heredity

Functions of DNA

  • Storing information: DNA carries the genetic instructions for the development and functioning of all living organisms.

  • Copying information: DNA can be replicated to pass genetic information from one generation to the next.

  • Gene expression: DNA provides the template for the synthesis of RNA and proteins.

Chemical Components of DNA

Nucleic Acids and Nucleotides

  • Nucleic acids are long, acidic molecules made from small monomers called nucleotides.

  • Each nucleotide consists of three components:

    • A 5-carbon sugar (deoxyribose)

    • A phosphate group

    • A nitrogenous base

Nitrogenous Bases

  • Purines: Adenine (A) and Guanine (G)

  • Pyrimidines: Cytosine (C) and Thymine (T) in DNA, Uracil (U) in RNA

Chargaff's Discoveries

Amount of Nitrogen Bases

Chargaff found that the amount of adenine (A) is always equal to the amount of thymine (T), and the amount of guanine (G) is always equal to the amount of cytosine (C). This is known as Chargaff's rule: \A = T and \G = C . The ratio of A to G and T to C varies between species.

Franklin's Picture

Importance of X-ray Diffraction

  • Rosalind Franklin used X-ray diffraction to study DNA structure.

  • She stretched DNA fibers, made them parallel, and aimed an X-ray beam at them.

  • The scattering pattern on film revealed an X shape, indicating a helical structure.

  • Dark spots showed that nitrogenous bases are stacked at regular intervals near the center.

Watson and Crick Model

Structure of DNA

  • DNA is made of two polynucleotide strands held together by hydrogen bonds.

  • Two hydrogen bonds form between adenine (A) and thymine (T).

  • Three hydrogen bonds form between guanine (G) and cytosine (C).

  • The two strands are antiparallel, meaning they run in opposite directions.

Antiparallel Strands

DNA strands are antiparallel. One strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction.

Base Pairing Rule

Adenine (A) always binds with thymine (T), and guanine (G) always binds with cytosine (C).

DNA Structure Model

Constructing a model of DNA involves illustrating the double helix structure, the antiparallel arrangement of the strands, and the specific base pairing.