Ch 10

DNA: The Chemical Nature of the Gene

10.1 Genetic Material Characteristics

Genetic material possesses several key characteristics essential for its role in heredity and function:

1. Complex Information: Genetic material must contain detailed and intricate information necessary for the development and functioning of an organism.
2. Faithful Replication: It should replicate accurately to assure that genetic information is evenly distributed during cell division.
3. Phenotype Encoding: It must encode the phenotype, which refers to the observable characteristics of an organism.
4. Capacity to Vary: Genetic material must have the ability to undergo variation, allowing for evolution and adaptation.

Concept Check 1

Importance of DNA Structure Discovery

The discovery of the structure of DNA was crucial because without understanding its structure, comprehending how genetic information is encoded and expressed would be impossible.

10.2 Encoding Genetic Information in DNA/RNA

Early Studies of DNA

Several pivotal studies contributed to our understanding of DNA:

• Miescher (1869): Discovered nuclein (now known as DNA) in the nuclei of white blood cells.
• Kossel: Identified that DNA contains four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
• Chargaff's Rules: Key findings that include:
  • Adenine (A) = Thymine (T) (A = T)
  • Guanine (G) = Cytosine (C) (G = C)

Timeline of Key Discoveries

• 1833: Brown describes the nucleus of the cell.
• 1869: Miescher discovers nuclein.
• 1884: Histones are isolated from the nucleus.
• 1900: Mendel's work is rediscovered.
• 1910: Levene proposes the tetranucleotide theory.
• 1928: Griffith demonstrates the transforming principle through his work with virulent and non-virulent bacteria.
• 1947: Ashbury begins X-ray diffraction studies of DNA.
• 1952: Hershey and Chase establish that DNA is the genetic material present in bacteriophages.
• 1953: Watson and Crick devise the established structure of DNA.

Chargaff’s Rules - Table 10.1

The base composition and ratios of bases in DNA from different organisms are summarized below:

Concept Check 2

Contributions of Levene

Levene contributed to our understanding of DNA structure through the following:

• Determined that the nucleus contains DNA.
• Identified that DNA has four nitrogenous bases.
• Established that DNA consists of nucleotides.
• Recognized that the nitrogenous bases in DNA are present in specific ratios.

10.2 DNA as Genetic Information Source

Transforming Principle Identification

The experiments conducted by different scientists led to the identification of DNA as the genetic material:

• Griffith's Experiment (Figure 10.3): Demonstrated transformation in bacteria, allowing a non-virulent bacterial strain to become virulent when exposed to heat-killed virulent bacteria.

  • Experiment Outline:
  1. Type IIIS (virulent) bacteria injected into a mouse resulted in the mouse's death.
  2. Type IIR (non-virulent) bacteria injected into a mouse resulted in the mouse living.
  3. Heat-killed type IIIS bacteria injected into a mouse resulted in the mouse surviving.
  4. A mixture of heat-killed type IIIS and live type IIR bacteria injected into a mouse led to the mouse's death, and type IIIS bacteria were recovered.
  • Conclusion: A substance from the heat-killed virulent bacteria transformed the type IIR bacteria into live virulent types.

• Avery, MacLeod, and McCarty’s Experiment (Figure 10.4): Followed up on Griffith's work to reveal the chemical nature of the transforming substance.

  • Methodology:
  1. Homogenize heat-killed type IIIS bacteria and filter to obtain bacterial filtrate.
  2. Treat the samples with enzymes (RNase, Protease, DNase) to determine the type of macromolecule responsible for transformation.
  3. Observe that only samples treated with DNase did not transform type IIR bacteria, indicating that DNA is the transforming substance.

• Hershey and Chase Experiment: Demonstrated that DNA is the genetic material in bacteriophages.

  • Key Steps:
  1. Used isotopes of sulfur (35S) to label phage proteins and phosphorus (32P) to label phage DNA.
  2. Infected E. coli with labeled phages, then separated the phage proteins from the bacterial cells.
  3. Observed radioactivity in the bacterial pellet for the 32P-labeled DNA, confirming that DNA, not protein, was transmitted to progeny phages (Figure 10.6).

Concept Check 3

Hypothetical Conclusion by Avery, MacLeod, and McCarty

If their results had shown that heat-killed bacteria treated with RNase and DNase transformed bacteria while samples treated with protease did not, they would conclude that:

• Protein is the genetic material.

10.2 Structure Understanding

Watson and Crick Discovery

James Watson and Francis Crick built on the accumulated knowledge of DNA structure, including the work of Rosalind Franklin's X-ray diffraction images, to create the three-dimensional model of DNA (Figure 10.7).

Concept Check 5

Tools Used by Watson and Crick

Watson and Crick utilized:

• X-ray diffraction images
• Laws of structural chemistry
• Models of DNA
  All of the above contributed to their breakthrough in determining DNA structure.

10.2: RNA as Genetic Information

• RNA as Genetic Material: Some viruses, like tobacco mosaic virus (TMV), show that RNA can carry genetic information instead of DNA.
• TMV Experiment (Table 10.9): Experiments that hybridized RNA and protein from different types of TMV exhibited that RNA dictates the type of protein in progeny viruses.

10.3 Structure of DNA

Primary Structure

• DNA Nucleotides: Contain a sugar (deoxyribose), a phosphate group, and one of the four nitrogenous bases (A, T, G, C).

Secondary Structure

• Double Helix Formation: The DNA structure consists of two complementary and antiparallel strands held together through base pairing and hydrogen bonds (Figures 10.12 to 10.14).
• Phosphodiester bonds form the backbone of DNA, linking nucleotides in a chain.
• A pairs with T (2 hydrogen bonds), and C pairs with G (3 hydrogen bonds).

Concept Check 3

Antiparallel Structure

The antiparallel nature of DNA refers to:

• The opposite direction of the two strands of nucleotides (5' to 3' and 3' to 5').

10.4 Special Structures in DNA/RNA

• Hairpin Structure: Found in single strands where complementary sequences lead to folds, common in RNA.
• H-DNA: Triplex structures formed under specific conditions, often in regions with repetitive purine/pyrimidine sequences.
• DNA Methylation: Involves adding methyl groups to nucleotides and is vital for regulating gene expression in eukaryotes.