ch 10

Chapter 10: The Chemical Nature of the Gene

Characteristics of Genetic Material

• Genetic Material must:

  • Contain complex information

  • Replicate faithfully

  • Encode the phenotype

  • Have capacity to vary

Evidence that DNA is the Genetic Material

• Why is DNA considered the genetic material?

  • DNA is organized into chromosomes, while RNA and proteins are distributed throughout the cell.

  • The amount of DNA present correlates with the number of chromosomes:

  • Diploid organisms contain twice as much DNA as haploid organisms.

  • The composition of DNA is consistent throughout an organism, whereas RNA and proteins differ from cell to cell.

  • DNA exhibits stability over time.

The Three Classic Experiments Establishing DNA as Genetic Material

• Griffith's Experiment:

  • Studied transformation in the bacteria Streptococcus pneumoniae.

  • Investigated two strains:

  • S strain (Type IIIS):

    • Virulent, smooth, encapsulated.

  • R strain (Type IIR):

    • Avirulent, rough, non-encapsulated.

Methodology of Griffith's Experiment

• Question: Can an extract from dead bacterial cells genetically transform living cells?

• Procedures:

  1. Inject Type IIIS (virulent) bacteria into a mouse.

  2. Inject Type IIR (nonvirulent) bacteria into a mouse.

  3. Inject heat-killed Type IIIS bacteria into a mouse.

  4. Inject a mixture of Type IIR bacteria and heat-killed Type IIIS bacteria into a mouse.

Results

• Mouse dies from Type IIIS injection.

• Mouse lives from Type IIR injection.

• Mouse lives from heat-killed Type IIIS injection.

• Mouse dies from mixture injection, leading to recovery of Type IIIS bacteria.

Conclusion

• A substance from heat-killed virulent bacteria transformed Type IIR bacteria into live, virulent Type IIIS bacteria.

• Avery, MacLeod, and McCarty Experiment:

  • Aim: Identify the transforming factor.

  • Methods:

  • Used Type IIIS bacterial filtrate and added RNase, Protease, and DNase to determine which component was responsible for transformation.

Results

• RNase (destroys RNA) – did not prevent transformation.

• Protease (destroys proteins) – did not prevent transformation.

• DNase (destroys DNA) – prevented transformation of Type IIR bacteria.

Conclusion

• Because only DNase destroyed the transforming factor, the transforming substance was identified as DNA.

• Hershey and Chase Experiment:

  • Provided additional support that DNA is the genetic material in bacteriophages.

  • Utilized E. coli and Bacteriophage T2 for differential labeling.

Experiment Methodology

• Question: Which component of the phage (DNA or protein) serves as the genetic material transmitted to progeny?

Procedures

• Conducted protein labeling with $^{35}S$ and DNA labeling with $^{32}P$ in T2 phage followed by infection into E. coli.

Results

• Observed that genetic information of phage T2 resides in its DNA (not proteins), as only the DNA was successfully transmitted to phage progeny.

Conclusion

• DNA and not protein is the genetic material in bacteriophages.

Structure of Nucleic Acids

• Composition of Nucleic Acids:

  • Composed of Nucleotides:

  • Phosphate group

  • A five-carbon sugar (pentose)

  • A nitrogenous base (cyclic compound)

Structural Components of DNA/RNA

• Key components include:

  • Phosphate

  • Sugar (ribose in RNA, deoxyribose in DNA)

  • Bases:

  • DNA Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

  • RNA Bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C)

Nitrogenous Bases

• Pyrimidines (1 ring):

  • Cytosine (C)

  • Thymine (T)

  • Uracil (U)

• Purines (2 rings):

  • Adenine (A)

  • Guanine (G)

Methylation of DNA

• Methylation can influence gene expression, affecting whether a gene is active or silenced.

Deoxyribonucleotides

• Nucleotides are linked by phosphodiester bonds to form a DNA structure:

  • dAMP (Deoxyadenosine 5'-monophosphate)

  • dGMP (Deoxyguanosine 5'-monophosphate)

  • dTMP (Deoxythymidine 5'-monophosphate)

  • dCMP (Deoxycytidine 5'-monophosphate)

Chargaff’s Rules

• Erwin Chargaff (1955) determined ratios of bases:

  • $[A] = [T]$ and $[G] = [C]$

  • $A + G = C + T$ (for double-stranded DNA).

Base Composition from Different Organisms

DNA Structure

• Double Helix:

  • Two strands coiled in a right-handed helix, forming a sugar-phosphate backbone.

  • Complementary strands with bases held together by hydrogen bonds.

  • Approximately 10 base pairs per turn, separated by $0.34 ext{ nm}$.

  • The diameter of the helix is $2 ext{ nm}$.

X-ray Diffraction Evidence

• Rosalind Franklin and Maurice Wilkins contributed to confirming the double helix structure through X-ray diffraction patterns, which showed a highly ordered two-strand configuration.

Variants of DNA Helices

• B-DNA: Physiological form, right-handed with 10 nucleotides per turn, 1.9 nm wide.

• A-DNA: Right-handed, found under high salt conditions or when partially dehydrated, with 11 nucleotides per turn, 2.3 nm wide.

• Z-DNA: Left-handed helix, characterized by repeating oligos of GCGC, with 12 nucleotides per turn, 1.8 nm wide.

DNA/RNA Secondary Structures

• Hairpin Structure: Forms when sequences of nucleotides on the same strand are inverted complements.

Triplex and Secondary Structures in Nucleic Acids

• H-DNA: Three-stranded DNA that occurs when one strand pairs with a double-stranded DNA from another part of the molecule, particularly in long sequences of only purines or only pyrimidines.

Genetic Information Pathways

• DNA Represents:

  • The repository of genetic information characterized by the sequence of nucleotides.

  • Must replicate faithfully to ensure that genetic information is accurately preserved.

  • Provides the instructions that will be translated into phenotype.

Central Dogma of Molecular Genetics

• Proposed by Francis Crick, depicting the flow of genetic information:

  • DNA replication $ o$ Transcription $ o$ mRNA $ o$ Translation $ o$ Protein.

• Modified Central Dogma:

  • Introduction of pre-mRNA and reverse transcription, leading to multiple protein products from a single gene.

• Flow of Information:

  • DNA $ o$ (by Transcription) $ o$ mRNA $ o$ (by Translation) $ o$ Proteins 1, 2, 3, …, n.