Study group

Hershey and Chase Experiment

• Overview: The Hershey and Chase experiment was fundamental in demonstrating that DNA is the genetic material.

  • Conducted by Martha Chase and Alfred Hershey in 1952.

  • Focused on bacteriophages (viruses that infect bacteria).

Key Processes

• Virus Mechanism: A virus attaches to bacteria, injecting its genetic material.

  • The bacteria then replicate the viral DNA.

  • This process results in the production of new viruses, which cause the bacterial cell to burst (lyse).

Components Used

• Isotopes for Labeling: The experiment utilized radioactive isotopes to track DNA and proteins.

  • Phosphorus-32 (32P) was used to label DNA (phosphate groups in DNA).

  • Sulfur-35 (35S) was used to label proteins (specifically in amino acids).

Experiment Design

• Two groups of bacteriophages were produced:

  1. Phages with radioactive DNA (32P).

  2. Phages with radioactive protein coat (35S).

• Bacteria were infected with these phages.

Results

• Centrifuging: After infection, the mixture was blended to separate the viral coats from the bacteria, then centrifuged.

  • Results showed that the bacteria contained phosphorus (32P), indicating that DNA entered the bacteria during infection.

  • Conversely, the protein coat, labeled with sulfur, remained outside the bacteria.

• Conclusion: This experiment provided solid evidence that DNA, and not protein, serves as the genetic material in viruses.

Structure of DNA vs. RNA

• DNA: Double-stranded helix structure, contains deoxyribose sugar, bases include adenine (A), thymine (T), cytosine (C), and guanine (G).

• RNA: Single-stranded molecule, contains ribose sugar, bases are adenine (A), uracil (U), cytosine (C), and guanine (G).

Base Pairing Rules

• Watson and Crick Findings: Each nucleotide base pairs specifically:

  • Adenine (A) pairs with Thymine (T) (DNA) or Uracil (U) (RNA).

  • Cytosine (C) pairs with Guanine (G).

Hydrogen Bonds

• A-T pairs form two hydrogen bonds.

• C-G pairs form three hydrogen bonds, contributing to the stability of DNA.

DNA Replication Facilitation

• Mechanism of Replication:

  • Double Helix Unwinding: The DNA strands separate under the action of enzymes (like helicase).

  • Template Strands: Each original strand serves as a template for a new complementary strand, enhancing accuracy.

Key Enzymes Involved

• Helicase: Unwinds DNA strands at the replication fork.

• DNA Polymerase: Synthesizes new strands by adding nucleotides complementary to the templates in a 5' to 3' direction.

• Ligase: Joins Okazaki fragments on the lagging strand.

Central Dogma of Molecular Biology

• Definition: Genetic information flows from DNA to RNA to Protein.

  • Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus.

  • Translation: mRNA is translated into proteins in the cytoplasm at ribosomes.

Transcription Process

• Occurs in the nucleus of eukaryotic cells.

• Reactants: DNA is the template; product is mRNA.

• Processing: Involves splicing, adding a 5' cap, and a poly-A tail.

Translation Process

• Occurs in the cytoplasm at ribosomes.

• Reactants: mRNA, tRNA, and ribosomes.

• Products: Polypeptide chains (proteins) are formed from amino acids brought by tRNA.

Types of Mutations and Consequences

• Point Mutation: A single nucleotide change that can lead to silent, missense, or nonsense mutations.

  • Silent Mutation: No change in protein function.

  • Missense Mutation: Alters one amino acid in a protein, potentially affecting functionality.

  • Nonsense Mutation: Creates a premature stop codon, leading to truncated proteins.

• Frame Mutation: Involves shifts in the reading frame, altering downstream amino acid sequence.

Epigenetic Regulation in Eukaryotes

• Definition: Modifications of DNA and histones that can affect gene expression without altering the DNA sequence.

• Key Modifications:

  • Methylation: Addition of methyl groups, typically represses gene expression.

  • Acetylation: Addition of acetyl groups, usually enhances gene expression by loosening DNA around histones, making it more accessible for transcription.

Conclusion

• The combination of knowledge from the Hershey-Chase experiment and understanding the molecular mechanisms of DNA, RNA, and mutations provides essential insights into genetics and cellular biology.