Chapters 13 and 14 Test

13-1 Identifying the Substance of the Gene

Bacterial Transformation

• Frederick Griffith (1928) conducted experiments to determine how bacteria produced pneumonia.

  • Strains of Bacteria:

    • S strain: Smooth colonies, disease-causing.

    • R strain: Rough colonies, harmless.

  • Experiment Steps:

    1. Injected mice with disease-causing bacteria (S strain): Mice died.

    2. Injected mice with harmless bacteria (R strain): Mice lived.

    3. Injected mice with heat-killed S strain: Mice lived.

    4. Injected mice with a mixture of heat-killed S strain and live R strain: Mice died.

  • Conclusion: A factor from the heat-killed S strain transformed the R strain into disease-causing bacteria.

  • Hypothesis: The transforming factor might be a gene.

Oswald Avery's Work (1944)

• Repeated Griffith’s experiments to identify the transforming substance.

  • Treated heat-killed bacteria extracts with enzymes that destroyed proteins, lipids, carbohydrates, and RNA: Transformation still occurred.

  • Treated extracts with enzymes that destroyed DNA: Transformation did not occur.

  • Conclusion: DNA was the transforming factor, responsible for storing and transmitting genetic information.

Hershey-Chase Experiment (1952)

• Studied bacteriophages (viruses that infect bacteria) to determine if genetic material was protein or DNA.

  • Method: Used radioactive isotopes:

    • 32P to label DNA.

    • 35S to label protein coat.

  • Results: Radioactive 32P was found in bacteria, proving DNA is the genetic material.

13-2 The Structure of DNA

Key Contributions:

• Erwin Chargaff (1940s): Discovered that the percentage of adenine (A) equals thymine (T), and cytosine (C) equals guanine (G). Known as Chargaff's Rules.

• Rosalind Franklin (1950s): Used X-ray diffraction to reveal DNA’s helical structure.

• James Watson and Francis Crick: Built the first accurate 3D model of DNA as a double helix.

DNA Components:

• Nucleotides: Monomers that form DNA, each consisting of:

  • 5-carbon sugar (deoxyribose).

  • Phosphate group.

  • Nitrogenous base (A, T, C, G).

• Base Pairing Rules:

  • Adenine (A) pairs with Thymine (T).

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

  • Hydrogen bonds hold base pairs together.

13-3 DNA Replication

Process Overview:

1. Unzipping DNA:

   • Enzyme: Helicase breaks hydrogen bonds, forming a replication fork.

2. Template Strands:

   • Each original strand serves as a template.

3. Binding Proteins:

   • Single-stranded binding proteins keep strands apart.

4. New Strand Formation:

   • DNA polymerase adds complementary nucleotides and proofreads.

   • Direction: 5’ to 3’.

5. Result: Two identical DNA molecules, each with one original and one new strand (semi-conservative model).

Key Points to Remember:

• Antiparallel Strands: 5’ and 3’ ends run in opposite directions.

• Leading vs. Lagging Strand:

  • Leading strand: Continuous synthesis.

  • Lagging strand: Synthesized in Okazaki fragments, joined by DNA ligase.

• Differences in Prokaryotes and Eukaryotes:

  • Prokaryotes: Single, circular DNA molecule.

  • Eukaryotes: Multiple replication forks across linear chromosomes.

Additional Details on DNA Replication:

• Replication Forks and Enzymes:

  • The replication fork is where the DNA separates, with helicase unwinding it.

  • DNA ligase joins Okazaki fragments on the lagging strand.

• Single-stranded Binding Proteins (SSBs): Prevent reannealing of the DNA strands during replication.

• Proofreading Function: DNA polymerase ensures accurate nucleotide matching to prevent mutations.

14-1 RNA and Protein Synthesis

RNA Structure and Function:

• RNA Components:

  • Ribose sugar.

  • Phosphate group.

  • Nitrogenous bases: Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).

• Comparison with DNA:

  • RNA is single-stranded, while DNA is double-stranded.

  • RNA contains uracil (U) instead of thymine (T).

Types of RNA:

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.

• Ribosomal RNA (rRNA): Combines with proteins to form ribosomes.

• Transfer RNA (tRNA): Transfers amino acids to ribosomes for protein synthesis.

14-2 Transcription and Translation

Transcription Process:

1. RNA polymerase binds to DNA at the promoter region.

2. DNA strands separate, and RNA polymerase synthesizes a complementary mRNA strand.

3. Transcription stops at a termination signal, and mRNA is edited (introns removed, exons spliced).

Translation Process:

1. mRNA attaches to a ribosome in the cytoplasm.

2. tRNA with anticodons binds to mRNA codons.

3. Each tRNA carries a specific amino acid, forming a polypeptide chain as the ribosome reads the mRNA.

4. Process continues until a stop codon is reached, releasing the completed protein.

Codons and Genetic Code:

• Codons are sequences of three mRNA nucleotides that code for specific amino acids.

• Start codon: AUG (methionine).

• Stop codons signal the end of translation.

14-3 Gene Regulation and Expression

Gene Regulation in Prokaryotes:

• Operons (e.g., lac operon in E. coli) control gene expression.

  • Promoter: Binding site for RNA polymerase.

  • Operator: Binding site for regulatory proteins.

  • When lactose is present, it binds to the repressor, allowing gene transcription.

Gene Regulation in Eukaryotes:

• More complex, involving transcription factors and enhancers.

• TATA Box: A DNA sequence that helps position RNA polymerase.

• Epigenetics: Chemical modifications that affect gene expression without altering DNA sequence.

14-4 Mutations

Types of Mutations:

• Gene Mutations:

  • Point Mutations: Substitution, insertion, deletion.

  • Frameshift Mutations: Shifts reading frame, affecting downstream amino acids.

• Chromosomal Mutations:

  • Deletion, duplication, inversion, translocation.

Effects of Mutations:

• Can be neutral, harmful, or beneficial.

• Mutagens: Chemical or physical agents that cause mutations (e.g., UV light, chemicals).

Polyploidy:

• Condition where organisms have extra sets of chromosomes; common in plants.