BIO 120 Chapter Four F2024

Chapter 4: Nucleic Acids and Information Flow

4.1 Overview of DNA

  • Common Structure: DNA has a uniform structure across all organisms.

  • Functions:

    • Storing Genetic Information

    • Self-Replication

  • Early Experiments: Key experiments laid the groundwork for understanding the structure and function of DNA.

4.2 Historical Experiments on DNA

  • F. Griffith's Experiment (1928):

    • Demonstrated that DNA is the genetic material.

    • Key Findings:

      • Mice injected with virulent S. pneumoniae died, while those injected with non-virulent strains survived.

      • Mixing heat-killed virulent bacteria with non-virulent bacteria led to the death of mice, suggesting that some genetic information remained.

  • Avery, MacLeod, and McCarty's Experiment:

    • Isolated DNA, RNA, and proteins to determine which was responsible for transformation.

    • Results:

      • Only the extract containing DNA transformed non-virulent bacteria into virulent forms.

      • Enzymatic degradation of DNA with DNase prevented transformation (confirming DNA as genetic material).

4.3 The Building Blocks of Nucleic Acids

4.3.1 Nucleotides

  • Composition: Nucleotides consist of:

    • Phosphate Group

    • Deoxyribose Sugar

    • Nitrogenous Base (A, G, T, or C)

  • Structure: DNA structure built from nucleotide subunits.

4.3.2 The Four DNA Bases

  • Purines: Adenine (A), Guanine (G)

  • Pyrimidines: Thymine (T), Cytosine (C)

4.4 DNA Structure

  • Phosphodiester Bonds: Connect nucleotides; form between the 5' phosphate and 3' hydroxyl group, creating a directional strand (5' to 3').

  • Base Pairing: Adenine pairs with Thymine (two hydrogen bonds), and Guanine pairs with Cytosine (three hydrogen bonds).

4.5 DNA Replication

  • Semiconservative Process: Each new DNA molecule consists of one old and one new strand.

  • Fidelity: Ensured through complementary base pairing, although errors can lead to mutations (which can be harmful, beneficial, or neutral).

4.6 The Central Dogma of Molecular Biology

  • Flow of Genetic Information:

    • DNA → RNA → Protein

  • Transcription: DNA is transcribed into RNA.

  • Translation: RNA is translated to form proteins.

4.7 Differences between Prokaryotic and Eukaryotic Transcription

  • Process Locations:

    • Prokaryotes: Cytoplasm

    • Eukaryotes: Nucleus (RNA processing occurs before transport to cytoplasm).

4.8 RNA and Its Structure

  • Comparison with DNA:

    • Sugar: RNA contains ribose (hydroxyl group) while DNA contains deoxyribose (hydrogen in place of hydroxyl).

    • Base: RNA contains Uracil (U) instead of Thymine (T).

4.9 RNA Processing in Eukaryotes

  • Modifications: Include the addition of a 5' cap, removal of introns, and addition of a poly(A) tail which stabilizes RNA and aids in translation.

4.9.1 Splicing and Alternative Splicing

  • Intron Removal: Introns are non-coding regions that are spliced out, while exons (coding regions) are joined together.

  • Alternative Splicing: Allows a single gene to encode multiple proteins through different splicing patterns.

4.10 Transcription Regulation

  • Gene Expression: Not all genes are active simultaneously. Transcription factors play a critical role in regulating gene expression based on cellular conditions.

4.11 Enhancers and Regulatory Sequences

  • Enhancer Sequences: Mutations in enhancer sequences can drastically alter transcription efficiency and gene expression.