Notes on Viruses, DNA Structure, RNA Types, and Gene Expression

Overview of Viruses

  • Definition: Viruses are biological entities that invade living cells to reproduce.

  • Structure:

    • Genetic material (DNA or RNA)

    • Protein coat (capsid)

    • In some viruses, a membranous envelope surrounds the protein coat.

  • Infection Process:

    • Viruses bind to host cell receptors.

    • Viral nucleic acid enters the host cell.

    • It can integrate into the host chromosome and remain dormant.

    • When activated, viral DNA uses host's machinery to replicate, destroying the host cell in the process and releasing new viral particles.

Structure of DNA

  • Historical Contributions: Key figures include Mendel, Erwin Chargaff, Rosalind Franklin, James Watson, and Francis Crick.

Chargaff’s Rules

  • DNA comprises four nucleotides differentiated by bases:

    • Purines: Adenine (A) and Guanine (G)

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

  • Proportions:

    • The amount of Adenine (A) equals Thymine (T), and Guanine (G) equals Cytosine (C).

    • These proportional relationships are crucial for understanding DNA structure.

Franklin's X-Ray Diffraction

  • Rosalind Franklin's X-ray diffraction studies led to evidence that DNA has a helical shape based on the diffraction pattern observed from DNA fibers.

Watson and Crick Model of DNA

  • Model Description:

    • Composed of two polynucleotide chains twisted into a double helix.

    • Sugar-phosphate backbone on the outside and nitrogenous bases on the inside, held together by hydrogen bonds.

  • Base Pairing:

    • Specific pairs of bases (A-T and G-C) maintain uniformity in structure.

DNA Replication

  • Process Overview:

    • Semiconservative replication: Each new DNA helix contains one original and one new strand.

    • Steps:

    1. Unzipping: Enzyme helicase unwinds the double helix, breaking hydrogen bonds between bases.

    2. Complementary Base Pairing: DNA polymerase facilitates pairing of new nucleotides to the original strands.

    3. Repair: DNA ligase repairs breaks in the backbone.

RNA Structure and Types

  • Definition: RNA (Ribonucleic acid) consists of nucleotides which include A, U, C, G, ribose sugar, and phosphate.

  • Types of RNA:

    • Messenger RNA (mRNA) is produced in the nucleus by a
      process called transcription
      • Messenger RNA carries genetic information from DNA to
      the cytoplasm

    • Transfer RNA (tRNA): Transports amino acids to ribosomes.

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

Gene Expression and Protein Synthesis

  • Process:

    1. Transcription: Conversion of DNA to mRNA in the nucleus.

    2. Translation: mRNA directs amino acid assembly into proteins.

  • Genetic Code: Codons (three-nucleotide sequences) represent specific amino acids, with 64 possible codons.

    • AUG signals start, while three stop codons signal termination of translation.

Transcription Process

  • Stages:

    1. Initiation: RNA polymerase binds to the promoter, unwinding the DNA helix.

    2. Elongation: RNA strand elongates as nucleotides are added.

    3. Termination: RNA polymerase reaches the terminator sequence, completing mRNA synthesis.

  • Eukaryotic mRNA Processing:

    • Involves capping, poly-A tail addition, and splicing out of introns to form mature mRNA before translation occurs in the cytoplasm.