4.3 Bio Notes

Nucleotides

  • Building Block of DNA: The basic unit of DNA is a nucleotide.
  • Molecular Structure:
    • Composed of three parts:
    1. Sugar:
      • Contains five carbon atoms.
      • Can be ribose or deoxyribose (lacks a hydroxyl group on 2' carbon).
      • Carbons are numbered 1' to 5'.
    2. Phosphate Group:
      • Attached to the 5' carbon of the sugar.
    3. Nitrogen-Containing Base:
      • Five different bases exist, but only four are used in DNA.
  • Types of Bases:
    • Purines: Adenine (A), Guanine (G)
    • Pyrimidines: Thymine (T), Cytosine (C), Uracil (U) (Note: Uracil is found in RNA, not DNA).
  • Functionality: Nucleotides also serve as energy carriers (e.g., ATP).

Nucleic Acids

  • Formation: Nucleic acids are polymers formed by nucleotides through condensation reactions (dehydration synthesis).
  • Bonding:
    • Phosphate of one nucleotide bonds to the -OH of another's 3' carbon, creating a phosphodiester bond.
  • Directionality:
    • Nucleic acid chains are asymmetrical with distinct ends:
    • 5' end (free phosphate)
    • 3' end (free hydroxyl group)

DNA Structure

  • Polymer Structure:
    • DNA is a polymer made exclusively of deoxyribose nucleotides.
    • Strands: A single chain of nucleotides.
    • Backbone consists of sugars and phosphates held together by covalent bonds.
  • Double-Stranded: Chromosomes consist of two strands of DNA that are complementary and anti-parallel.
    • Base Pairing:
    • A pairs with T (2 Hydrogen bonds)
    • G pairs with C (3 Hydrogen bonds)
    • This complementarity allows one strand's information to dictate the other strand.
  • Twisted Structure: DNA forms a double helix structure with alternating major and minor grooves.
    • Major groove: larger gap.
    • Minor groove: smaller gap.

Functions of DNA

  • Genetic Information Storage:
    • DNA contains the information necessary to synthesize RNA and proteins, organized into sequences known as genes.
    • Genes are segments of DNA that code for specific proteins.
  • Gene Expression: The process by which information from genes is used to synthesize RNA and proteins.
  • Chromosomal Organization:
    • Human cells contain 23 pairs of chromosomes, totaling 46 per cell, comprising about 3.2 billion base pairs.
    • Approximately 20% of this DNA is coding DNA (genes), while 80% is non-coding DNA with various functions.

DNA Replication

  • Semi-Conservative Process:
    • Replication of DNA results in two double-stranded molecules, each containing one original and one new strand.
  • Replication Basics:
    • Occurs during the S phase of interphase before cell division.
    • Involves unwinding the DNA strands and synthesizing new complementary strands using original strands as templates.
  • Initiation of Replication:
    • Begins at replication origins where replication bubbles form.
    • Bidirectional replication occurs through forks originating from the bubbles until the entire chromosome is copied.

DNA Polymerase Functionality

  • Synthesis:
    • DNA polymerase synthesizes DNA by catalyzing nucleotide incorporation onto a growing strand.
    • It also proofreads newly synthesized DNA to correct mismatched bases, ensuring fidelity in DNA replication.
    • Proofreading Mechanism: If an incorrect base is added, polymerase identifies the error, halts synthesis, and uses its exonuclease activity to remove the wrong base before continuing synthesis.

Key Terms and Concepts to Remember

  • Nucleotide Components: Sugar, Phosphate Group, Nitrogenous Base.
  • DNA Structure Terms: Strand, Directionality, Backbone, Complementarity, Double Helix, Antiparallel, Major Groove, Minor Groove.
  • DNA Functionality: Gene, Genetic Sequence, Gene Expression, Coding DNA, Non-Coding DNA.
  • Replicative Concepts: Replication Bubble, Replication Fork, Bidirectional Replication.
  • Role of DNA Polymerase: DNA Synthesis and Proofreading process.