Oxford IB Biology Textbook 2023 Fixed pages 16 - 25

Structure and Function of Nucleic Acids

Hereditary Information Storage

  • Nucleic acids allow the storage of hereditary information, akin to how binary code encodes data in computers.

    • Binary code uses two options (0 and 1) while DNA codons use four possible values, leading to 64 values with three symbols for codons, compared to 256 possibilities in binary with eight symbols.

    • This capacity has prompted research into DNA computers.

DNA Representation in Binary Code

  • Example: The term "DNA" would be represented in binary code through ASCII values:

    • D: 01000100

    • N: 01001110

    • A: 01000001

Structure of DNA and Accurate Replication

  • The structure of DNA is crucial for accurate replication due to complementary base pairing.

    • A is always paired with T, and C is paired with G.

    • Each chromosome consists primarily of DNA, forming two chromatids connected by a centromere after replication.

Components of Nucleotides

Defining Nucleotides

  • Nucleotides are the building blocks of nucleic acids, composed of:

    1. Sugar: A five-carbon pentose sugar.

    2. Phosphate group: Acidic and negatively charged.

    3. Nitrogenous base: Contains nitrogen and can have single or double rings.

  • The linkage of the base and phosphate occurs via covalent bonds to the pentose sugar.

Sugar-Phosphate Backbone

  • Nucleotides are linked through covalent bonds between the phosphate of one nucleotide and the pentose sugar of the next, creating a strong backbone in nucleic acids.

  • The sugar-phosphate backbone conserves the sequence of nitrogenous bases along the molecule.

Base Pairing and Genetic Coding

Bases in Nucleic Acids

  • DNA and RNA contain four bases, with three bases being identical and one differing:

    • DNA: A, C, G, T

    • RNA: A, C, G, U

  • The vast number of possible base combinations allows for infinite genetic coding possibilities.

Importance of Distinct Base Structures

  • Each nitrogenous base has a unique chemical structure, essential for maintaining the fidelity of DNA replication and function in coding genetic information.

RNA Formation

Polymerization of RNA

  • RNA is formed through condensation reactions among nucleotides, creating a single, unbranched polymer.

  • The process involves the elimination of water as nucleotides bond together.

The Role of Directionality in Nucleic Acids

  • Nucleotides are connected in a consistent direction, giving a strand a distinct 3' and 5' end.

    • This directionality is vital for processes like DNA replication and transcription.

Structural Characteristics of DNA

DNA Double Helix

  • DNA consists of two antiparallel strands of nucleotides forming a double helix.

    • The strands link via hydrogen bonds between complementary bases:

      • A pairs with T

      • G pairs with C

  • These interactions allow for the stability and integrity of the genetic code during replication.

Antiparallel Orientation

  • The two strands of DNA run in opposite directions, an arrangement that is crucial for base pairing and replication processes.

Genetic Code and Its Universality

Universal Genetic Code

  • The genetic code, comprising codons of three bases, is nearly universal among living organisms, which suggests a common ancestry.

    • There are 64 codons corresponding to various amino acids or signals for protein synthesis.

  • The conservation of this genetic code across diverse life forms emphasizes the unity of life and underlines evolutionary relationships.

Implications of Genetic Code Conservation

  • The near universality of the genetic code has profound philosophical and practical implications for understanding life and evolution.