Chapter 11 - Molecular Structure of DNA and RNA

Molecular Structure of DNA and RNA

Overview of DNA and RNA Structure

  • Definition of a Nucleic Acid:

    • Nucleic acids are large macromolecules, primarily DNA and RNA, that store and transmit genetic information.

  • The Four Levels of DNA Complexity:

    1. Nucleotide Structure: Building blocks of nucleic acids.

    2. Single Strands: Nucleotides linked to form a linear strand.

    3. Double Helix: Two strands of DNA interact to form a helical structure.

    4. Higher Order Structures: Further folding and interactions with proteins result in complex forms like chromosomes.

Structure of DNA and RNA

  • DNA and RNA Characteristics:

    • Types of nucleic acids with various levels of complexity.

    • Nucleotides serve as the repeating units forming the strands.

    • DNA strands coalesce to create a double helical structure, which folds and bends to form chromosomes.

Nucleotide Structure

  • Components of a Nucleotide:

    • Phosphate Group

    • Pentose Sugar:

    • Deoxyribose in DNA.

    • Ribose in RNA.

    • Nitrogenous Base:

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

    • Pyrimidines: Thymine (T), Cytosine (C), and Uracil (U) in RNA.

    • The nucleotide is the basic structural unit of both DNA and RNA.

Nucleotide Examples

  • Nucleoside vs. Nucleotide:

    • Nucleoside: Consists of a base + sugar.

    • Examples:

      • Adenosine (Adenine + Ribose)

      • Deoxyadenosine (Adenine + Deoxyribose)

    • Nucleotide: Consists of a base + sugar + phosphate(s).

    • Examples:

      • Adenosine Monophosphate (AMP)

      • Adenosine Diphosphate (ADP)

      • Adenosine Triphosphate (ATP)

Structure of DNA Strands

  • DNA Backbone:

    • Formed by alternating phosphate and sugar (deoxyribose) groups.

    • Bases project from the backbone conferring directionality (5′ to 3′).

    • Phosphodiester Linkage:

    • Connects the 5′ carbon of one nucleotide to the 3′ carbon of another nucleotide.

DNA Strand Information

  • Defining Characteristic:

    • The sequence of nucleotides within DNA/RNA carries information.

    • The bases cannot be shuffled or rearranged.

DNA Double Helix Structure

  • Formation of Helix:

    • Two strands twist around a common axis.

    • Stabilized by hydrogen bonding between complementary base pairs.

    • Base Pairing Rules:

    • Adenine (A) pairs with Thymine (T) through 2 hydrogen bonds.

    • Guanine (G) pairs with Cytosine (C) through 3 hydrogen bonds.

    • Antiparallel Orientation:

    • One strand runs in the 5′ to 3′ direction, while the other runs in the 3′ to 5′ direction.

    • Twist Features:

    • Typically, 10 bases and 3.4 nm per complete twist of the helix.

    • Grove Structures:

    • There are two grooves, a major groove and a minor groove.

RNA Structure

  • Characteristics of RNA:

    • RNA strands are usually several hundred to thousands of nucleotides long, synthesized by transcription from DNA.

    • Primary structure resembles DNA, except:

    • Uracil (U) replaces Thymine (T).

    • Ribose replaces Deoxyribose.

RNA Tertiary Structure

  • Factors Influencing Tertiary Structure:

    • Complementary base-pairing within the RNA strand.

    • Base-stacking interactions.

    • Interactions with small molecules, ions, and proteins.

    • Various RNA secondary structures can occur, including double-helical regions.

Interactive Questions

  • iClicker Questions:

    • Q1: Distinction between nucleotide and nucleoside:

    • Option c is correct: Nucleotides include a phosphate group; nucleosides do not.

    • Q2: Consequence of replacing a phosphate group with a hydroxyl group in DNA:

    • Option b is correct: It disrupts phosphodiester bond formation, preventing strand elongation.

    • Q3: How does RNA folding get affected by a base substitution resulting in A-U pairs:

    • Option C is correct: The local helix weakens due to fewer hydrogen bonds.

    • Q4: If the anticodon loop of tRNA is misfolded:

    • Option C is likely affected most immediately regarding codon-anticodon recognition with mRNA.

Summary

  • DNA and RNA serve pivotal roles in genetic information storage and transmission. Their structural complexities allow for intricate forms of interaction and function. Understanding the precise characteristics of their nucleotide components, strand formation, and overall helical structures is essential for grasping the basics of molecular biology.