M

Molecular Structure of DNA and RNA

Molecular Structure of DNA and RNA

Chapter Overview

  • Focus on understanding the characteristics of genetic material and the discovery of DNA as the genetic material.

  • Part 1: Discovery that DNA is the genetic material.

  • Part 2: Discovery of the double helix.

  • Part 3: Structure of DNA.

  • Part 4: Structure of RNA.

Characteristics of Genetic Material

  • Essential properties:

    • Must contain information necessary for development and function.

    • Must be passed from parents to offspring.

    • Must be capable of being copied, structure must allow this.

    • Must vary between individuals to account for phenotypic differences.

  • Argument Against DNA Function: Question of how a four-letter alphabet can convey enough information to support life.

Discovering DNA as the Genetic Material

  • Experiments:

    • Initial observations of rough (R) and smooth (S) bacteria:

    • R strain has no capsule and is destroyed by the immune system.

    • S strain has a capsule and is virulent.

    • Conclusion: Rough bacteria can acquire the information to form a capsule from dead smooth bacteria (Griffith’s transformation principle).

    • Key Experiment — Avery, MacLeod, and McCarty (1944):

    • Isolated and tested DNA, RNA, proteins, and carbohydrates from smooth strains.

    • Only purified DNA could transform R strain to S strain, demonstrating DNA as the transforming substance.

Chargaff's Rule

  • Erwin Chargaff (1950):

    • Determined that nucleotide composition varies across species and established:

    • Amount of adenine (A) = amount of thymine (T).

    • Amount of guanine (G) = amount of cytosine (C).

    • This leads to: A + G = C + T (total purines = total pyrimidines).

X-ray Diffraction and the Double Helix

  • X-ray Diffraction:

    • Conducted by Rosalind Franklin:

    • Produced data indicating the helical structure of DNA with 2 or more strands and 10 bases per turn.

  • Watson-Crick Model (1953):

    • Utilized Chargaff’s rule, X-ray diffraction data, and 3-D modeling to propose the double-helical structure of DNA.

    • Implications: If strands are complementary, one serves as a template for the other.

Structure of DNA

  • Building Blocks of DNA:

    • Nucleotides: Composed of a phosphate group, deoxyribose sugar (in DNA), and nitrogenous bases (adenine, thymine, cytosine, guanine).

  • Bonds in DNA:

    • Held together by phosphodiester bonds between the phosphate group of one nucleotide and the 3' hydroxyl group of another.

  • Antiparallel Nature:

    • Strands run in opposite directions: 5' to 3' and 3' to 5'.

Sequence of DNA

  • The sequence of DNA (nucleotide order) dictates genetic information.

  • Example: A DNA strand may have the sequence 5’ AACACC 3’, with the complementary strand being 3’ TTGTGG 5'.

RNA Structure

  • Comparison with DNA:

    • RNA has ribose sugar instead of deoxyribose (in DNA).

    • Uracil (U) replaces thymine (T).

  • RNA is single-stranded and can form complex three-dimensional structures, contributing to its biological functions.

  • Fragility of RNA: Due to its single-stranded nature, RNA is more susceptible to degradation than the double-stranded DNA.

  • Secondary Structures: RNA can form regions like bulge loops, internal loops, and stem-loops which play roles in its function.

DNA as Genetic Information Molecule

  • The sequence of nucleotides encodes information needed to produce proteins, which perform structural, enzymatic, and regulatory functions in cells.

  • Organized into chromosomes, DNA is heritable, passed from parents to offspring, playing a fundamental role in inheritance.

Chromosome Structure

  • Description of DNA in metaphase chromosomes: contains radial loops and is much more compact than its relaxed form.

  • Measurements:

    • Chromatin (2 nm in diameter)

    • Nucleosomes (11 nm in diameter)

    • Condensed structure (30 nm fibers)

Differences Between DNA and RNA

  • Structural Differences:

    • DNA: Double-stranded, deoxyribose sugar, thymine base.

    • RNA: Single-stranded, ribose sugar, uracil base.

Analyzing RNA Virus Base Composition

  • Example: An RNA virus with specific percentages of bases (14.1% A, 14.0% U, 36.2% G, 35.8% C) could indicate if it is double or single-stranded based on the ratio successiveness of complementary bases; potential implications of base pairing should be analyzed in context.

Conclusion

  • Understanding the molecular structure of DNA and RNA is crucial to comprehending how genetic information is stored, replicated, and transmitted across generations. This knowledge underlines the intricate relationship between molecular biology, genetics, and evolutionary biology.