Molecular Structure of DNA and RNA: Foundations and Key Experiments

Molecular Structure of DNA and RNA

Introduction

The molecular structure of DNA and RNA forms the basis of genetic inheritance and cellular function. Understanding their structure and the experiments that established DNA as the genetic material is fundamental in genetics.

Criteria for Genetic Material

Essential Properties

• Information: The genetic material must contain the information necessary to construct an entire organism.

• Transmission: It must be passed from parent to offspring, ensuring continuity of genetic information.

• Replication: The material must be capable of being copied accurately for inheritance.

• Variation: It must be able to account for the known phenotypic variation within and between species.

Experimental Identification of DNA as Genetic Material

Griffith's Transformation Experiment

• Background: Frederick Griffith worked with Streptococcus pneumoniae bacteria, which exist in two forms: smooth (S, virulent) and rough (R, non-virulent).

• Key Observations:

  • Live S strain: killed mice.

  • Live R strain: mice survived.

  • Heat-killed S strain: mice survived.

  • Live R + heat-killed S: killed mice; live S bacteria recovered from mice.

• Conclusion: A "transforming principle" from the dead S bacteria converted R into S type, suggesting transfer of genetic information.

Avery, MacLeod, and McCarty Experiment

• Purpose: To identify the "transforming principle" as DNA, RNA, or protein.

• Method: Treated extracts from S cells with enzymes that degrade proteins, RNA, or DNA.

• Results: Only extracts treated with DNase (which destroys DNA) lost the ability to transform R cells into S cells.

• Conclusion: DNA is the genetic material responsible for transformation.

Hershey and Chase Experiment

• Background: Used bacteriophage T2, which infects Escherichia coli.

• Method: Labeled phage DNA with radioactive phosphorus (32P) and protein with radioactive sulfur (35S). Allowed phages to infect bacteria, then separated phage coats from cells.

• Results: 32P (DNA) entered bacterial cells; 35S (protein) did not.

• Conclusion: DNA, not protein, is the genetic material transmitted by phages.

Structure of Nucleotides

Components of a Nucleotide

• Phosphate group

• Pentose sugar: Deoxyribose in DNA, ribose in RNA

• Nitrogenous base: Purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA; cytosine, uracil in RNA)

Definition: A nucleotide is the repeating structural unit of nucleic acids, consisting of a sugar, a phosphate group, and a nitrogenous base.

Comparison of DNA and RNA Nucleotides

Key Differences

• Sugar: DNA contains deoxyribose; RNA contains ribose.

• Bases: DNA uses thymine; RNA uses uracil instead of thymine.

• Strand Structure: DNA is typically double-stranded; RNA is usually single-stranded.

Structural Features of DNA

Primary Structure

• Nucleotides are linked by phosphodiester bonds between the 5' phosphate and 3' hydroxyl groups of adjacent sugars, forming a sugar-phosphate backbone.

Secondary Structure: The Double Helix

• Two strands coil around a central axis, forming a right-handed double helix.

• Strands are antiparallel (one runs 5' to 3', the other 3' to 5').

• Bases pair via hydrogen bonds: adenine (A) with thymine (T), guanine (G) with cytosine (C).

• There are approximately 10 base pairs per complete turn of the helix.

Key Experiments Leading to the Double Helix Model

• Pauling: Proposed helical structures for proteins, inspiring similar models for DNA.

• Franklin and Wilkins: Used X-ray diffraction to show DNA is helical, with more than one strand and 10 base pairs per turn.

• Chargaff: Determined that the amount of A equals T and G equals C in DNA (Chargaff's rules).

• Watson and Crick: Integrated all data to propose the double helix structure in 1953.

Structural Features of the Double Helix

• Major and minor grooves allow protein binding and regulation.

• Base stacking stabilizes the helix via hydrophobic interactions.

• Alternative forms (A-DNA, B-DNA, Z-DNA) exist under different conditions.

Structural Features of RNA

Primary and Secondary Structure

• RNA is usually single-stranded but can form complex secondary structures (hairpins, bulges, internal loops) via intramolecular base pairing.

• RNA uses ribose sugar and uracil instead of thymine.

• RNA molecules can fold into tertiary structures, often with the help of proteins or metal ions.

Summary Table: Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double-stranded (usually)	Single-stranded (usually)
Helix	Right-handed double helix (B-form)	Varied; often forms secondary structures

Key Equations and Rules

• Chargaff's Rule:

$[A] = [T]$ $[G] = [C]$

• Phosphodiester Bond Formation:

$\text{Nucleotide}_n + \text{Nucleotide}_{n+1} \rightarrow \text{Nucleotide}_n - \text{phosphodiester bond} - \text{Nucleotide}_{n+1}$

Applications and Importance

• Understanding DNA and RNA structure is essential for fields such as molecular biology, genetics, biotechnology, and medicine.

• Knowledge of these structures underpins techniques like PCR, DNA sequencing, and gene editing.