DNA Structure and Analysis: Foundations of Genetic Material

DNA Structure and Analysis

Introduction

This chapter explores the experimental evidence and molecular details that established DNA as the hereditary material. It covers the chemical structure of DNA and RNA, the historical experiments that identified DNA as the genetic material, and the key features of nucleic acid structure.

Characteristics of Hereditary Material

Essential Properties

• Replication: The genetic material must be able to make accurate copies of itself.

• Information Storage: It must store all the information necessary for the structure and function of an organism.

• Expression: The information must be expressed to direct cellular processes.

• Transmission: It must be passed from parent to offspring.

• Mutation: It must be capable of undergoing changes to allow evolution.

Historical Experiments Identifying DNA as Genetic Material

Griffith's Transformation Experiment

Frederick Griffith demonstrated that a 'transforming principle' from dead virulent bacteria could convert non-virulent bacteria into a virulent form, suggesting the transfer of genetic material.

Avery, MacLeod, and McCarty Experiment

These researchers fractionated bacterial extracts and showed that only the DNA fraction could transform non-virulent bacteria, providing strong evidence that DNA is the genetic material.

Hershey-Chase Blender Experiment

Using bacteriophages labeled with radioactive isotopes, Hershey and Chase demonstrated that DNA, not protein, enters bacterial cells and directs viral replication, confirming DNA as the genetic material.

Additional Evidence for DNA as Genetic Material

• Organelle Genetics: DNA is present in mitochondria and chloroplasts, which perform genetic functions.

• Mutagenesis: DNA and RNA absorb UV light at 260 nm, the same wavelength that induces mutations, while proteins absorb at 280 nm.

Chemical Structure of DNA and RNA

Nucleotides: The Building Blocks

Nucleic acids are polymers of nucleotides, each consisting of a phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

Nitrogenous Bases

• Purines: Adenine (A) and Guanine (G) – double-ring structures.

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), and Uracil (U, in RNA) – single-ring structures.

Phosphodiester Bonds

Nucleotides are linked by phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' hydroxyl of the next, giving the strand directionality (5' to 3').

Key Experiments Leading to the DNA Double Helix Model

Chargaff's Rules

Erwin Chargaff found that in DNA, the amount of adenine equals thymine (A = T) and the amount of guanine equals cytosine (G = C), suggesting base pairing.

Organism	Adenine (%)	Thymine (%)	Guanine (%)	Cytosine (%)
Escherichia coli	26.0	23.9	24.9	25.2
Streptococcus	29.8	31.6	20.5	18.0
Yeast	31.7	32.6	18.3	17.4
Turtle red blood cells	28.0	27.9	22.0	21.3
Salmon sperm	29.2	29.2	20.8	20.4
Chicken red blood cells	28.0	28.4	21.6	21.0
Human liver cells	30.3	30.3	19.5	19.9

Rosalind Franklin's X-ray Diffraction

Franklin's X-ray diffraction images revealed that DNA is a helical molecule with a regular structure, about 2 nm in diameter, and 10 base pairs per turn.

Watson and Crick Model

Watson and Crick integrated Chargaff's rules and Franklin's data to propose the double helix model of DNA, with two antiparallel strands held together by specific base pairing (A-T and G-C).

Key Features of the DNA Double Helix

• Double Helix: Two strands form a right-handed helix.

• Complementary Base Pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds).

• Antiparallel Strands: The two strands run in opposite directions (5' to 3' and 3' to 5').

• Major and Minor Grooves: The helix has alternating wide (major) and narrow (minor) grooves important for protein binding.

• 10 Base Pairs per Turn: Each complete turn of the helix contains 10 base pairs.

DNA and RNA: Similarities and Differences

Comparison Table

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded (usually)	Single-stranded (usually)
Function	Genetic information storage	Information transfer, catalysis

Structural Forms of DNA

B-DNA and Z-DNA

• B-DNA: The most common form in cells; right-handed helix, bases perpendicular to axis.

• Z-DNA: Left-handed helix, zigzag backbone, can form under certain conditions.

RNA Structure and Function

Primary and Secondary Structure

RNA is typically single-stranded but can form secondary structures such as hairpins, bulges, and internal loops due to complementary base pairing (A-U, C-G).

Major Classes of RNA

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation; has a cloverleaf secondary structure and L-shaped tertiary structure.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

Summary Table: Key Experiments and Discoveries

Year	Discovery
1860s	Mendel's work published
1900s	Chromosome theory of inheritance proposed
1940s	DNA shown to carry genetic information
1950s	Watson-Crick model of DNA
1970s	Recombinant DNA technology developed
1990s	Genomics begins, Human Genome Project initiated
2000s	Applications of genomics, gene editing technologies

Key Terms

• Virulent: Extremely poisonous or deadly.

• Transforming Principle: Substance capable of transferring genetic information.

• Mutagenesis: The process of generating mutations.