DNA as the Hereditary Molecule

Essential Characteristics of Hereditary Material

The hereditary material must possess several key features to fulfill its biological role:

• Localization: Found in the nucleus and as a component of chromosomes.

• Stability: Present in a stable form within cells.

• Complexity: Sufficiently complex to encode information for structure, function, development, and reproduction.

• Replication: Capable of accurate self-replication, ensuring daughter cells inherit identical information.

• Mutability: Able to undergo mutations at a low rate, providing genetic variation for evolution.

Discovery and Early Evidence

• Friedrich Miescher (1869): Isolated DNA from nuclei, termed it "nuclein."

• Edmund Wilson (1895): Linked DNA to hereditary material, noting equal chromosome contribution from sperm and egg.

• Mendel’s Principles (1900): Rediscovered, leading to the chromosomal theory of inheritance.

• DNA Chemistry (1920): Identified as a polynucleotide with four bases: adenine (A), thymine (T), cytosine (C), guanine (G).

Experimental Evidence for DNA as Hereditary Material

• Griffith’s Transformation Experiment: Demonstrated that a "transformation factor" could transfer hereditary information between bacterial strains.

• Avery, MacLeod, and McCarty: Identified DNA as the transformation factor by selectively destroying DNA, RNA, proteins, or lipids and observing effects on transformation.

• Hershey-Chase Experiment (1952): Used radioactive labeling to show that DNA, not protein, is responsible for bacteriophage infection of bacteria.

DNA Structure

Nucleotides and Their Components

DNA is composed of nucleotides, each consisting of:

• Deoxyribose sugar (five carbons: 1', 2', 3', 4', 5')

• Phosphate group(s) (attached to 5' carbon)

• Nitrogenous base (attached to 1' carbon)

Types of DNA Bases

• Pyrimidines: Single ring (thymine, cytosine)

• Purines: Double ring (adenine, guanine)

Polynucleotide Chain Assembly

DNA polymerase catalyzes the formation of phosphodiester bonds between the 3' hydroxyl group of one nucleotide and the 5' phosphate of another, creating a sugar-phosphate backbone.

Double Helix Structure

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

• Antiparallel Strands: One strand runs 5' to 3', the other 3' to 5'.

B-Form DNA

• Most common form in cells

• Diameter: 20 Å

• Base pairs spaced at 3.4 Å intervals

• Major and minor grooves alternate, providing binding sites for proteins

Forms of DNA

DNA can exist in several structural forms:

DNA Replication

Semiconservative and Bidirectional Replication

DNA replication ensures the integrity of genetic information. The process is semiconservative and bidirectional in most organisms.

• Semiconservative: Each daughter duplex contains one parental and one newly synthesized strand.

• Conservative: One duplex contains both parental strands, the other both daughter strands.

• Dispersive: Each duplex contains interspersed parental and daughter segments.

Meselson-Stahl Experiment

This experiment used isotopic labeling and density gradient centrifugation to demonstrate semiconservative replication in E. coli.

• After one replication cycle in 14N medium, DNA was hybrid (14N/15N).

• After two cycles, half was hybrid, half was light (14N/14N).

Origins of Replication

Bacterial DNA

• Replication is bidirectional from a single origin (oriC).

• Replication bubble forms, with forks at each end.

Eukaryotic DNA

• Multiple origins of replication per chromosome.

• Example: Drosophila melanogaster chromosomes.

Replication Origins and Consensus Sequences

• oriC in E. coli: Contains 245 bp, with three 13-mer and four 9-mer repeats.

• Consensus sequences: Conserved among related bacterial species.

Replication Initiation in E. coli

• DnaA: Binds 9-mer sequences, bends DNA, breaks H bonds in 13-mer region.

• DnaB: Helicase, unwinds DNA.

• DnaC: Delivers DnaB to the origin.

• SSB: Single-stranded binding protein, prevents reannealing.

DNA Replication Mechanism

• Primase: Synthesizes RNA primers required for DNA polymerase to initiate synthesis.

• DNA Polymerase III: Main enzyme for strand elongation.

• Replisome: Protein complex at replication fork, contains two pol III enzymes.

Leading and Lagging Strands

• Leading strand: Synthesized continuously in direction of fork movement.

• Lagging strand: Synthesized discontinuously as Okazaki fragments, opposite to fork movement.

RNA Primer Removal and Okazaki Fragment Ligation

• DNA Polymerase I: Removes RNA primers (5'-3' exonuclease) and replaces with DNA (5'-3' polymerase).

• DNA Ligase: Seals gaps between DNA fragments.

DNA Proofreading and Error Correction

Proofreading Activity

• Most DNA polymerases possess 3'-5' exonuclease activity for proofreading.

• Replication errors are rare (about 1 in a billion nucleotides in E. coli).

• Incorrect nucleotides are removed and replaced, ensuring high fidelity.

Supercoiling and Topoisomerases

DNA Supercoiling

• Unwinding during replication causes torsional stress and supercoiling.

• Topoisomerases: Enzymes that relieve supercoiling by controlled cleavage and rejoining of DNA.

Telomeres and Telomerase

Telomere Function and Replication

• Telomeres are repetitive DNA sequences at chromosome ends, important for stability and longevity.

• Telomerase is a ribonucleoprotein that extends telomeres in germ line and certain other cells.

• Telomerase uses its RNA template to add DNA repeats to chromosome ends.

Telomeres, Aging, and Cancer

• Telomere length affects cell longevity and reproductive success.

• Activation of telomerase in somatic cells may promote longevity, but is also associated with cancer cell proliferation.

Summary Table: Key Enzymes and Proteins in DNA Replication

Additional info: These notes expand on the original slides and text, providing definitions, examples, and context for Genetics students. All included images directly reinforce the explanations and are referenced in the relevant paragraphs.