Overview: Life’s Operating Instructions

Discovery of DNA as Genetic Material

DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for life. The discovery of its structure and function was a pivotal moment in biology, providing insight into heredity and cellular function.

• DNA Structure: In 1953, James Watson and Francis Crick described the double helix model of DNA, explaining how genetic information is stored and replicated.

• Inheritance: DNA is inherited from both parents, with half of your DNA coming from each.

• Function: DNA directs the synthesis of proteins, which determine an organism’s traits.

• Replication: Accurate copying of DNA is essential for inheritance and cell division.

Concept 16.1: DNA is the Genetic Material

The Search for the Genetic Material of Heredity

Early experiments sought to identify whether proteins or DNA were the genetic material. Proteins were initially favored due to their complexity, but key experiments shifted the focus to DNA.

• Chromosomes: Composed of DNA and proteins; genes are located on chromosomes.

• Hershey-Chase Experiment: Used bacteriophages (viruses that infect bacteria) to show that DNA, not protein, is the genetic material transmitted to bacteria.

• Griffith’s Experiment: Demonstrated transformation in bacteria, suggesting a "transforming principle" (later identified as DNA).

Griffith’s Experiment: Genetic Information Can Be Transferred

• Procedure: Injected mice with different strains of Streptococcus pneumoniae (smooth, S, and rough, R).

• Findings: Mice injected with heat-killed S strain and live R strain died, indicating that some "transforming principle" from the dead S cells converted R cells into virulent S cells.

• Conclusion: The genetic material responsible for transformation was later identified as DNA.

Avery, McCarty, and MacLeod: DNA is the Transforming Principle

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

• Result: Only extracts treated with DNA-destroying enzymes lost the ability to transform R cells, confirming DNA as the genetic material.

Hershey and Chase: DNA, Not Protein, Enters Bacterial Cells

• Experiment: Used radioactive isotopes to label DNA (with phosphorus-32) and protein (with sulfur-35) in bacteriophages.

• Result: Only DNA entered bacterial cells and directed viral replication, confirming DNA as the genetic material.

Concept 16.2: The Structure of DNA

Watson and Crick’s Double Helix Model

Watson and Crick, using data from Rosalind Franklin’s X-ray diffraction studies, proposed the double helix structure of DNA.

• Double Helix: Two antiparallel strands twisted into a helix.

• Backbone: Sugar-phosphate backbone on the outside; nitrogenous bases on the inside.

• Base Pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Hydrogen Bonds: Hold the base pairs together (A-T: 2 bonds; G-C: 3 bonds).

Chargaff’s Rules

• The amount of A equals T, and the amount of G equals C in DNA from any species.

• Base composition varies between species.

Summary Table: DNA Structure

Concept 16.3: DNA Replication

Models of DNA Replication

• Semiconservative Model: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

• Conservative Model: Parental strands reassociate after replication; new molecule is entirely new.

• Dispersive Model: Each strand is a mix of old and new DNA.

Meselson-Stahl Experiment: Used isotopes of nitrogen to demonstrate that DNA replication is semiconservative.

Mechanism of DNA Replication

• Origin of Replication: Specific sequence where replication begins.

• Replication Fork: Y-shaped region where new DNA strands are synthesized.

• Enzymes Involved:

  • Helicase: Unwinds the DNA double helix.

  • Single-Strand Binding Proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves strain ahead of the replication fork.

  • Primase: Synthesizes RNA primers to initiate synthesis.

  • DNA Polymerase: Adds nucleotides to the growing DNA strand in the 5' to 3' direction.

  • Ligase: Joins Okazaki fragments on the lagging strand.

Leading and Lagging Strands

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, later joined by DNA ligase.

Directionality of DNA Synthesis

• DNA polymerase can only add nucleotides to the 3' end of a pre-existing strand.

• Replication always proceeds in the 5' to 3' direction.

Summary Table: Enzymes in DNA Replication

Concept 16.4: Proofreading and Repair of DNA

Proofreading and Repair Mechanisms

• Proofreading: DNA polymerases check and correct errors during DNA synthesis.

• Mismatch Repair: Enzymes remove and replace incorrectly paired nucleotides.

• Nucleotide Excision Repair: Removes bulky DNA lesions, such as thymine dimers caused by UV light.

Specific Repair Mechanisms

• DNA Photolyase: Repairs UV-induced thymine dimers (in some organisms).

• Nucleotide Excision Repair: Removes and replaces damaged DNA segments.

Telomeres and Telomerase

Role of Telomeres

• Telomeres: Repetitive DNA sequences at the ends of eukaryotic chromosomes that protect genes from erosion during replication.

• Telomerase: Enzyme that extends telomeres in germ cells and some cancer cells, preventing shortening and loss of genetic information.

Summary Table: Telomeres and Cellular Aging

Key Equations and Concepts

• Base Pairing Rule:

• Direction of DNA Synthesis:

• Semiconservative Replication: Each daughter DNA molecule contains one parental and one new strand.

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Tables have been reconstructed to summarize key comparisons and enzyme functions.