DNA Structure and Replication

Discovery of DNA as Genetic Material

The identification of DNA as the hereditary material was a pivotal moment in biology. Early experiments distinguished DNA from proteins as the carrier of genetic information.

• Griffith-Avery Experiment: Demonstrated transformation in bacteria, suggesting a 'transforming principle' (later identified as DNA).

• Hershey-Chase Experiment: Used bacteriophages to show that DNA, not protein, enters bacterial cells and directs viral replication.

• Watson and Crick: Deduced the double helix structure of DNA, integrating chemical and physical data.

Example: The Hershey-Chase experiment used radioactive isotopes to label DNA and protein, confirming DNA as the genetic material.

DNA Structure

DNA is a double-stranded helical molecule composed of nucleotides. Its structure is essential for its function as genetic material.

• Nucleotide: Consists of a phosphate group, deoxyribose sugar, and a nitrogenous base (A, T, G, C).

• Double Helix: Two antiparallel strands held together by hydrogen bonds between complementary bases.

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

• Major and Minor Grooves: The helical structure creates grooves that are important for protein binding.

Example: Chargaff's rules state that the amount of A equals T, and G equals C in DNA from any species.

Key Features Defining DNA Structure

• Double-stranded helix

• Antiparallel orientation (5' to 3' and 3' to 5')

• Right-handed helix

• Major and minor grooves

DNA Packaging in Chromosomes

To fit within the nucleus, DNA is highly compacted with proteins into chromatin and chromosomes.

• Nucleosome: Basic unit of DNA packaging, consisting of DNA wrapped around histone proteins.

• Chromatin: The complex of DNA and proteins that forms chromosomes.

• Levels of Compaction: DNA wraps around histones, forming nucleosomes ('beads on a string'), which further coil and fold into higher-order structures.

DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It ensures genetic continuity between generations of cells.

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

• Origin of Replication (ORI): Specific sequence where replication begins.

• Replication Fork: The Y-shaped region where the DNA is split into two strands for copying.

Steps of DNA Replication

1. Unwinding the DNA: Helicase separates the two DNA strands at the ORI, creating a replication bubble.

2. Stabilization: Single-stranded binding proteins (SSBP) prevent re-annealing of the strands.

3. Relieving Tension: Topoisomerase prevents supercoiling ahead of the replication fork.

4. Primer Synthesis: Primase synthesizes a short RNA primer to provide a starting point for DNA polymerase.

5. DNA Synthesis: DNA polymerase adds nucleotides to the 3' end of the primer, synthesizing the new strand in a 5' to 3' direction.

6. Leading and Lagging Strands: The leading strand is synthesized continuously, while the lagging strand is synthesized in short Okazaki fragments.

7. Primer Removal and Ligation: DNA polymerase I removes RNA primers and replaces them with DNA; DNA ligase seals the gaps between fragments.

Leading vs. Lagging Strand Synthesis

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork in Okazaki fragments.

• Okazaki Fragments: Short DNA segments on the lagging strand, later joined by DNA ligase.

Limitations and Solutions in DNA Replication

• DNA Polymerase Limitations: Cannot initiate synthesis without a primer; can only add nucleotides to the 3' end.

• End Replication Problem: Linear chromosomes shorten with each replication due to incomplete synthesis of 5' ends.

• Telomeres: Repetitive DNA sequences at chromosome ends that protect coding regions.

• Telomerase: Enzyme that extends telomeres in germ cells and some stem cells.

Proofreading and DNA Repair

High fidelity in DNA replication is achieved through proofreading and repair mechanisms.

• Proofreading: DNA polymerase checks and corrects errors during synthesis.

• Mismatch Repair: Enzymes correct errors missed by DNA polymerase.

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

Key Terms and Enzymes

• Helicase: Unwinds the DNA double helix.

• Single-strand binding protein (SSBP): Stabilizes unwound DNA.

• Primase: Synthesizes RNA primers.

• DNA Polymerase: Synthesizes new DNA strands.

• Ligase: Joins Okazaki fragments.

• Topoisomerase: Relieves supercoiling.

• Telomerase: Extends telomeres.

Summary Table: Leading vs. Lagging Strand

Additional info:

• Mutations arising from replication errors are a source of genetic variation, which is essential for evolution by natural selection.

• Shortening of telomeres is associated with cellular aging and cancer biology.