DNA Structure and Replication: Study Notes

DNA Structure and Genetic Material

What Are Genes Mad Of?

Early 20th-century scientists debated whether genes were composed of proteins or DNA. The Hershey-Chase experiment provided evidence that DNA, not protein, is the genetic material.

• Genes: Segments of DNA that encode instructions for building proteins.

• Hershey-Chase Experiment: Used bacteriophage T2 and Escherichia coli to show that DNA enters the bacterial cell and directs viral replication.

• Bacteriophages (Phages): Viruses that infect bacteria, composed of DNA enclosed in a protein coat.

• Key Result: Only DNA, not protein, entered the host cell, proving DNA is the genetic material.

DNA Structure

Watson and Crick built models to confirm the double helix structure of DNA, revealing its molecular organization.

• Double Helix: Two sugar-phosphate backbones with nitrogenous bases paired in the interior.

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

• Nucleotide Alphabet: DNA is composed of four bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

DNA Replication

Models of DNA Replication

Three models were proposed to explain how DNA replicates:

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

• Conservative Model: One molecule contains both parental strands, and the other contains two new daughter strands.

• Dispersive Model: New DNA strands are mixtures of parental and newly synthesized segments.

Table: DNA Replication Models

Model	Description
Semiconservative	Each daughter DNA has one parental and one new strand
Conservative	One DNA is all parental, one is all new
Dispersive	Each strand is a mix of old and new DNA segments

Meselson-Stahl Experiment

This experiment confirmed the semiconservative model using isotopes of nitrogen.

• Method: Grew E. coli in heavy nitrogen (15N), then switched to light nitrogen (14N), and analyzed DNA density after replication.

• Result: DNA molecules after replication contained one heavy and one light strand, supporting the semiconservative model.

Molecular Mechanism of DNA Replication

Initiation and Enzymes Involved

DNA replication begins at specific sites called origins of replication, forming replication bubbles and forks.

• Origin of Replication: Site where DNA strands separate and replication begins.

• Replication Fork: Y-shaped region where parental DNA is unwound.

• Bacteria: Typically have a single origin of replication.

• Eukaryotes: Have multiple origins per chromosome.

Key Enzymes and Proteins

• Helicase: Unwinds the DNA double helix at the replication fork.

• Single-Strand Binding Proteins (SSBP): Stabilize unwound DNA strands.

• Topoisomerase: Relieves strain caused by unwinding.

• Primase: Synthesizes short RNA primers needed to start DNA synthesis.

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

Leading and Lagging Strands

DNA polymerase can only synthesize DNA in the 5' to 3' direction, resulting in continuous and discontinuous synthesis.

• Leading Strand: Synthesized continuously toward the replication fork.

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

• Okazaki Fragments: Short DNA segments on the lagging strand, each initiated by an RNA primer.

• DNA Ligase: Joins Okazaki fragments to form a continuous strand.

Table: Leading vs. Lagging Strand

Strand	Synthesis	Direction	Features
Leading	Continuous	5' to 3'	Single primer
Lagging	Discontinuous	5' to 3'	Multiple primers, Okazaki fragments

Basic Rules of Replication

• Semi-conservative: Each new DNA has one old and one new strand.

• Starts at the origin: Replication begins at specific DNA sequences.

• Synthesis direction: Always 5' to 3'.

• RNA primers: Required to initiate synthesis.

Accuracy and Repair Mechanisms

Proofreading and DNA Repair

DNA replication is highly accurate due to several mechanisms:

• Hydrogen Bonding: Correct base pairs (A-T, G-C) are more stable.

• DNA Polymerase Proofreading: Removes mismatched nucleotides.

• Nucleotide Excision Repair: Damaged DNA is cut out and replaced by nucleases and repair enzymes.

Telomeres and Chromosome Ends

Telomere Structure and Function

Telomeres are repetitive nucleotide sequences at the ends of eukaryotic chromosomes, protecting them from degradation.

• Telomeres: Series of short, G-rich repeats at chromosome ends.

• 3' Overhang: The very end of the lagging strand lacks a complementary sequence.

• Telomerase: Enzyme that extends telomeres, preventing chromosome shortening.

• Cellular Aging: Telomere shortening is associated with aging and cell senescence.

• Cancer: Most cancer cells have high telomerase activity, allowing unlimited division.

Table: Telomere Features

Feature	Description
Sequence	G-rich repeats
Function	Protect chromosome ends
Enzyme	Telomerase
Role in Aging	Shortening leads to senescence
Role in Cancer	High telomerase in most cancers

Additional info: These notes expand on the original slides and text, providing definitions, tables, and context for key processes in DNA structure and replication, suitable for General Biology students.