DNA and the Gene: Synthesis and Repair – Study Notes

DNA and the Gene: Synthesis and Repair

Testing Early Hypotheses about DNA Replication

Understanding how DNA replicates was a foundational question in molecular biology. Three main hypotheses were proposed:

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

• Conservative replication: The parental DNA molecule remains intact, and an entirely new molecule is synthesized.

• Dispersive replication: Parental DNA is fragmented, and each daughter molecule contains interspersed segments of old and new DNA.

The Meselson–Stahl experiment provided evidence supporting the semiconservative model by using isotopic labeling and density gradient centrifugation to distinguish old and new DNA strands.

A Model for DNA Synthesis

DNA synthesis is catalyzed by the enzyme DNA polymerase, which adds nucleotides to the 3′ end of a growing DNA strand. DNA polymerases can only synthesize DNA in the 5′ → 3′ direction. The building blocks for DNA synthesis are deoxyribonucleoside triphosphates (dNTPs), which have high potential energy due to their three phosphate groups. This energy drives the formation of phosphodiester bonds between nucleotides.

• Directionality: DNA polymerase adds nucleotides only to the 3′ end, so synthesis always proceeds 5′ → 3′.

• Energy: Hydrolysis of dNTPs releases energy, making the reaction exergonic.

Origin and Direction of DNA Replication

DNA replication begins at specific locations called origins of replication. In bacteria, there is a single origin per chromosome, while eukaryotic chromosomes have multiple origins. Replication proceeds bidirectionally, forming replication bubbles with two replication forks moving in opposite directions.

• Bacterial chromosomes: One origin, one replication bubble.

• Eukaryotic chromosomes: Multiple origins, multiple replication bubbles.

Opening and Stabilizing the DNA Helix

Several proteins are required to open and stabilize the DNA double helix during replication:

• DNA helicase: Unwinds the double helix by breaking hydrogen bonds between strands.

• Single-strand DNA-binding proteins (SSBPs): Bind to separated strands to prevent them from re-annealing.

• Topoisomerase: Relieves tension ahead of the replication fork by cutting and rejoining DNA strands.

Synthesis of the Leading Strand

The leading strand is synthesized continuously toward the replication fork. DNA polymerase cannot initiate synthesis de novo; it requires a short RNA primer synthesized by primase (an RNA polymerase). DNA polymerase then extends the primer by adding dNTPs to the 3′ end.

• Primer: Short RNA sequence base-paired to the template strand.

• Continuous synthesis: Leading strand is synthesized in one continuous piece.

Synthesis of the Lagging Strand

The lagging strand is synthesized discontinuously, away from the replication fork, as short fragments called Okazaki fragments. Each fragment begins with an RNA primer. DNA polymerase synthesizes DNA until it reaches the next primer, and the fragments are later joined by DNA ligase.

• Discontinuous replication: Multiple primers are required; synthesis occurs in short segments.

• Okazaki fragments: Short DNA fragments synthesized on the lagging strand.

• DNA ligase: Joins Okazaki fragments into a continuous strand.

Proteins Required for DNA Synthesis in Bacteria

DNA replication involves a coordinated set of proteins, each with a specific function:

Protein	Structure	Function
Helicase	Enzyme	Separates DNA strands
SSBPs	Protein	Stabilizes single-stranded DNA
Topoisomerase	Enzyme	Relieves twisting forces
Primase	Enzyme	Synthesizes RNA primer
DNA polymerase III	Enzyme	Extends leading/lagging strand
Sliding clamp	Protein	Holds DNA polymerase in place
DNA polymerase I	Enzyme	Removes RNA primer, replaces with DNA
DNA ligase	Enzyme	Joins Okazaki fragments

The Replisome: DNA-Synthesizing Machine

The replisome is a large, dynamic complex of enzymes and proteins responsible for DNA synthesis at the replication fork. It coordinates the activities of all proteins involved in replication, ensuring efficient and accurate DNA synthesis.

Replicating the Ends of Linear Chromosomes

Replication of linear eukaryotic chromosomes presents a unique challenge at the ends, known as telomeres. The lagging strand cannot be fully replicated, leading to progressive shortening of chromosomes with each cell division. Telomeres consist of repetitive, non-coding DNA sequences.

• End replication problem: After removal of the final RNA primer, DNA polymerase cannot fill the gap, resulting in chromosome shortening.

Telomerase and Telomere Replication

Telomerase is an enzyme that extends telomeres by adding repetitive DNA sequences, using an RNA template carried within the enzyme. This prevents chromosome shortening in cells where telomerase is active (e.g., gametes, stem cells).

• Mechanism: Telomerase binds to the overhang, extends it, and allows normal DNA synthesis to complete the lagging strand.

• Biological significance: Most somatic cells lack telomerase, leading to limited cell divisions. Cancer cells often reactivate telomerase, enabling unlimited division.

Correcting Mistakes in DNA Synthesis

DNA polymerase has high fidelity but occasionally incorporates incorrect bases. Several mechanisms ensure accuracy:

• Proofreading: DNA polymerase detects and removes mismatched nucleotides using its exonuclease activity.

• Mismatch repair: Specialized enzymes recognize and repair mismatches missed during replication.

Repairing Damaged DNA

DNA is constantly exposed to damaging agents such as UV light, X-rays, and chemicals. One common form of damage is the formation of thymine dimers due to UV exposure, which creates kinks in the DNA and blocks replication.

Nucleotide Excision Repair

The nucleotide excision repair system recognizes and removes damaged DNA segments. The process involves:

1. Detection of DNA damage (e.g., thymine dimers).

2. Removal of the damaged DNA segment.

3. Filling in the gap using the undamaged strand as a template.

4. Sealing the new DNA with DNA ligase.

Summary Table: Key Enzymes and Functions in DNA Replication and Repair

Enzyme/Protein	Function
DNA polymerase	Synthesizes new DNA, proofreads
Primase	Synthesizes RNA primers
Helicase	Unwinds DNA helix
SSBPs	Stabilize single-stranded DNA
Topoisomerase	Relieves supercoiling
DNA ligase	Joins DNA fragments
Telomerase	Extends telomeres
Exonuclease	Removes mismatched bases
Nucleotide excision repair complex	Removes and replaces damaged DNA

Key Equations:

• Phosphodiester bond formation (generalized): $\text{(dNMP)}_n + \text{dNTP} \rightarrow \text{(dNMP)}_{n+1} + \text{PP}_i$