Reactive Oxygen Species and DNA damage are closely linked processes in cellular biology. ROS such as superoxide, hydrogen peroxide, and hydroxyl radicals can damage DNA through oxidative stress, leading to mutations and genomic instability.
Reactive Oxygen Species (ROS) are highly unstable, chemically reactive molecules that contain oxygen. They are produced normally inside cells as by-products of aerobic respiration and also by exposure to radiation, heavy metals, and toxins. The three main ROS are:
- Hydrogen peroxide (H₂O₂)
- Superoxide anion (O₂•⁻)
- Hydroxyl radical (•OH)
Among these, the hydroxyl radical (•OH) is the most reactive and most damaging to DNA.
The Fenton Reaction
H₂O₂ alone is relatively stable and does not directly damage DNA efficiently. However, in the presence of transition metal ions such as Fe²⁺ (ferrous iron) or Cu²⁺ (cupric copper), it undergoes the Fenton reaction to generate the highly destructive hydroxyl radical:
Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻
The •OH produced in this reaction is an extremely reactive free radical. It has a very short half-life but attacks any biological molecule in its immediate vicinity — and DNA is a primary target.
Three Types of DNA Damage Caused by ROS
1. Double-Strand Breaks (DSBs)
The •OH radical attacks the phosphodiester backbone of DNA by abstracting a hydrogen atom from the deoxyribose sugar. If two such attacks occur on opposite strands at nearby positions, the result is a double-strand break (DSB). DSBs are the most dangerous form of DNA damage because both strands are severed simultaneously, making faithful repair difficult. Unrepaired DSBs lead to chromosomal deletions, translocations, and cell death.
2. Single-Strand Breaks (SSBs)
When •OH attacks the backbone on only one strand, it causes a single-strand break (SSB). SSBs are more common than DSBs but less lethal individually. However, if two SSBs occur close together on opposite strands, they effectively become a DSB.
Repair of SSBs: The enzyme PARP (Poly ADP-Ribose Polymerase) rapidly detects SSBs and binds to them. PARP recruits the base excision repair (BER) machinery to restore the broken strand using the intact complementary strand as a template.
3. Oxidized Bases
•OH also attacks the purine and pyrimidine bases directly, chemically modifying them. This produces oxidized bases that are highly mutagenic.
Most Common Oxidized Base: 8-Oxoguanine (8-oxoG)
The most frequently formed and most studied oxidized base is 8-oxoguanine (8-oxoG), produced by oxidation of guanine at the C-8 position.
Why is 8-oxoG dangerous?
Normal guanine pairs with cytosine (G:C). However, 8-oxoG adopts a syn conformation and mispairs with adenine (instead of cytosine) during DNA replication.
This leads to the following mutational consequence:
G:C → T:A transversion (a purine:pyrimidine to pyrimidine:purine switch)
This is a transversion mutation — the most common mutation caused by oxidative DNA damage.
Another oxidized base: Thymine glycol is formed by oxidation of thymine. It blocks DNA polymerase and stalls replication, but is less mutagenic than 8-oxoG.
Summary Table
| ROS Type | Primary DNA Damage | Mutation Type |
|---|---|---|
| •OH (from Fenton) | DSBs, SSBs | Chromosomal breaks |
| •OH on guanine | 8-oxoguanine | G:C → T:A transversion |
| •OH on thymine | Thymine glycol | Replication block |
Conclusion
ROS, especially the hydroxyl radical generated via the Fenton reaction, cause three major types of DNA damage: DSBs, SSBs, and oxidized bases. The most important oxidized lesion is 8-oxoguanine, which produces G:C to T:A transversion mutations. Cells repair SSBs through the PARP-BER pathway and DSBs through HR or NHEJ. Failure to repair ROS-induced DNA damage leads to mutations, cancer, and premature ageing.
Reactive Oxygen Species and DNA damage are critical in understanding mutation and disease development. The hydroxyl radical generated via the Fenton reaction causes severe DNA damage, including strand breaks and base modifications like 8-oxoguanine, leading to transversion mutations.
📌 Exam Key Points: Fenton reaction formula must be written exactly. 8-oxoG → mispairs with A → transversion (not transition). PARP → SSB repair. DSBs → most dangerous. Thymine glycol = second oxidized base to name.
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