11. Which of the following is the most pertinent biologicaleffect of the depletion of ozone layer in stratosphere?
    (1) Denaturation of cellular proteins
    (2) Formation of pyrimidine dimmers
    (3) Increase in the photosynthetic rates in plants
    (4) Cross-linking of deoxyribose sugar of DNA

Depletion of the ozone layer in the stratosphere is a major environmental concern with profound biological implications. The ozone layer acts as Earth’s natural sunscreen, absorbing most of the sun’s harmful ultraviolet-B (UVB) radiation. When this layer is depleted, more UVB reaches the Earth’s surface, affecting all forms of life. Among the potential biological effects—denaturation of cellular proteins, formation of pyrimidine dimers, increased photosynthetic rates in plants, and cross-linking of deoxyribose sugar in DNA—the formation of pyrimidine dimers stands out as the most pertinent and well-documented effect. This article explores why, how UVB damages DNA, and what this means for organisms and ecosystems.

The Role of the Ozone Layer

The stratospheric ozone layer is crucial for protecting life on Earth from the sun’s intense ultraviolet radiation. Ozone molecules absorb the majority of UVB (280–320 nm), preventing it from reaching the surface where it can cause cellular damage. When ozone is depleted—primarily due to human-made chemicals like chlorofluorocarbons (CFCs)—more UVB penetrates the atmosphere, increasing the risk to biological systems.

How Increased UVB Affects Living Organisms

UVB radiation is energetic enough to cause direct damage to biological molecules, especially DNA, proteins, and lipids. The most significant and well-studied biological effect is the formation of pyrimidine dimers in DNA. This process occurs when adjacent pyrimidine bases (thymine or cytosine) in a DNA strand absorb UVB energy, forming covalent bonds between them. This creates a structural distortion in the DNA molecule, which can interfere with replication and transcription, leading to mutations, cell death, or cancer.

Evaluating the Options

Let’s examine each of the given options to determine which is the most pertinent biological effect of ozone layer depletion:

1. Denaturation of Cellular Proteins

While UVB can damage proteins by causing their denaturation (loss of structure and function), this is not the primary or most pertinent effect. Proteins are generally more resistant to UVB than DNA, and most cellular damage from UVB is mediated through DNA rather than protein denaturation.

2. Formation of Pyrimidine Dimers

This is the correct and most pertinent biological effect. UVB radiation is absorbed by DNA, causing adjacent pyrimidine bases (thymine or cytosine) to form covalent bonds, creating pyrimidine dimers. These dimers disrupt the DNA helix, block DNA replication and transcription, and are a major cause of UV-induced mutations and skin cancers. The formation of pyrimidine dimers is the most direct and significant biological consequence of increased UVB exposure due to ozone depletion.

3. Increase in the Photosynthetic Rates in Plants

Increased UVB exposure typically has the opposite effect: it inhibits photosynthesis and reduces plant growth. UVB damages chloroplasts, reduces chlorophyll content, and impairs photosynthetic efficiency. Some plants may develop protective mechanisms, but overall, photosynthetic rates do not increase due to ozone depletion.

4. Cross-linking of Deoxyribose Sugar of DNA

UVB radiation can cause various types of DNA damage, but cross-linking of the deoxyribose sugar is not a common or well-documented effect. The main type of UV-induced DNA damage is the formation of pyrimidine dimers.

Why Pyrimidine Dimers Matter Most

Pyrimidine dimer formation is the most pertinent biological effect for several reasons:

• Direct DNA Damage: UVB photons are absorbed directly by DNA, leading to the formation of cyclobutane pyrimidine dimers and (6-4) photoproducts.

• Mutation and Cancer Risk: Unrepaired pyrimidine dimers can cause mutations during DNA replication, increasing the risk of skin cancer and other genetic disorders.

• Widespread Impact: All organisms exposed to sunlight, including humans, animals, and plants, are vulnerable to UVB-induced DNA damage.

• Well-Documented Mechanism: The link between UVB, pyrimidine dimer formation, and increased cancer risk is supported by extensive laboratory and epidemiological evidence.

Additional Biological Effects of Ozone Depletion

While pyrimidine dimer formation is the most pertinent effect, ozone depletion also leads to other biological consequences:

• Skin Cancer: Increased UVB exposure is directly linked to higher rates of non-melanoma and malignant melanoma skin cancers.

• Eye Damage: UVB can cause cataracts and other eye disorders by damaging lens proteins and retinal cells.

• Immune Suppression: UVB exposure can suppress the immune system, reducing the body’s ability to fight infections and cancer.

• Effects on Plants and Ecosystems: UVB impairs plant growth, reduces crop yields, and disrupts aquatic ecosystems by damaging phytoplankton and other microorganisms.

The Science Behind Pyrimidine Dimer Formation

When DNA absorbs UVB photons, the energy can cause adjacent pyrimidine bases to form covalent bonds, creating cyclobutane pyrimidine dimers or (6-4) photoproducts. These lesions distort the DNA helix, blocking DNA polymerases during replication and transcription. If not repaired, these distortions can lead to mutations, cell death, or uncontrolled cell growth (cancer).

Cells have repair mechanisms, such as nucleotide excision repair, to fix pyrimidine dimers. However, excessive or chronic UVB exposure can overwhelm these systems, leading to permanent genetic damage.

Summary Table

Key Takeaways

• The most pertinent biological effect of ozone layer depletion is the formation of pyrimidine dimers in DNA due to increased UVB exposure.

• Pyrimidine dimers disrupt DNA structure, leading to mutations and increased cancer risk.

• Other effects, such as protein denaturation or cross-linking of deoxyribose, are less significant or not well documented.

• Increased UVB also causes skin cancer, cataracts, immune suppression, and reduced plant growth.

Conclusion

Depletion of the ozone layer leads to increased UVB radiation at the Earth’s surface, which causes the formation of pyrimidine dimers in DNA—the most pertinent and well-documented biological effect. This DNA damage underlies the increased risk of skin cancer and other health problems associated with ozone depletion. Protecting the ozone layer remains essential for safeguarding human health and the integrity of ecosystems worldwide.