Molecular Basis of Mutations in Relation to UV Light and Chemical Mutagens

Introduction

Mutations are heritable changes in the genetic material (DNA or RNA) that can modify gene structure and function. These alterations can occur spontaneously during replication or be induced by external factors known as mutagens. Two major categories of mutagens are: (1) Physical mutagens (e.g., ultraviolet radiation, X-rays) and (2) Chemical mutagens (e.g., base analogues, alkylating agents, intercalating compounds). Mutations play a dual role in biology — they are a source of genetic diversity essential for evolution, yet a cause of genetic diseases and cancer when unregulated.

1. UV Light–Induced Mutations

Nature of UV Radiation

Ultraviolet (UV) light is a form of non-ionizing radiation present in sunlight. It is divided into: UV-A (320–400 nm), UV-B (280–320 nm), and UV-C (100–280 nm). UV-B is biologically active and mutagenic, while UV-C is absorbed by the ozone layer.

Mechanism of DNA Damage

UV light induces the formation of pyrimidine dimers, mainly thymine dimers, between adjacent pyrimidine bases on the same DNA strand. These covalent linkages distort the DNA double helix, block replication and transcription, and lead to base substitutions or frameshift mutations.

Biological Consequences

Replication errors, mutation accumulation, cell cycle arrest, apoptosis, and skin cancers such as melanoma can result from UV exposure.

DNA Repair Mechanisms

DNA Repair Mechanisms

DNA is constantly exposed to damaging agents such as UV light, radiation, and chemical mutagens. To maintain genetic stability, cells have evolved repair mechanisms that detect and correct these damages before they become permanent mutations. The three key repair mechanisms are Photoreactivation, Nucleotide Excision Repair (NER), and SOS Repair.

1. Photoreactivation (Light-Dependent Repair)

Definition: Photoreactivation is a direct DNA repair mechanism that specifically corrects thymine dimers formed due to UV radiation.

Enzyme Involved: Photolyase enzyme

Mechanism:
1. UV light induces the formation of pyrimidine dimers between adjacent thymine bases on the same DNA strand.
2. The enzyme photolyase binds to the thymine dimer in the dark.
3. When the enzyme absorbs visible light energy (blue or near-UV light), it becomes activated and breaks the abnormal covalent bonds between the thymine bases.
4. The DNA returns to its normal helical structure without removing or replacing any bases.

Occurrence: Common in bacteria, fungi, and some animals such as amphibians and fish, but absent in placental mammals including humans.

Importance: Provides a simple, quick, and energy-efficient way to correct UV-induced DNA damage.

2. Nucleotide Excision Repair (NER)

Definition: NER is a universal DNA repair mechanism that removes damaged or distorted DNA segments and replaces them with newly synthesized DNA.

Steps in NER:
1. Damage recognition by proteins (UvrA, UvrB in bacteria; XPC in humans).
2. DNA around the lesion is unwound by helicase enzymes.
3. The damaged section (12–30 nucleotides) is cut out by endonucleases.
4. DNA polymerase fills the gap using the complementary strand as a template.
5. DNA ligase seals the new segment into the strand.

Occurrence: Found in all organisms, including humans.

Importance: Prevents mutations and maintains DNA integrity. Defects in NER cause genetic disorders like xeroderma pigmentosum (XP), leading to extreme UV sensitivity and skin cancer.

3. SOS Repair (Error-Prone Repair in Bacteria)

Definition: SOS repair is an emergency DNA repair mechanism in bacteria that allows replication to continue despite severe DNA damage.

Mechanism:
1. Extensive DNA damage causes single-stranded DNA regions to form.
2. RecA protein binds to these regions and becomes activated.
3. Activated RecA promotes cleavage of the LexA repressor, which normally suppresses SOS genes.
4. Once LexA is inactivated, over 40 SOS genes are expressed, including error-prone DNA polymerases (Pol IV and Pol V).
5. These polymerases replicate across damaged DNA but often insert incorrect bases, leading to mutations.

Importance: Allows cell survival under extreme damage conditions, although it is error-prone and mutagenic.

2. Chemical Mutagen–Induced Mutations

Chemical mutagens interact with DNA to modify bases, alter pairing properties, or distort the double helix.

a. Base Analogues

Molecules similar to normal DNA bases but with abnormal base-pairing. Example: 5-Bromouracil (5-BU) mimics thymine but can pair with guanine, leading to transition mutations.

b. Alkylating Agents

Compounds like ethyl methanesulfonate (EMS) and mustard gas add alkyl groups to DNA bases, causing base mispairing or strand breaks.

c. Intercalating Agents

Flat molecules such as ethidium bromide and acridine dyes insert between base pairs, causing insertions or deletions (frameshift mutations).

d. Deaminating Agents

Nitrous acid removes amino groups from bases. Example: Cytosine deamination produces uracil, which pairs with adenine.

e. Reactive Oxygen Species (ROS)

Generated during metabolism or radiation, ROS modify bases (e.g., 8-oxo-guanine), cause strand breaks, and produce mutagenic lesions.

3. Significance of Studying Mutations

Understanding mutation mechanisms has vast applications in genetics, toxicology, cancer biology, pharmacology, and evolutionary biology. It helps design DNA-protective agents, antioxidants, and therapies for repair-deficient disorders.

FAQs (Frequently Asked Questions)

Q: What is the most common DNA lesion caused by UV light?
A: Formation of thymine dimers.

Q: Can all UV-induced DNA damage be repaired?
A: Most can be repaired by photoreactivation or NER, but excessive damage can overwhelm repair systems.

Q: Why are base analogues mutagenic?
A: They mimic natural bases but form abnormal base pairs, causing point mutations.

Q: How do intercalating agents induce frameshift mutations?
A: By inserting between DNA bases, they cause addition or loss of nucleotides.

Q: What disease results from defective nucleotide excision repair?
A: Xeroderma pigmentosum, leading to UV sensitivity and skin cancer.

MCQs (Multiple Choice Questions)

1. UV light mainly induces which DNA lesion?

A. DepurinationB. Thymine dimer formationC. Base substitutionD. DNA methylation
Answer: Thymine dimer formation

2. Which enzyme is responsible for photoreactivation?

A. DNA polymerase
B. Photolyase
C. Ligase
D. Helicase
Answer: Photolyase

3. 5-Bromouracil is an example of a:

A. Base analogue
B. Alkylating agent
C. Deaminating agent
D. Intercalating dye

Answer: Base analogue

4. Ethidium bromide causes:

A. Base substitution
B. Frameshift mutation
C. Depurination
D. Oxidative damage

Answer: Frameshift mutation

5. Cytosine deamination produces:

A. Thymine
B. Uracil
C. Adenine
D. Guanine

Answer: Uracil

Worksheet (Student Practice Section)

A. Define the following:
1. Mutation
2. Base analogue
3. Thymine dimer
4. Photoreactivation

B. Differentiate between:
1. Transition and Transversion mutations
2. Physical and Chemical mutagens
3. Alkylating and Intercalating agents

C. Short Answer Questions:
1. Explain the molecular mechanism of UV-induced thymine dimer formation.
2. Write the role of reactive oxygen species in DNA damage.
3. Describe two DNA repair mechanisms with examples.

D. Diagram Practice:
Draw a labeled diagram showing thymine dimer formation and its repair by photolyase.

References

1. Lodish, H., Berk, A., & Zipursky, S. L. (2016). Molecular Cell Biology. 8th Edition. W. H. Freeman and Company.

2. Griffiths, A. J. F. et al. (2020). Introduction to Genetic Analysis. 12th Edition. W. H. Freeman.

3. Alberts, B. et al. (2022). Molecular Biology of the Cell. 7th Edition. Garland Science.

4. Friedberg, E. C. (2003). DNA Repair and Mutagenesis. ASM Press.

5. National Cancer Institute (NCI). Mutations and Cancer. www.cancer.gov