Which sequence is the best target for damage by UV radiation: AGGCAA, CTTTGA, or GUAAAU? Why aren’t all bacteria killed when they are exposed to sunlight?

Verified step by step guidance

1. Step 1: Understand that UV radiation primarily causes damage by inducing the formation of thymine dimers in DNA, which are covalent linkages between adjacent thymine bases. This disrupts DNA replication and transcription.
2. Step 2: Analyze each sequence to identify which contains the most adjacent thymine (T) bases, as these are the primary targets for UV-induced damage. The sequences are: AGGCAA, CTTTGA, and GUAAAU (note that GUAAAU is RNA, not DNA).
3. Step 3: Recognize that the sequence CTTTGA contains three consecutive thymine bases (TTT), making it the most susceptible to UV damage due to the higher likelihood of thymine dimer formation.
4. Step 4: Understand that not all bacteria are killed by sunlight because many have DNA repair mechanisms, such as photoreactivation and nucleotide excision repair, which can reverse or remove UV-induced damage.
5. Step 5: Additionally, factors like bacterial pigmentation, biofilm formation, and the presence of protective enzymes can reduce UV damage, allowing some bacteria to survive sunlight exposure.

Key Concepts

UV Radiation and DNA Damage

Ultraviolet (UV) radiation primarily damages DNA by causing the formation of thymine dimers, which are covalent bonds between adjacent thymine bases. Sequences rich in thymine or adjacent pyrimidines are more susceptible to UV-induced damage, disrupting DNA replication and transcription.

Nucleic Acid Composition and Susceptibility

DNA sequences containing thymine (T) are more vulnerable to UV damage compared to RNA sequences or DNA sequences lacking thymine. For example, the sequence CTTTGA has multiple thymine bases, making it a better target for UV-induced thymine dimer formation than AGGCAA (which has fewer thymines) or GUAAAU (which is RNA and contains uracil instead).

Bacterial DNA Repair Mechanisms and UV Resistance

Not all bacteria are killed by sunlight because many possess DNA repair systems, such as photoreactivation and nucleotide excision repair, which can recognize and fix UV-induced DNA damage. These mechanisms allow bacteria to survive and recover from UV exposure, contributing to their resilience in sunlight.