thymine dimers
Summary
Thymine dimers are covalent bonds between two adjacent thymine bases on the same DNA strand, formed by ultraviolet light. They distort the helix and block replication, and in humans they are mainly removed by nucleotide excision repair.
Detail
Meaning: UV light, mainly UV-B and UV-C, makes adjacent pyrimidines on the same strand bond covalently, forming a cyclobutane ring. Thymine pairs are the classic case; cytosine can also take part. The bulky lesion kinks the helix, so DNA and RNA polymerases stall or misread it, and unrepaired damage can lead to mutations.
On the MCAT: A passage describes UV-exposed cells or a repair-defective cell line and asks which pathway fixes the damage. Nucleotide excision repair does: enzymes remove a short stretch of the damaged strand, DNA polymerase refills the gap using the intact strand as template, and DNA ligase seals the nick. Bacteria also have photolyase, which uses visible light to reverse the dimer directly; placental mammals lack it. Defective excision repair, as in xeroderma pigmentosum, causes extreme UV sensitivity and a high skin cancer risk.
Common confusion: A thymine dimer is not a mispair between strands and not a change in hydrogen bonding. It is a covalent bond between neighboring bases on one strand. Mismatch repair corrects replication errors, so it is the wrong choice for a UV lesion.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.