Lec 14: telomere replication and dna repair processes. Telomere: a series of short repetitive dna sequences located at the ends of eukaryotic chromosomes. Telomerase: an enzyme that adds nucleotides to telomeres, especially in cancer cells. Endo & exonuclease: (endo) an enzyme that cleaves a polynucleotide chain by separating nucleotides other than the two end ones. (exo) an enzyme that removes successive nucleotides from the end of a polynucleotide molecule. Pyrimidine dimer: pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in. Ultraviolet light induces the formation of covalent linkages by reactions localized on the c=c double bonds. In dsrna (double-stranded rna), uracil dimers may also accumulate as a result of uv radiation. Glycosylase: dna glycosylases are a family of enzymes involved in base excision repair, classi ed under. Base excision repair is the mechanism by which damaged bases in dna are removed and replaced. Dna glycosylases catalyze the rst step of this process.