Complex sentence: In both DNA and RNA one of the pyrimdines is cytosine (C), but the second major pyrimidine is not the same in both: it is thymine (T) in DNA and uracil (U) in RNA.
Split into simpler sentences: In both DNA and RNA, one of the pyrimdines is cytosine (C). However, the second major pyrimidine is not the same in both: it is thymine (T) in DNA and uracil (U) in RNA.