#18 is incorrect, the picture depicts a DNA molecule and represents both Adenine and Uracil nucleases. Uracil is only found in RNA molecules, in the DNA, Uracil is replaced by Thymine. Let’s push that a little further… Uracil is found in RNA, which, contrarily to DNA, isn’t a double helix, it’s single. In addition to that, when the bases of RNA connect to other homologous bases, it’s only for a short period of time (when synthesizing proteins for example). In the mentioned context, what seems like a cute, romantic message, would in fact mean that both lovers would’t be able to pursue a long term relationship, ever. Sorry for science.