A modified or isolated composition comprising a gel. The gel includes a plurality of first polymers, each of said first polymers having first functional groups comprising a first nucleic acid sequence attached as side-chains thereto. The gel includes a plurality of second polymers, each of said second polymers having second functional groups comprising a second nucleic acid sequence attached as side-chains thereto, said first and said second functional groups forming a reversible cross-link between said first and said second polymers, wherein said cross-link comprises a plurality of hydrogen bonds between complementary base portions of said first and said second nucleic acid sequences.
