Sentence:  For example, tobacco cell cultures grown at 5oC, resulted in a loss of viability relative to control cells and exhibited DNA <WSD>laddering</WSD> (Koukalova et al., 1997). 
Meaning of the word marked with <WSD> tags:  A regular pattern of oligonucleotide sizes on electrophoretic gels, resulting from the fragmentation of DNA strands during cell death.