Genetically modified tomato. Conceptual computer artwork of a tomato in front of a computer screen and a helix of DNA (deoxyribonucleic acid). This represents the genetic modification of food crops. Genetically modified (GM) tomatoes are available in some countries. Genetic modification is where an organism's DNA is altered to create inherited changes in that organism. The changes can include improved shelf-life and resistance to disease. The structure of DNA is a double helix held together by cross-linking struts called nucleotide bases. The sequence of these bases is the genetic code that determines inherited characteristics.