Base pairs, of equal width, occur in the center of the helix while the phosphates occur on the outside surface of the structure. The backbone of the helix is punctuated by major and minor grooves. Chromosomes carry the information required for life under all conditions. Mitochondria and chloroplasts are organelles believed to have arisen by endosymbiosis. They contain DNA, but cannot exist independently.
Viruses are non-cellular genetic elements that enlist specific cells for their own replication.
There are two types - insertion sequences and transposons. Insertion sequences contain only the genetic information conferring mobility and no other genes. Transposons contain additional genes. Transposable elements provide evidence that genetic material is not stable, but instead fluid in nature.