They confer a negative charge. Adenine pairs with thymine via two hydrogen bonds while three hydrogen bonds occur between cytosine and guanine. Ten base pairs exist per helical turn spanning 34 Angstroms. 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.