In the DNA molecule, four types of nitrogenous bases are found: cytosine (C), and guanine (G), adenine (A) and thymine (T). In DNA the bases pair up with each other in the following ways: A pairs with T by two hydrogen bonds, while C pairs with G by three hydrogen bonds. In RNA (i.e., transcript sequence), T is replaced by uracil (U). Moreover, the RNA nucleotide sequence is read in the direction 5' to 3'.
Fossils can only be formed when mud, silt or tar is in the area to preserve the fossil, If erosion does not occur and allows the fossil to form, and Oxygen and microorganisms cannot decay the bones.
