Genetic information—DNA—is composed of two long chains of nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogen-containing base. Hydrogen bonding, which occurs between specific bases known as base pairs, links the two chains together in the famous double helix shape.
In a living person, DNA strands are millions of base pairs long. Degraded over time, the DNA found in ancient remains is badly fragmented, sometimes to less than 100 base pairs long.
Not only is ancient DNA fragmented, it may also be contaminated—that is, mixed with DNA from other organisms. Most of the DNA found in an ancient bone will actually belong to bacteria living in the soil. Touching remains with ungloved fingers can easily contaminate them with DNA from the cells that continually slough off of your skin.
Fragmentation and contamination make genetic analysis of ancient remains very tricky work.