First, they made a map of the human genome through marks on the genome code to position the sequences of letters for later. Then they broke copies of the genome into pieces, each 150,000 letters of long. Then they put the fragments into a bacterial chromosome that could be grown so when divided, the DNA would be replicated to make a library of DNA clones. Then they broke the cloned fragments into smaller chunks, each around 1000 to 2000 base pairs long. From here they were able to determine the base pairs, and then the entire human DNA sequence.