Nevertheless, even the most powerful assembly software relies more on elegance and simplicity than on brute force. A program used to assemble the human genome would fit easily on the hard disk of a typical personal computer. But you couldn't actually run the assembler from the computer on your desk. Genome scientists venture daily into uncharted territory. When a genome has never been sequenced before, there is nothing to tell its explorers whether they have sequenced it correctly.
Some sequences are particularly difficult to copy or to sequence and get left out.
But a combination of redundancy and careful checking helps ensure that errors in genome sequencing are kept to an absolute minimum. One trick for eliminating errors is to sequence the genome more than once. That way, if the sequencing machine gets a base wrong, or if a piece of DNA slips through the cracks and doesn't get sequenced, there are likely to be other, correct reads that will provide the sequence. In addition to identifying DNA bases, software on automatic sequencing machines can evaluate the probability that a base really is the base it appears to be. Error probabilities for all of the bases in a read are added together for an estimate of the number of errors in the sequence.