Hello
Could someone please explain a few reasons about why bacterial artificial chromosomes were so crucial to the success of the hierarchical shotgun sequencing. I know that the use of bac libraries is one reason but I think I am missing something. Also some help on how bac librarise are used in shotgun sequencing would be useful too.

While I haven't heard the term heirarchical BACs very often. The importance of BAC, and later YACs was to anchor the sequence. The human genome, for instance, is mostly repetitive sequence, LINEs, SINEs, etc. Each LINE element is several kb long and present tens or hundreds of times in the human genome and they are all largely identical. Similar situations can arise with recently duplicated genes or intergenic sequence. Shotgun sequencing gives reads of about 1000 bases and may not be able to place each repetitive element uniquely. If you are shotgun sequencing one BAC it restricts the potential locations to a few hundred kb. You can use this analogy, if I told you that I live in on a particular street in Portland you wouldn't be able to find me. There are many Portlands in the US and around the world. You need to restrict the potential locations so we have states. Hopefully there is only one Portland in the state so you can place my location uniquely. The BAC restricts the number of locations the sequence be from just like the state. Once you have shotgun sequenced a whole BAC you will have several hundred kb of ordered sequence which you hopefully will be able to uniquely place on the chromosome.

Hopefully this helps