How do you read bps on gel ladders?

Answer 1

Are you referring to size standards on DNA/RNA gels? If so, whichever standard you use should be pre-defined as to the size of each fragment. You can then compare where your sample fragments line up in relation and get a reasonably good estimate of their size in base pairs (bp). For instance, lambda-HinDIII markers go (reading from the lowest, in base pairs): 600, 2,000, 2,200, 4,300, 6,000-something, and larger. If your band of interest is a bit lower than the 600 band, it's somewhere about 450-550bp. If you don't know what the fragment sizes are on the size standards, though, you're SOL.

Answer 2

Gel ladders vary from gel ladder to gel ladder. Get a hold of the manual for the gel ladder, and there will be a legend. Look for bright marks to orient you.

If you can't find the manual, get the name, and it will be online at the company website.