If you want to be able to work with DNA then you are going to have to be able to transform bacteria. This is about how and why this is done. The reason for using bacteria is that the bacteria cells go very fast. The bacteria can quickly amplify plasmid DNA. This plasmid DNA can be ligated to get the desired DNA that you want. These plasmids need to contain a resistant to a form of antibiotics as will be explained later.
A very common bacterium that is used is Escherichia coli or more commonly named E. coli. The plasmids are inserted into the bacteria by either a calcium chloride/heat shock treatment or electroporation. These methods make holes in the bacteria so that the DNA can enter. The main problem is that only a very small amount of bacteria will take the DNA, so a few extra steps are needed.
The first step is to remove all the bacteria that did not take any plasmid at all. This is done by placing the bacteria on a agar plate that has been coated with an antibody, such as ampicillin, depending on the resistance of the plasmid. This means that all the bacteria that do not take the plasmid, they will not be able to reproduce. This means that all the bacteria that is left will form colonies with the plasmid. With only this step there is no way to check to see if the plasmid has the desired DNA, so another step is needed.
The next step is done by adding a lacZ inducer and a substrate that will be formed from the lacZ. The lacZ is broken by the adding of the new DNA and so the desired plasmids will not form the substrate. The substrate turns the bacteria blue so it becomes very easy to tell the two types of bacteria. Then the colorless bacteria is removed and used to do as you wish with it.
So in case if you ever are thinking about how to go about producing large DNA then I hope this helps.