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A base analog is a chemical that can substitute a normal nucleobase in nucleic acids.
Some common examples are 5-bromo-2-deoxyuridine (BrdU) and 5-fluro-2-deoxyuridine.
Base analogs can be used in the sequencing of DNA. Certain base analogs can be added to the growing DNA chain, but due to the lack of a hydroxyl group, no further bases can be added. That stops the growing chain. If a piece of DNA is replicated in a solution containing some of those base analogs, chains will be made that end after whatever analog has been added is used.
When the above experiment is run using analogs for all four bases (separately) and then are run on a gel, the DNA sequence can be read off. Every time a base analog was added, it will terminate the chain, and there will be DNA pieces of that length. That way every different length of DNA strand will be formed, and which lane any given piece is in is determined by what base analog was used to terminate the chain of that length, and thus what base is in that position.
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