Linker DNA is typically attached to double-stranded cDNA before it is cloned. Linker DNA is typically of known sequence and has unique restriction enzyme sites. Which of the following is a good description of linker DNA?
1) DNA that is used to link two DNA fragments together
2) DNA that is used to attach cDNA to a vector
3) DNA that contains unique restriction enzyme sites
4) DNA that is used to clone cDNA