Sentence:  The 5′ junction is checked using a target gene-specific primer of ~21 nucleotides, which <WSD>anneals</WSD> upstream of the cassette integration site, together with a primer which <WSD>anneals</WSD> downstream of the first "loxP" site in the PCR cassette. 
Meaning of the word marked with <WSD> tags:  To make a double-stranded nucleic acid by pairing a single strand with a complementary strand.