The reaction in a test tube for making complementary DNA (cDNA) from RNA is called reverse transcription. This reaction doesn't "generate" anything in the sense of producing a new substance. Instead, it uses existing molecules to create a new DNA strand. Here's what happens:

1. Starting materials: You begin with a template RNA molecule and the enzyme reverse transcriptase. You also need deoxynucleotides (dNTPs), which are the building blocks for DNA, and a primer – a short sequence of DNA that will bind to the RNA template and provide a starting point for the reverse transcriptase.

2. Reverse transcription process:

* The primer binds to the RNA template.

* Reverse transcriptase uses the RNA template and dNTPs to synthesize a complementary DNA strand. This strand is the cDNA.

3. Result: The end product is a cDNA molecule that is complementary to the original RNA template.

Important Note: The reaction itself doesn't "generate" anything new in terms of molecules. It simply uses existing molecules to create a new DNA sequence based on the RNA template.