are you for real? If a gene is just 500 bp you can just sequence it directly (dideoxy method allows for an 800 bp fragment to be sequenced).
To produce it commercially is a lot tricker, hybridoma cells are probably the way to go. unfortunately because antibody molecules are so big and complex (and as a consequence so much larger than 500 bp) you can’t just put them in E.coli and wait for the bugs to make it, like you can do with insulin
Ok, here’s what you do:
1.you sequence the human DNA which has already been done.
2. Build a supercomputer capable of performing LOTS of calculations.
3. Design an optimized program for predicting gene products based on the DNA sequence.
4. Design another program to fold the proteins from #3 into the correct shape. Don’t forget VDJ rearrangements!
5. Use a dock program to fit a model of the antibody to the proteins from #4.
6. Take the best match from #5 and that’s your sequence.
To produce the antibody, just use supercomputer from #2 to build a replicator.