Explain in a few sentences:
a) How is cDNA made?
b) Why does the DNA on a microarray have to be denaturated?
c) Explain why it is necessary to make cDNA libraries of different tissues although all somatic cells
of an organism contain the same DNA.

a) RNA can be transformed to DNA:
- Cells like to add polyA-tails to freshly transcribed RNA
- Hybridize with polyT-primer
- Make complementary DNA copy with reverse transcriptase
- Degrade RNA with RNase H
- Synthesize a second cDNA strand using DNA polymerase; RNA fragment acts as primer

b) The strands on a microarray need to be single-stranded (denaturated cDNA), so they can fish their labeled complementary sequence probes out of the target mixture in a hybridization reaction.

c) cDNA is produced from fully transcribed mRNA found in the nucleus and therefore contains only the expressed genes of an organism.