Complex sentence: The synthesizing process starts with the "four bottles" of the base pairs A, G, C and T and synthesizes oligonucleotides that are then synthesized into specific gene cassette of about 1000 base pairs.
Split into simpler sentences: The synthesizing process starts with the "four bottles" of the base pairs A, G, C and T and then oligonucleotide synthesis. Those oligonucleotide are then synthesized into specific gene cassette of about 1000 base pairs.