The outside face of the card shows an electron micrograph of a gene being heavily transcribed into RNA, an early and fundamental step of gene expression. Because of its shape, this is often referred to as a "Christmas Tree." The gree tint on the micrograph enhances this association.
The inside of the card explains:
The electron micrograph on the front documents the first step in gene expression. The DNA gene positioned vertically in the center is being transcribed into RNA by RNA polymerase enzymes. The polymerases that have traveled furthest down the gene have produced the longest RNA chains.