Prokaryotic cells (e.g. Bacteria) don't contain a nucleus, so it depends on what type of cell you're talking about. For eukaryotic cells, however, I would agree that the nucleus is the most important part.

It might be a brown widow spider. They range from dark brown to light tan and have the same hourglass marking on the abdomen. While their venom is twice as toxic as a black widow's, they don't inject as much and so are less dangerous.