Cells that lack a nucleus are called prokaryotes. In prokaryotes, DNA does not take on the familiar double helix shape of eukaryotic DNA. Instead, it exists as a single, circular molecule called a nucleoid.

The nucleoid is not enclosed by a membrane like the nucleus in eukaryotes. It is a region within the cytoplasm where the DNA is concentrated and organized. The DNA is supercoiled, meaning it's twisted upon itself to fit within the small space of the prokaryotic cell. This supercoiling allows for efficient packaging and helps to regulate gene expression.

So, in summary:

* Shape: Circular

* Structure: Nucleoid (not a membrane-bound nucleus)

* Organization: Supercoiled