Prokaryotic cells are characterised by the absence of a nuclear membrane. This means that the genetic material, which is present in the form of a single circular chromosome, is not enclosed within a membrane-bound nucleus. Here's a detailed explanation of this characteristic feature of prokaryotic cells:

Absence of a nuclear membrane:
- Prokaryotic cells are unicellular organisms that lack a membrane-bound nucleus.
- Instead of a nucleus, the genetic material is present in the cytoplasm in the form of a single circular chromosome.
- This chromosome is not enclosed within a membrane-bound structure, but is rather free-floating in the cytoplasm.
- In addition to the chromosome, prokaryotic cells may also contain plasmids, which are smaller circular pieces of DNA that are separate from the main chromosome.
- The absence of a nuclear membrane in prokaryotic cells means that there is no physical barrier separating the genetic material from the rest of the cell's contents.

Presence of chromatin material:
- While prokaryotic cells do not have chromatin material in the sense of eukaryotic cells, they do have a form of DNA packaging.
- In prokaryotic cells, the DNA is packaged by proteins called histones, which help to condense the DNA into a compact structure.
- This compacted DNA is known as nucleoid, and is visible under a microscope as a dense region within the cytoplasm.
- However, it is important to note that nucleoid is not the same as chromatin material, which is the complex of DNA and proteins found in eukaryotic cells.

Other characteristics of prokaryotic cells:
- Prokaryotic cells are generally smaller and simpler in structure than eukaryotic cells.
- They lack membrane-bound organelles such as mitochondria, chloroplasts, and endoplasmic reticulum.
- Instead, they have simpler structures such as ribosomes, which are responsible for protein synthesis.
- Prokaryotic cells are found in domains Bacteria and Archaea, and are the most abundant and diverse forms of life on Earth.