**True Nucleus is Absent in:**

The true nucleus is absent in certain cells known as prokaryotic cells. Prokaryotes are a group of organisms that include bacteria and archaea. These cells lack a well-defined nucleus surrounded by a nuclear membrane. Instead, their genetic material is present in the form of a nucleoid region, which is not enclosed by a membrane. Here are some details explaining the absence of a true nucleus in prokaryotic cells:

**1. Prokaryotic Cell Structure:**
Prokaryotic cells are relatively simple in structure compared to eukaryotic cells. They lack various membrane-bound organelles found in eukaryotic cells, including a nucleus. Instead, their genetic material is present as a single circular DNA molecule located in the nucleoid region.

**2. Nucleoid Region:**
The nucleoid region is a dense area within the prokaryotic cell where the genetic material is found. It contains the DNA molecule, which carries the instructions for cellular functions and heredity. However, unlike the true nucleus of eukaryotic cells, the nucleoid is not surrounded by a nuclear membrane.

**3. Lack of Nuclear Membrane:**
In eukaryotic cells, the nucleus is enclosed by a double-layered nuclear membrane, also known as the nuclear envelope. This membrane acts as a protective barrier, separating the genetic material from the rest of the cellular components. However, prokaryotic cells lack this nuclear membrane, resulting in the absence of a true nucleus.

**4. Other Prokaryotic Features:**
Apart from the absence of a true nucleus, prokaryotic cells possess other unique features. These include the presence of cell walls made of peptidoglycan (in bacteria), smaller size, simpler internal structure, lack of membrane-bound organelles (such as mitochondria and endoplasmic reticulum), and the presence of ribosomes that are smaller and structurally different from eukaryotic ribosomes.

In conclusion, the true nucleus is absent in prokaryotic cells such as bacteria and archaea. Instead, these cells have a nucleoid region where their genetic material is located. The absence of a nuclear membrane and other distinguishing features make prokaryotic cells distinct from eukaryotic cells.