When you're checking out a plant or animal cell through a microscope, you can usually see the nucleus pretty easily. It's the most noticeable part inside the cell. The nucleus was actually the first organelle (a tiny structure inside the cell) that scientists found. In eukaryotic cells (which are more complex than prokaryotic cells, like bacteria), the nucleus holds the DNA. DNA is like a blueprint for how the cell works, so the nucleus is kind of like the boss of all the cell's activity. That's why it's so important! And that's also why scientists study it a lot. In fact, the nucleus is one of the most important parts of the cell to understand. And now you know a little bit more about it! By the way, did you know that the nucleus is made up of different parts, too? There's a lot to learn about this tiny powerhouse!

Cell Nucleus Structure

The cell nucleus is a defining feature of eukaryotic cells. Eukaryote actually means "true nucleus," which sets it apart from prokaryotic cells found in bacteria and archaea, which don't have a nucleus. In prokaryotic cells, the genetic material is located in a part of the cytoplasm called the nucleoid. When you look at cells under a microscope, it's simple to tell which ones are eukaryotic and which ones are prokaryotic because of the nucleus (and other organelles that are unique to eukaryotic cells, as we mentioned in our article about Eukaryotic Cells). Understanding the differences between these types of cells is essential for many areas of research, from medicine to ecology. By studying these differences, scientists can gain insight into how cells function, evolve, and adapt in different environments.

Cell Nucleus Diagram

The nucleus is a rounded or elliptical structure that's surrounded by two membranes (you can see what it looks like in figures 1 and 2). Each of these membranes is made up of a double layer of phospholipids, just like the plasma membrane (which only has one layer). The two membranes together form a structure called the nuclear envelope, with the inner membrane facing the inside of the nucleus and the outer membrane facing the cytoplasm. This outer membrane is actually connected to the endoplasmic reticulum in the cytoplasm, and both organelles work together as part of the endomembrane system of the cell. The nucleus is the organelle in eukaryotic cells that's enclosed by these membranes and contains the genetic material (chromosomes) that controls the cell's activity.

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The nuclear envelope is covered in nuclear pores that allow ions and molecules to move between the nucleus and the cytoplasm. These pores are surrounded by proteins that form a complex to control the passage of proteins, RNA molecules, and other large components. The inner part of the nuclear envelope is covered in a mesh-like structure called the nuclear lamina (except where the pores are), which supports the envelope and maintains the shape of the nucleus.

The functions of the nuclear envelope and its lamina are not entirely clear, but they seem to be involved in fundamental processes for the cell. Mutations in the LMNA gene, which carries the information for a type of nuclear lamins, can cause at least eight human diseases that present with symptoms of premature aging.

The nucleus contains most of the cell's genetic material, as well as one or more nucleoli. The nucleolus is a dense mass of granules and fibers that is not surrounded by a membrane. It is composed of regions of chromatin involved in ribosome synthesis, along with RNA molecules and proteins. The DNA in eukaryotic cells is organized into multiple linear chromosomes that are associated with proteins to maintain their structure.

Mitochondria and chloroplasts have some genes of their own, as these organelles most likely derived from ancestral free-living bacteria that were engulfed by an ancestral host cell through endosymbiosis and therefore had their own genetic material initially.

Cell Nucleus Function

The cell nucleus has two main functions:

The storage of most of the cell's genetic material. Duplication of the genetic material (chromosomes) during the interphase of the cell cycle, to be distributed to the daughter cells during cell division (see figure 2), including the transcription of the specifications for protein synthesis from DNA to RNA.

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The nucleus is known as the cell's control center since it encloses the genetic material that specifies all the cell's activities. The genetic material is organized into chromosomes that contain hundreds to thousands of smaller segments, each of which is a gene. The sequence of nucleotides in a gene serves as a code for making a protein. The process of going from a gene to a protein involves two steps: transcription and translation. Transcription occurs in the nucleus, where the DNA sequence is transcribed into messenger RNA (mRNA). This segment of mRNA leaves the nucleus through nuclear pores and is translated into the corresponding protein by ribosomes in the cytoplasm.

The nucleolus also transcribes segments of DNA that encode ribosomal RNA (rRNA), which is involved in the production of another cell organelle, the ribosome. Proteins from the cytoplasm enter the nucleus through nuclear pores and are assembled with rRNA molecules into ribosome subunits. Ribosomes are composed of one large and one small subunit, which leave the nucleus to the cytoplasm, where ribosome assembly is completed. Enzymes and other proteins required inside the nucleus, such as those associated with DNA or required to make ribosome subunits, are synthesized in the cytoplasm by ribosomes and enter the nucleus through nuclear pores. RNA molecules and ribosome subunits synthesized in the nucleus must leave through the pores to participate in protein synthesis in the cytoplasm.

Plant and Animal Cell Nucleus: Is There a Difference?

All eukaryotic cells have a nucleus, including animal, plant, fungi, and protist cells. There are no major differences in the structure or functions of the nucleus among eukaryotes. However, some eukaryotic cells can lose their nucleus as part of their development or due to errors during cell division.

Mature red blood cells in mammals are an example of cells that do not have a nucleus. They do not perform the functions of the nucleus, such as transcribing DNA to RNA or synthesizing proteins. Instead, their function is to transport oxygen to every cell in our body. RBCs eject their nucleus and other organelles when they mature to have more space for hemoglobin molecules. As a result, they cannot divide and have a lifespan of 100-120 days, after which they are continuously replaced. RBCs in other vertebrates do have a nucleus, but it is inactive. 

Cell Nucleus - Key Takeaways

The cell nucleus is a crucial organelle that controls cellular activity and contains most of the genetic material in a eukaryotic cell. It is surrounded by a nuclear envelope, which is composed of two bilayered membranes and is part of the cell's endomembrane system.

The nucleus has two primary functions: duplication of the genetic material (chromosomes) for distribution during cell division and transcription of protein synthesis specifications from DNA to RNA. Transcription occurs in the nucleus, where a DNA segment is transcribed into a molecule of messenger RNA (mRNA), which is then translated into the corresponding protein in the cytoplasm. The nucleolus is a substructure within the nucleus that transcribes segments of the DNA that encode ribosomal RNA (rRNA) and assembles them with proteins to form the subunits of ribosomes. Ribosomes are essential for protein synthesis and are composed of one large and one small subunit. All the proteins required inside the nucleus, such as those associated with DNA or required for ribosome subunit production, are synthesized in the cytoplasm by ribosomes and enter the nucleus through the nuclear pores. In contrast, RNA molecules synthesized in the nucleus must exit into the cytoplasm to participate in protein synthesis.

References

Leslie Mounkes and Colin Stewart, Aging and nuclear organization: lamins and progeria, Current Opinion in Cell Biology, 2004

Cristin Carry, How red blood cells nuke their nuclei, February 10, 2008. Whitehead Institute.

https://wi.mit.edu/news/how-red-blood-cells-nuke-their-nuclei

Cell Nucleus

Do prokaryotic cells have a nucleus?

No, prokaryotic cells do not have a “real” nucleus delimited by a membrane. They do have a nuclear region where the genetic material is concentrated.

Do plant cells have a nucleus?

Yes, plant cells are eukaryotic cells and have a nucleus.

Do all cells have a nucleus?

No, not all cells have a nucleus. Prokaryotic cells do not have a nucleus, while eukaryotic ones do. However, some eukaryotic cells can lose their nucleus as part of their development, like mature mammalian red blood cells (erythrocytes).

Do eukaryotic cells have a nucleus?

Yes, eukaryotic cells have a nucleus. Some eukaryotic cells can lose their nucleus as part of their development, like mature mammalian red blood cells (erythrocytes), but they originally had a nucleus.

What does the nucleus do in a cell?

The cell nucleus encloses the genetic material and has two main functions: duplication of the genetic material (chromosomes) to be distributed during cell division; and transcription of protein synthesis’ specifications from DNA to RNA.

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