Cell Nucleus

Cell Nucleus Diagram

The nucleus (plural, nuclei) has a spherical or elliptical shape and is delimited by two membranes (see figures 1 and 2). Each of these membranes is a phospholipid bilayer, like the one that composes the plasma membrane (the plasma membrane has one bilayer, the nucleus has two bilayers)

The structure formed by the inner (facing the inside of the nucleus) and outer (facing the cytoplasm) bilayered membranes is called the nuclear envelope. The outer membrane is continuous with the endoplasmic reticulum in the cytoplasm, and both organelles work together as part of the endomembrane system of the cell.

Diagram of a eukaryotic cell's nucleus and its components. The endoplasmic reticulum is continuous with the outer membrane of the nuclear envelope

There are nuclear pores all over the nuclear envelope through which ions and molecules move between the nucleus' interior and the cytoplasm. Proteins surround each nuclear pore forming a pore complex that controls the passage of proteins, RNA molecules, and larger components.

The inner part of the nuclear envelope is covered by the nuclear lamina (except where the pores are), a mesh-like structure composed of protein filaments. The lamina gives support to the nuclear envelope and maintains the nucleus shape.

We find most of the cell’s genetic material and one or more nucleoli (singular nucleolus) inside the nucleus. The nucleolus looks like a dense mass of granules and fibers, not delimited by a membrane. This mass is composed of regions of chromatin involved in ribosome synthesis, associated with RNA molecules and proteins. The nuclear components are immersed in a semi-solid fluid that fills the inside of the nucleus, called the nucleoplasm.

The DNA in eukaryotic cells (plant, animal, fungi, and protist cells) is organized in multiple linear chromosomes. DNA is an extremely large and thin molecule that could easily get tangled and break. Thus, it is associated with proteins that help maintain its structure. The complex of DNA and proteins is called chromatin and makes up the chromosomes. In a cell that is not dividing or reproducing, the chromosomes are somewhat loose and look like a granular agglomerate of threads. It is only during the division process that the chromatin condenses (it coils up and thickens) and becomes visible (figure 2, B-D, shows the nuclei of plant cells that are dividing).

We say that the nucleus contains most of the cell’s genetic material (nuclear DNA) because mitochondria and chloroplasts have some genes of their own (both these organelles most probably 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.

Figure 2. Section of an onion root tip. Stained cell nuclei are visible in different cell cycle stages. A) interphase, B) early stage of cell division, C and D) cells undergoing mitosis. Source: modified from Berkshire Community College Bioscience Image Library, public domain, flickr.com.

The nucleus is the cell's control center because it encloses the genetic material, which contains the information that specifies all the cell activity. A chromosome comprises hundreds to thousands of smaller segments containing the genetic information for specific traits (for example, the color of an animal’s fur). Each of these segments is a gene. The sequence of nucleotides in a gene works as a code (called the genetic code) for making a protein.

Genes do not synthesize proteins directly. The process to go from gene to protein is called gene expression (because proteins are the corresponding expression of genes) and involves two steps, transcription and translation. The nucleus is the site where transcription occurs. The nucleus transcribes (or copies) the nucleotide sequence from a gene to the corresponding RNA sequence, which is called messenger RNA -mRNA- (remember from nucleic acids structure that RNA is similar to a DNA molecule, except that in RNA, the sugar part is ribose instead of deoxyribose and has a uracil base instead of thymine).

In other words, the DNA sequence serves as a template that is transcribed into mRNA. This segment of mRNA then leaves the nucleus through the nuclear pores and will be translated into the corresponding protein by ribosomes in the cytoplasm.

The nucleolus also transcribes segments of the DNA that encode ribosomal RNA -rRNA-. As the name indicates, the rRNA is involved in the production of another cell organelle, the ribosome. Proteins from the cytoplasm enter the nucleus through the nuclear pores and, in the nucleolus, are assembled with the rRNA molecules into ribosome subunits. Ribosomes are composed of one large and one small subunit. The ribosome subunits leave the nucleus to the cytoplasm, where the ribosome assembly is completed.

All the enzymes and other proteins needed inside the nucleus (like the proteins associated with the DNA or the ones required to make the ribosome subunits) are synthesized in the cytoplasm by ribosomes and then enter the nucleus through the nuclear pores. On the other hand, 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 protists cells. There are no major differences in the structure or functions of the nucleus among eukaryote organisms. Some eukaryotic cells can lose their nucleus as part of their development (like red blood cells in mammals), or sometimes errors occur during cell division, and one of the daughter cells can keep both nuclei while the other has no nucleus (is anucleated).

Do Blood Cells Have a Nucleus?

Mature red blood cells in mammals do not have a nucleus. Red blood cells (RBCs), like all eukaryotic cells, do have a nucleus when formed. However, their function is to transport oxygen to every cell in our body. The more hemoglobin (the molecule that binds to oxygen) an RBC can carry, the more efficient it will be. Hence, RBCs eject their nucleus and other organelles when they mature to have more space for hemoglobin molecules.

What does it mean for a cell not to have a nucleus? As expected, the nucleus’ functions are not performed in mature RBCs. They do not transcribe DNA to RNA nor synthesize proteins (in consequence, they do not have most organelles to do this either). RBCs cannot divide either and thus are produced in the bone marrow; they have a lifespan of 100-120 days and are continuously being replaced. RBCs in other vertebrates (fish, reptiles, birds) are bigger than in mammals, and they do have a nucleus, but it is inactive.

References

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

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

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