2.4: The Nucleus

By the end of the section, you will be able to:

• Describe the structure and features of the nuclear membrane
• List the contents of the nucleus
• Explain the organization of the DNA molecule within the nucleus

The nucleus is the largest and most prominent of a cell’s organelles (Figure \(\PageIndex{1}\)). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure \(\PageIndex{2}\)), so they are referred to as multinucleated cells. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure \(\PageIndex{3}\)). Without nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.

Figure \(\PageIndex{1}\): The Nucleus. The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell. (Image credit: "Nucleus" by OpenStax is licensed under CC BY 4.0)

Figure \(\PageIndex{2}\): Multinucleate Muscle Cell. Unlike cardiac muscle cells and smooth muscle cells, which have a single nucleus, a skeletal muscle cell contains many nuclei, and is referred to as “multinucleated.” These muscle cells are long and fibrous (often referred to as muscle fibers). During development, many smaller cells fuse to form a mature muscle fiber. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic to mature muscle cells. LM × 104.3. (Image credit: "Multinucleate Muscle Tissue Micrograph" by OpenStax is licensed under CC BY 4.0/ Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Figure \(\PageIndex{3}\): Red Blood Cell Extruding Its Nucleus. Mature red blood cells lack a nucleus. As they mature, erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast before and after ejecting its nucleus, respectively. (Image credit: "RBC Extruding Nucleus Micrograph" by OpenStax is licensed under CC BY 4.0/ Modification of micrograph provided by the Regents of University of Michigan Medical School © 2012)

Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA, deoxyribonucleic acid. The nucleus sends “commands” to the cell via molecular messengers that translate the information from DNA. Each nucleated cell in your body (with the exception of germ cells, also known as eggs and sperm) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA.

Organization of the Nucleus and Its DNA

Like most other cellular organelles, the nucleus is surrounded by a membrane. The membrane surrounding the nucleus, called the nuclear envelope, consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA (ribonucleic acid), and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.

Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.

The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure \(\PageIndex{4}\)). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.

Figure \(\PageIndex{4}\): DNA Macrostructure. Strands of DNA are wrapped around supporting histones. These proteins are increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready to divide. (Image credit: "DNA Macrostructure" by OpenStax is licensed under CC BY 4.0)

Concept Review

The nucleus is the command center of the cell, containing the genetic instructions for all of the materials a cell will make (and thus all of its functions it can perform). The nucleus is encased within a membrane of two interconnected lipid bilayers, side-by-side. This nuclear envelope is studded with protein-lined pores that allow materials to be trafficked into and out of the nucleus. The nucleus contains one or more nucleoli, which serve as sites for ribosome synthesis. The nucleus houses the genetic material of the cell: DNA. DNA is normally found as a loosely contained structure called chromatin within the nucleus, where it is wound up and associated with a variety of histone proteins. When a cell is about to divide, the chromatin coils tightly and condenses to form chromosomes.

Review Questions

Q. The nucleolus specifically functions in the formation of which of the following structures?

A. chromatin

B. histones

C. ribosomal subunits

D. nucleosomes

Answer

Answer: C

Q. Place the following structures in order from least to most complex organization: chromatin, nucleosome, DNA, chromosome

A. DNA, nucleosome, chromatin, chromosome

B. nucleosome, DNA, chromosome, chromatin

C. DNA, chromatin, nucleosome, chromosome

D. nucleosome, chromatin, DNA, chromosome

Answer

Answer: A

Glossary

chromatin

substance consisting of DNA and associated proteins

chromosome

condensed version of chromatin

histone

family of proteins that associate with DNA in the nucleus to form chromatin

nuclear envelope

membrane that surrounds the nucleus; consisting of a double lipid-bilayer

nuclear pore

one of the small, protein-lined openings found scattered throughout the nuclear envelope

nucleolus

small region of the nucleus that functions in ribosome synthesis

nucleosome

unit of chromatin consisting of a DNA strand wrapped around histone proteins

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology