Structure of the Cell Nucleus

Nuclear Envelope

The nucleus is surrounded by a barrier, formed by two membranes, called the nuclear envelope. This barrier separates the contents of the nucleus from the cytoplasm. However, molecules can still move between the nucleus and the cytoplasm because the nuclear envelope is pierced by hundreds of hollow structures called nuclear pores that allow traffic between the cytoplasm and nucleus. The nuclear pores are suspended in a framework found on the inside of the nuclear envelope. This framework, composed of lamin proteins, governs the overall shape of the nucleus. If the lamin proteins change their function, this causes the nuclear envelope to change its shape or to even disappear, as occurs during cell division! These features help explain why cell nuclei look so different in different types of cells.


Chromosomes are the most important parts of the cell nucleus. Each chromosome has two components: 1) an axis formed by protein fibers, and 2) long loops of a molecule called DNA that hang off of the chromosome’s axis. There are 46 chromosomes in each human cell. Each chromosome is plastered onto the interior of the nuclear envelope in its own special place. Information on the DNA is used to control the function of the cell. This information is copied onto other molecules (messenger RNA molecules) and sent to the cytoplasm of the cell. In the cytoplasm, this information is used to make the proteins that operate the cell machinery.

How, exactly, is this done?


DNA is partly composed of thousands of sugar molecules, all linked together into long strands by molecules of phosphate. The name of each sugar molecule is deoxyribose. The molecules of phosphate that join the sugars together are very acidic. Thus, DNA represents an acidic molecule found in the nucleus that contains deoxyribose. In other words, DNA is deoxyribonucleic acid.

Attached to each deoxyribose sugar in each strand of DNA are 4 types of molecules called nucleotides, or bases. These bases are called adenine (A), cytosine (C), guanine (G), and thymidine (T). The sequence of bases found on the strands of sugars, which can be thousands of bases long, represents a coded form of information that is used to control the cell. The encoded information on the DNA is called a gene, and there are about 20,000 active genes in human chromosomes. How is the information in the DNA of a gene used?

The sequence of bases on the DNA is copied onto a similar molecule called messenger ribonucleic acid (mRNA). Then, the mRNA travels through the nuclear pores into the cytoplasm. In the cytoplasm, tiny machines called ribosomes use the information on the mRNA to link together hundreds of amino acids into long chains. A long chain of amino acids is called a protein. Proteins are the building blocks of the cell. They can form rigid structures inside the cell, which help organize its structure. Alternatively, they can form enzymes that accelerate chemical reactions and provide the cell with energy.

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Written by John Young

John K. Young is a retired professor of Cell Biology. He worked in the Department of Anatomy at Howard University College of Medicine in Washington, DC for 35 years, teaching his students about cells. During his career, Dr. Young published scientific articles about a part of the brain called the hypothalamus and also wrote a number of books about cells and about the brain.

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