what is a chromosome

What is a chromosome?

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In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

DNA and histone proteins are packaged into structures called chromosomes.

Chromosomes have a p arm, a q arm, and a centromere. They are made up of DNA wrapped around histone proteins.

Credit: U.S. National Library of Medicine

For more information about chromosomes:

MedlinePlus Genetics provides information about each human chromosome written in lay language.From the National Institutes of Health

basic introduction to chromosomes is available from the National Human Genome Research Institute.From the National Institutes of Health

The Centre for Genetics Education offers a fact sheet that introduces genes and chromosomes.

The University of Utah’s Genetic Science Learning Center offers a description of chromosomes, including how scientists tell them apart.

Topics in the Cells and DNA chapter

Other chapters in Help Me Understand Genetics

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.


Listen to pronunciation(KROH-muh-some)A structure found inside the nucleus of a cell. A chromosome is made up of proteins and DNA organized into genes. Each cell normally contains 23 pairs of chromosomes.

More Information
Structure of DNA; drawing shows a chromosome, nucleosome, histone, gene, and nucleotide base pairs: guanine, cytosine, adenine, and thymine. Also shown is a cell and its nucleus.

Structure of DNA. Most DNA is found inside the nucleus of a cell, where it forms the chromosomes. Chromosomes have proteins called histones that bind to DNA. DNA has two strands that twist into the shape of a spiral ladder called a helix. DNA is made up of four building blocks called nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). The nucleotides attach to each other (A with T, and G with C) to form chemical bonds called base pairs, which connect the two DNA strands. Genes are short pieces of DNA that carry specific genetic information.

What is a chromosome? Our cells contain many structures that are essential for living. One of these key structures is a chromosome.

Chromosomes are found in the nucleus of every cell of our body and are made up of DNA, tightly coiled around proteins. We have 23 pairs of chromosomes altogether, and they can only be easily observed during cell division.

So why is our DNA kept in this special packaging? The chromosomes unique structure allows our genetic data to be discreetly kept inside our nuclei. This is important because we have an abundance of genetic data. If the DNA molecules in a single cell were unwound, they would stretch out to six feet long.

The chromosomes unique structure has a few key parts. Each chromosome has two short arms called p arms and two long arms called q arms. These arms are held together at the center by the centromere. The tips of the chromosome are capped by sections of DNA called telomeres. Telomeres protect chromosomes during DNA replication. By keeping our DNA secure, chromosomes make daily activities possible. For an organism to function properly, certain cells must constantly divide and replace older worn out cells with new ones. Chromosomes allow DNA to be accurately copied during these cell divisions.

So one more time. Chromosomes are found in the nuclei of our cells and allow DNA to be accurately copied during cell division. This ensures that our inner workings proceed smoothly and efficiently.

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