Skip to main content
Biology LibreTexts

2.34: Chromosomes

  • Page ID
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)

    f-d:b5a9e53ee193281564fc88a1550baac3b18155075ee915ec5d8c2aa2 IMAGE_TINY IMAGE_TINY.1

    How is it assured that every cell in your body has the same DNA?

    Chromosomes, like those shown here, must form prior to cell division, to ensure that each daughter cell receives a complete set of genetic material. Essentially, each new cell receives half of each "X-shaped" chromosome.


    In eukaryotic cells, the nucleus divides before the cell itself divides. The process in which the nucleus divides is called mitosis. Before mitosis occurs, a cell’s DNA is replicated. This is necessary so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To understand how this happens, you need to know more about chromosomes.

    Chromosomes are coiled structures made of DNA and proteins. Chromosomes are the form of the genetic material of a cell during cell division. It is this coiled structure that ensures proper segregation of the chromosomes during cell division. During other phases of the cell cycle, DNA is not coiled into chromosomes. Instead, it exists as a grainy material calledchromatin.

    The vocabulary of DNA: chromosomes, chromatids, chromatin, transcription, translation, and replication is discussed at (18:23).

    Chromatids and the Centromere

    DNA condenses and coils into the familiar X-shaped form of a chromosome, shown in Figure below, only after it has replicated. (You can watch DNA coiling into a chromosome at the link below.) Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies are called sister chromatids. They are attached to one another at a region called the centromere. A remarkable animation can be viewed at

    Diagram of a chromosome

    Chromosome. After DNA replicates, it forms chromosomes like the one shown here.

    Chromosomes and Genes

    The DNA of a chromosome is encoded with genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.

    Human Chromosomes

    Human cells normally have two sets of chromosomes, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape that also contain the same genes. The chromosomes in a pair are known as homologous chromosomes.


    • Chromosomes are coiled structures made of DNA and proteins.
    • Chromosomes form after DNA replicates; prior to replication, DNA exists as chromatin.
    • Chromosomes contain genes, which code for proteins.
    • Human cells normally have 46 chromosomes, made up of two sets of chromosomes, one set inherited from each parent.
    • See Chromosomes at for a detailed summary.

    Explore More

    Use this resource to answer the questions that follow.

    1. What is a chromosome? What makes up a chromosome?
    2. How many chromosomes do people usually have?
    3. Can changes in the number of chromosomes affect health and development?


    1. What are chromosomes? When do they form?
    2. Identify the chromatids and the centromere of a chromosome.
    3. Explain how chromosomes are related to chromatin. Why are chromosomes important for mitosis?
    4. How many chromosomes are in a normal human cell?

    2.34: Chromosomes is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

    CK-12 Foundation
    CK-12 Foundation is licensed under CK-12 Curriculum Materials License
    • Was this article helpful?