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2.4: Cell Nucleus

Difficulty Level: At Grade Created by: CK-12
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Where is your DNA?

You may know that a criminal can easily leave DNA at a crime scene. How? DNA is found in every cell of your body. In each cell there is a nucleus, which is home to your DNA. So if a criminal has a cut, and blood is left at the crime scene, or a hair falls out and is left behind, then DNA will also be left at the scene.

The Nucleus

The nucleus is only found in eukaryotic cells. It contains most of the genetic material (the DNA) of the cell. The genetic material of the nucleus is like a set of instructions. These instructions tell the cell how to build molecules needed for the cell to function properly. That is, the DNA tells the cell how to build molecules needed for life. The nucleus is surrounded by the nuclear envelope, a double membrane (two phospholipid bilayers) that controls what goes in and out of the nucleus. The nucleus also has holes embedded in the nuclear envelope. These holes are known as nuclear pores, and they allow things to flow in and out of the nucleus. 


Inside of the nucleus, you will find the chromosomes. Chromosomes are strands of DNA wrapped around proteins. They contain genes, or small units of genetic material (DNA) that contains the code for the creation of a protein. Human cells have 46 chromosomes (23 pairs). There are hundreds to thousands of genes on each chromosome. 


The nucleus of many cells also contains a central region called the nucleolus. The job of the nucleolus is to build ribosomes. These ribosomes flow out the nuclear pores into the cytoplasm. Ribosomes are organelles that make proteins in the cytoplasm. See the composition of the nucleus pictured below (Figure below).

Position of the nucleus inside a cell

In eukaryotic cells, the DNA is kept in the nucleus. The nucleus is surrounded by a double membrane called the nuclear envelope. Within the nucleus is the nucleolus.


  • The nucleus contains the genetic material of the cell.
  • The genetic material of the cell is found in chromosomes, DNA wrapped around proteins.
  • The nucleolus, which makes ribosomes, is also within the nucleus.

Explore More

Use the resources below to answer the following questions.

  1. What is chromatin? What molecules make up chromatin?
  2. What is the nuclear membrane? What kind of membrane is this membrane?
  3. What is the nucleolus? What molecules do you find there?
  4. How is the inside of the nucleus connected to the cytosol? Why is this connection vital for the cell?


  1. What is contained in the nucleus of a cell?
  2. What is a chromosome? Where are chromosomes located?
  3. How many chromosomes do humans have?
  4. What is the function of the nucleolus?

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chromosome Strands of DNA wrapped around proteins.
gene Unit of DNA that contains code for the creation of one protein.
nuclear envelope Double membrane surrounding the nucleus of a cell.
nucleolus Area within the nucleus where ribosomes are made.
nucleus Eukaryotic cell structure that contains the genetic material, DNA.
ribosome Organelle that makes proteins.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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