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3.3: Protein Synthesis and Gene Expression

Difficulty Level: At Grade Created by: CK-12
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How do you build a protein?

Your body needs proteins to create muscles, regulate chemical reactions, transport oxygen, and perform other important tasks in your body. But how are these proteins built? They are made up of units called amino acids. Just like there are only a few types of blocks in a set, there are a limited number of amino acids. But there are many different ways in which they can be combined.

Introduction to Protein Synthesis

A monomer is a molecule that can bind to other monomers to form a polymer. Amino acids are the monomers of a protein. The DNA sequence contains the instructions to place amino acids into a specific order.

When the amino acid monomers are assembled in that specific order, proteins are made, a process called protein synthesis. In short, DNA contains the instructions to create proteins. But DNA does not directly make the proteins. Proteins are made on the ribosomes in the cytoplasm, and DNA (in an eukaryotic cell) is in the nucleus. So the cell uses an RNA intermediate to produce proteins.

Each strand of DNA has many separate sequences that code for a specific protein. Insulin is an example of a protein made by your cells (Figure below). Units of DNA that contain code for the creation of a protein are called genes.

Amino acid sequence of insulin

Insulin. Each blue or purple bead represents a different amino acid. Just 20 different amino acids are arranged in many different combinations to make thousands of proteins.

Cells Can Turn Genes On or Off

There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. Does every human cell make the same proteins? No. In a multicellular organism, such as us, cells have specific functions because they have different proteins. They have different proteins because different genes are expressed in different cell types (which is known as gene expression).

Imagine that all of your genes are "turned off." Each cell type only "turns on" (or expresses) the genes that have the code for the proteins it needs to use. So different cell types "turn on" different genes, allowing different proteins to be made. This gives different cell types different functions. 

Once a gene is expressed, the protein product of that gene is usually made. For this reason, gene expression and protein synthesis are often considered the same process.


  • DNA contains the instructions to assemble amino acids in a specific order to make protein.
  • Each cell type only "turns on" (or expresses) the genes that have the code for the proteins it needs to use.
  • Gene expression and protein synthesis are usually considered the same molecular process.

Explore More

Use the resource below to answer the questions that follow.

  1. What is the cell structure used in the assembly of proteins?
  2. What is the molecule that delivers the amino acids?
  3. What ends protein synthesis?


  1. What is a gene?
  2. What is an amino acid?
  3. If every human cell has the same genes, how can they look and function so differently?
  4. What is the relationship between DNA and proteins?

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amino acid Building blocks of proteins.
gene Unit of DNA that contains code for the creation of one protein.
gene expression Process of using a gene to make a gene product.
protein synthesis The process of making a protein.

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