Introns: Definition & Functions

Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Did you know that not every letter in your DNA sequence codes for a protein? In this lesson you will learn about one specific type of DNA sequence that does not code for proteins called introns.

Introns are Part of Genes

Every person has about 3 billion letters of DNA in each one of their cells. The letters, called nucleotides or bases, are organized into genes. But not every DNA letter within a gene does the same thing. Bases that are within regions called introns were once thought to be just 'extra' DNA with no specific purpose. But now we know they are vital to human development. Before we get into more detail about what introns actually do, let's first briefly review what genes are and how they are organized.

How a Gene Codes for a Protein

'Gene' can have several meanings. It is something you inherit, like the gene that gives you attached or detached earlobes. A gene is also a set of instructions to make something. Typically that 'something' is a protein. Finally, a gene is a piece of DNA. A piece of DNA is the instructions to make a protein. And when that protein is made it can have a direct effect on your earlobes.

DNA is made up of just 4 different bases. Those 4 bases are abbreviated A, C, G and T. At the right time and place, part of the DNA code is copied into a long molecule called messenger RNA (mRNA). The mRNA then finds a ribosome, which 'reads' the informational code in the mRNA and assembles a protein out of amino acids. Different combinations of A, C, G and T dictate the order of the amino acids and the function of the protein. Sometimes along the way, parts of the mRNA molecule are removed which affects the exact protein that is made.

It's similar to a toy factory that uses buttons, tabs, stickers, gears and 16 other widgets as toy components. When the component parts are assembled in different orders and amounts you can get lots of different toys. The factory worker (ribosome) needs to read the order of the instructions (DNA and mRNA) to know how to assemble a specific toy (protein). If mRNA instructions have something added or left out the end product toy will be different.

Gene Structure

The 4 major parts of a gene are:

  • Promoter - controls the expression of a gene, which means it controls when, where (in what cells) and how much of the protein is made
  • Exon - these are the bases that actually code for the amino acids in the protein that is being made. The code is copied onto the mRNA strand that travels to the ribosome.
  • Introns - these bases are copied into the mRNA, but are sometimes 'edited out' or spliced out of the mature mRNA before it is read by the ribosomes.
  • Untranslated regions - bases that are copied into the mRNA strand, but do not code for amino acids. They regulate the interaction of the mRNA with the ribosome.

Functions of Introns

The term 'intron' is derived from 'intragenic region of a gene'. Therefore an intron is a DNA sequence within a gene. You may be wondering why genes have extra DNA within them. Not all species have introns, but most higher eukaryotes (i.e. animals and plants) do. Introns play several important roles, which are described below.

There are approximately 22,000 genes in the human body. That is far less than was originally thought before the human genome was sequenced. How can something as complex as the human body have so few instructions? The answer is introns! By having a DNA sequence that may or may not be 'edited out' before the final product, the human body can create different protein products from one gene.

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