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Stop Codon Mutation: Sequence & Overview

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  • 0:00 What are Mutations?
  • 0:25 What are Stop Codons?
  • 2:00 Nonsense Mutations
  • 2:40 Nonstop Mutations
  • 3:16 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

Imagine if a traffic light stayed on red and never changed - or maybe it never even turned red at all. This would cause a lot of problems! Stop codons are like the stoplights for making proteins. Learn about stop codon mutations in this lesson.

What Are Mutations?

DNA is the unit of heredity of all organisms and is arranged into genes. Genes, including any mutations in them, can be passed on from parents to children. A mutation is any permanent change in a DNA sequence. Mutations don't have to be bad; some are beneficial while others have no effect. However, in this lesson, we will focus on potentially detrimental mutations involving stop codons.

What Are Stop Codons?

We'll get to what a stop codon is in a little bit. First, we need to learn more about DNA and genes. Genes are sequences of DNA that code for proteins, and proteins are made up of many amino acids. Nucleotides are the repeating units of a DNA sequence. There are four nucleotides, each with a different nitrogenous base: thymine (T), adenine (A), guanine (G), and cytosine (C). The order of these nitrogenous bases is what determines the sequences of amino acids in a protein.

The first step in making a protein, called transcription, is to use the DNA sequence of a gene to make an RNA molecule. RNA is made up of nucleotides and nitrogenous bases just like DNA, except that thymine is replaced with uracil (U). Then, this RNA molecule is used to assemble a chain of amino acids, or a protein, in a process called translation. Amino acids are specified by a 3-base sequence called a codon. Amino acids continue to be added to the chain until a stop codon is reached. A stop codon does not specify an amino acid but instead stops translation, just like a stoplight halts traffic.

Transcription makes a single-stranded RNA molecule from a double-stranded DNA molecule. Translation uses each codon to determine which amino acid belongs in the protein. (CAG codes for glutamine and CCC codes for proline. UAA is a stop codon.)
DNA to RNA to Protein

There are three different stop codons in RNA and DNA:

RNA DNA
UAG TAG
UAA TAA
UGA TGA

There are two types of mutations involving stop codons: nonsense mutations and nonstop mutations.

Nonsense mutations

Nonsense mutations occur when a codon that is supposed to specify an amino acid is changed to a stop codon instead. This results in a protein that is shorter than normal because it cannot finish adding all the necessary amino acids.

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