Mutate a Gene

A mutation is a permanent change in a DNA sequence. Some mutations can affect the protein a gene codes for.

The short DNA sequence below comes from the middle of a much longer gene. Change the DNA letters and see how it affects the resulting mRNA and protein. How do I edit the gene?

How do I edit the gene?

  1. Point mutation (change one DNA letter)
    1. Select a base
    2. In the menu that pops up, choose the letter you want to change it to
  2. Deletion mutation
    1. Select a base
    2. In the menu that pops up, choose the trashcan icon (see left image)
  3. Insertion mutation
    1. Select an arrow (between bases)
    2. In the menu that pops up, choose which base you want to add (see right image)
select the trashcan icon from the pop up menu to delete a mutationuse the arrows to select a base

What are the colors on the amino acids showing?

What are the colors on the amino acids showing?

Amino acids are different colors to start. These colors have to do with each amino acid’s chemical properties:

red = acidic (positive)
blue = basic (negative)
grey = hydrophobic
yellow = sulfur molecules; these can form bonds with each to make disulfide bridges, which help to stabilize a protein's three-dimensional structure

After you make a mutation, color highlights appear around some amino acids. These colors indicate the type of change you made:

green = silent mutation (the amino acid didn't change)
red = amino acid substitution (the amino acid changed)
blue = frame shift (multiple amino acids changed)

Genes are instructions for building proteins, and they are written in a literal code. To read a gene, a cell first makes an mRNA copy. Then it reads the mRNA letters to build a protein.

For a refresher on how cells read genes, visit Transcribe and Translate a Gene .

Codons and Reading Frame

Cells read a gene’s DNA code as a series of three-letter “words” called codons. Each codon codes for a specific amino acid. Genes also have a reading frame. Take this DNA sequence as an example:

GCATGCTGCGAAACTTTGGCTGA

You could divide it into codons in 3 ways, each with a different reading frame:

  1. GCA TGC TGC GAA ACT TTG GCT GA
  2. G CAT GCT GCG AAA CTT TGG CTG A
  3. GC ATG CTG CGA AAC TTT GGC TGA

Only one of these reading frames will code for a working protein. How can you tell which is the right one? It’s a little like finding the right spacing for the letters in a sentence. For example:

thesunwashotbuttheoldmandidnotgethishat

Only one reading frame makes a sentence that follows the rules of the English language: The sun was hot but the old man did not get his hat.

There are rules for reading genes too. For example, all protein-coding sequences begin with the codon “ATG” (AUG in mRNA). The correct reading frame will contain this codon.

Why isn't the start codon in DNA complementary to AUG?

DNA sequence

The order of DNA letters in a gene specifies the order of amino acids in the protein it codes for.

Types of Mutations

DNA mutations

Mutation is a process that causes a permanent change in a DNA sequence. Changes to a gene’s DNA sequence, called mutations, can change the amino acid sequence of the protein it codes for—but they don’t always.

Point Mutations
A point mutation is a change to single DNA letter. They fall into three categories:

  • Missense mutations cause a single amino acid change in the protein.
  • Nonsense mutations make a premature "stop" codon. Any codons after that are not translated, and the resulting protein is missing amino acids.
  • Silent mutations code for the same amino acid as before.

Insertion and Deletion Mutations
Insertion mutations add one or more DNA bases. Deletion mutations remove one or more DNA bases. Unless they happen in multiples of 3 bases, insertions and deletions shift the reading frame—which is why they’re also called frameshift mutations. A frameshift changes the grouping of bases into codons, affecting all the amino acids downstream of the mutation. The cell will keep reading until it reaches a stop codon, which may happen earlier or later than in the original sequence.

The Universal Genetic Code

All living things read genes the same way. They read codons to build proteins according to a Universal Genetic Code. Each 3-letter codon codes for a specific amino acid (or, in three cases, a “stop” codon).

When mutation changes a gene’s DNA sequence, you can use a codon look-up table to predict changes to the protein it codes for. Some codon changes are more impactful than others. For example, since multiple codons code for the same amino acid, some mutations will not change the protein. And when the protein does change, there is a greater impact when the substituted amino acid has a different size and chemical properties (e.g., Glu to Gly) than when it is similar to the original (e.g., Glu to Asp).

Related Content

To see how a protein’s amino acid sequence affects its function, visit the interactive Form is Function: Why a Protein's Shape Matters.

Because all living things read genes the same way, they can also read each other’s genes! To learn more, watch the video Same Gene, Different Organism.

Universal Genetic Code

This graphic is based on the codon look-up table in Miller and Levine's Biology textbook. For more information, see their description. Click image for full-size table and instructions.

It's not a mistake when we say that ATG is a start codon. Scientists generally consider AUG to be a start codon in mRNA sequence and ATG to be a start codon in a DNA sequence.

But...
If AUG on an mRNA molecule means "start,"
and mRNA is copied from a DNA template,
and the DNA template is complementary to the mRNA copy,
then why isn't a DNA start codon TAC?

The key thing to remember is that DNA is double stranded.

Here's a DNA sequence, with the start codon in red:

GC ATG CTG CGA AAC TTT GGC TGA

We've shown the sequence of just one of the DNA strands. It's a shortcut, and it's tidier to look at, and it's how DNA sequences are typically written. If we wanted to, we could include the sequences of both strands:

GC ATG CTG CGA AAC TTT GGC TGA
CG TAC GAC GCT TTG AAA CCG ACT

While our shorthand version shows just the top strand, it's actually the bottom strand that RNA polymerase reads to build an mRNA molecule. And if we're being literal about the actual nucleotides in the DNA strand that are read to build the mRNA's AUG start codon, we might consider the start codon on a DNA molecule to be TAC.

But that's not quite right. The chemical structure of DNA gives it a polarity, and the two complementary DNA strands are anti-parallel. That is, the 5' (5-prime) and 3' (3-prime) ends of the two DNA strands face in opposite directions:

5' GC ATG CTG CGA AAC TTT GGC TGA 3'
3' CG TAC GAC GCT TTG AAA CCG ACT 5'

The scientific standard is to write a nucleotide sequence from 5' to 3'. That means we'd have to write the sequence of the bottom strand like this:

5' TCA GCC AAA GTT TCG CAG CAT GC 3'

It would be more accurate to say that the DNA sequence of the "start codon" on the bottom strand is CAT. But that's an inconvenient way to talk about a protein-coding DNA sequence: everything's not only complementary but also backwards.

For the sake of ease and clarity, scientists tend to ignore the bottom strand (they call it the "non-coding" or "antisense" strand). Instead, they refer to the sequence of the "coding" or "sense" strand: the one that's almost identical to mRNA—the difference of course being that every T in DNA is replaced by a U in RNA. They know there's another strand, and they know how to figure out what its sequence is if they need to.


APA format:

Genetic Science Learning Center. (2019, June 10) Mutate a Gene. Retrieved March 24, 2024, from https://learn.genetics.utah.edu/content/genetics/mutate/

CSE format:

Mutate a Gene [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2019 [cited 2024 Mar 24] Available from https://learn.genetics.utah.edu/content/genetics/mutate/

Chicago format:

Genetic Science Learning Center. "Mutate a Gene." Learn.Genetics. June 10, 2019. Accessed March 24, 2024. https://learn.genetics.utah.edu/content/genetics/mutate/.