HSC Biology: Types of DNA Mutations – Base Mutations

8 Mins Read | Written by Simon Tang

Key Points Summary

  • Mutations are a change in the nucleotide sequence of DNA.
  • Mutations are broken into two different types: base and chromosomal mutations.
  • Base mutations involve changes to individual bases in the DNA sequence.
  • Examples include silent, missense, nonsense and frameshift.

Content

Introduction: What are mutations?

A mutation is a change in the nucleotide sequence of DNA. DNA is the genetic material that codes for who we are, and depending on when mutations occur during our development, these changes in our DNA could lead to physical changes to our body. 

What are the different types of mutations?

Mutations are best divided into two sections: Base and chromosomal mutations. This blog goes through base mutations, but if you want to read about chromosomal mutations, click here.

  • Base mutations occur when bases in a DNA sequence are added, deleted or exchanged for a different base. Remembering that DNA is just a long string of nucleotides (A, T, C, G), base mutations could be like changing a C to a G. 
  • Chromosomal mutations are when sections of whole chromosomes are affected. This is on a much larger scale than base mutations, as thousands, or potentially millions, of bases are affected. 

Types of Base Mutations

Silent mutations

A silent mutation is a type of substitution mutation (one base is replaced with another). This results in no change to the resulting protein and thus has almost no effect on an organism. 

To understand silent mutations, recall that there are redundancies in the genetic code; that is, a single amino acid may be encoded for by multiple, different codons

  • This means that if the base substitution results in a different codon encoding for the same amino acid, there will be no change to the polypeptide sequence and therefore the mutation is silent. See below the genetic table, which shows which codons code for which amino acids.

Sarah Greenwood, CC BY-SA 4.0 via Wikimedia Commons
Silent Mutations
For example, consider the following mRNA sequence:5’ – AUG CAU AAG UCA AUC AGA GGU UGA –3’
This would encode for the following polypeptide according to the table above:N – Met-His-Lys-Ser-Ile-Arg-Gly-STOP – C
Now suppose there was a T to G substitution mutation in the DNA, resulting in a corresponding change in the mRNA:5’ – AUG CAU AAG UCA AUC AGA GGG UGA –3’
Since both ‘GGU’ and ‘GGG’ encode for the same amino acid (glycine), the polypeptide sequence remains unchanged:N – Met-His-Lys-Ser-Ile-Arg-Gly-STOP – C

Missense mutations

When base substitutions do change the amino acid sequence, we call this a missense mutation. A missense mutation involves a change in one of the amino acids in a polypeptide sequence, but the whole polypeptide is still produced.

Missense Mutations
For example, consider the same original sequence as previously…5’ – AUG CAU AAG UCA AUC AGA GGU UGA –3’
…but now, the T to G mutation occurs at a different location:5’ – AUG CAU AAG GCA AUC AGA GGU UGA –3’
The original sequence had ‘UCA’ as the fourth codon, encoding for serine. The mutated sequence has ‘GCA’ instead, encoding for alanine. This will cause a change in the polypeptide sequence:N – Met-His-Lys-Ala-Ile-Arg-Gly-STOP – C

Note, however, that there are still seven amino acids in this sequence before the stop codon. The length of the polypeptide has not changed.

Nonsense mutations

Sometimes, a base substitution results in an early (premature stop codon). In such a case, polypeptide synthesis stops early and the protein is truncated. This type of base substitution is a nonsense mutation.

Nonsense Mutations
For example, consider the same original sequence as previously…5’ – AUG CAU AAG UCA AUC AGA GGU UGA –3’
…and supposed there is an A to T mutation:5’ – AUG CAU AAG UCA AUC UGA GGU UGA –3’
This results in a premature stop codon: ‘UGA’ where previously that codon was encoded for arginine. Instead of:N – Met-His-Lys-Ser-Ile-Arg-Gly-STOP – C
The polypeptide will be:N – Met-His-Lys-Ser-Ile- STOP – C

Truncated proteins are highly unlikely to function properly so, in most cases, nonsense mutations result in non-functional genes.

Frameshift Mutations

Sometimes, instead of simply substituting a single base, a base is inserted or deleted in the sequence. This results in a frameshift mutation that changes the reading frame during polypeptide synthesis. Frameshift mutations mean that any codon after the mutation will be read incorrectly. This results in drastic changes in the protein product, which renders the protein non-functional.

Frameshift Mutations
Once again, consider the previous sequence:5’ – AUG CAU AAG UCA AUC AGA GGU UGA –3’
Which would encode for the following polypeptide:N – Met-His-Lys-Ser-Ile-Arg-Gly-STOP – C
Now suppose there is an insertion in the second codon – an extra adenine is added:5’ – AUG CAAU AAG UCA AUC AGA GGU UGA –3’
This completely changes the reading frame…5’ – AUG CAA UAA GUC AAU CAG AGG UUG A –3’
…which would result in an entirely different protein product, including a premature stop codon:N – Met-Gln-STOP – C
Similarly, a deletion would also cause a frameshift. Suppose an adenine is deleted from the second codon:5’ – AUG C˽U AAG UCA AUC AGA GGU UGA –3’
Reading frame will become:5’ – AUG CUA AGU CAA UCA GAG GUU GA –3
The polypeptide sequence would then be:N – Met-Leu-Ser-Gln-Ser-Glu-Val – C

Conclusions

So there you have it! Base substitutions in a nutshell. Click here if you want to learn about chromosomal mutations, the next and final types of mutations you need to know. If you enjoyed this blog and want to know more about how we teach biology at Dymocks Tutoring, give us a ring at (02) 8774 2610 or book a free trial with us!

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