Types and Examples of DNA Mutations

How Changes in the Nucleotide Sequence Affect Amino Acid Coding

Mutation
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DNA mutations occur when there are changes in the nucleotide sequence that makes up a strand of DNA. These alterations can be caused by random mistakes in DNA replication or by environmental influences such as UV rays or chemicals. Changes at the nucleotide level go on to influence the transcription and translation from gene to protein expression. Changing even just one nitrogen base in a sequence can alter the amino acid that is expressed by that DNA codon which can lead to a completely different protein being expressed. These mutations can range from being completely harmless to potentially fatal.

Point Mutations

A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a sequence of DNA or RNA.
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A point mutation—the change of a single nitrogen base in a DNA sequence—is usually the least harmful type of DNA mutations. Codons are a sequence of three nitrogen bases in a row that is "read" by messenger RNA during transcription. That messenger RNA codon is then translated into an amino acid that goes on to make a protein that will be expressed by the organism. Depending on the placement of a nitrogen base in the codon, a point mutation may have no effect on the protein.

Since there are only 20 amino acids and a total of 64 possible combinations of codons, some amino acids are coded for by more than one codon. Often, if the third nitrogen base in the codon is changed, the amino acid won't be affected. This is called the wobble effect. If the point mutation occurs in the third nitrogen base in a codon, then it has no effect on the amino acid or subsequent protein and the mutation does not change the organism.

At most, a point mutation will cause a single amino acid in a protein to change. While this usually is not a deadly mutation, it may cause issues with that protein's folding pattern and the tertiary and quaternary structures of the protein.

One example of a point mutation that is not harmless is with the incurable blood disorder sickle cell anemia. This happens when a point mutation causes a single nitrogen base in a codon for one amino acid in the protein glutamic acid to code for the amino acid valine instead. This single small change causes a normally round red blood cell to instead be sickle-shaped.

Frameshift Mutations

Frameshift mutations are generally much more serious and often more deadly than point mutations. Even though only a single nitrogen base is affected, as with point mutations, in this instance, the single base is either completely deleted or an extra one is inserted into the middle of a DNA sequence. This change in sequence causes the reading frame to shift—hence the name "frameshift" mutation.

A reading frame shift changes the three-letter codon sequence for messenger RNA to transcribe and translate. That not only changes the amino acid—all subsequent amino acids are changed as well. This significantly alters the protein and can cause severe problems, even possibly leading to death.

Insertions

One type of frameshift mutation is called an insertion. As the name implies, an insertion occurs when a single nitrogen base is accidentally added in the middle of a sequence. This throws off the reading frame of the DNA and the wrong amino acid is translated. It also pushes the entire sequence down by one letter, changing all codons that come after the insertion, completely altering the protein.

Even though inserting a nitrogen base makes the overall sequence longer, that does not necessarily mean the amino acid chain length will increase. In fact, quite the opposite may be true. If the insertion causes a shift in the codons to create a stop signal, a protein may never be produced. If not, an incorrect protein will be made. If the altered protein is essential to sustain life, then most likely, the organism will die.

Deletions

Deletion is one last type of frameshift mutation and occurs when a nitrogen base is taken out of the sequence. Again, this causes the entire reading frame to change. It alters the codon and will also affect all amino acids that are coded for after the deletion. As with an insertion, nonsense and stop codons may also appear in the wrong places,

DNA Mutation Analogy

Much like reading text, the DNA sequence is "read" by messenger RNA to produce a "story" or an amino acid chain that will be used to make a protein. Since each codon is three letters long, let's see what happens when a "mutation" occurs in a sentence that uses only three-letter words.

THE RED CAT ATE THE RAT.

If there was a point mutation, the sentence would change to:

THC RED CAT ATE THE RAT.

The "e" in the word "the" mutated into the letter "c". While the first word in the sentence is no longer the same, the rest of the words still make sense and remain what they're supposed to be.

If an insertion were to mutate the above sentence, then it might read:

THE CRE DCA TAT ETH ERA T.

The insertion of the letter "c" after the word "the" completely changes the rest of the sentence. The second word no longer makes sense, nor do any words that follow it. The entire sentence has changed into nonsense.

A deletion would do something similar to the sentence:

THE EDC ATA TET HER AT.

In the example above, the "r" that should have come after the word "the" has been deleted. Again, it changes the entire sentence. While some of the subsequent words remain intelligible, the meaning of the sentence has completely changed. This demonstrates that even when codons are changed into something that isn't total nonsense, it still completely changes the protein into something that is no longer functionally viable.