A gene is essentially a sentence made up of the bases A, T, G, and C that describes how to make a protein. Any changes to those instructions can alter the gene's meaning and change the protein that is made, or how or when a cell makes that protein.

Gene mutations that involve changes in one or a few nucleotides are called point mutations because they occur at a single point in the DNA sequence. There are three types of point mutations.

Substitutions

In a substitution mutations, one base is changed to a different base. Substitutions usually affect no more than a single amino acid and sometimes they have no effect at all. A substitution mutation is equivalent to changing one letter in a sentence, such as this example, where we change the 'c' in cat to an 'h':

Original The fat Cat ate the wee rat.
Substitution The fat hat ate the wee rat.

Image shows original DNA sequence with matching mRNA and amino acids. Second image shows a substitution mutation with the changed DNA, mRNA and Amino Acids.

Insertion

Mutations that result in the addition of extra DNA are called insertion mutation.

Original The fat cat ate the wee rat.
Insertion The fat cat aat eth ewe era t.

Image shows original DNA sequence with matching mRNA and amino acids. Second image shows an insertion mutation with the changed DNA, mRNA and Amino Acids.

Deletion

Mutations that result in missing DNA are called deletion mutation. In this example, the deletion eliminated the letter c.

Original The fat cat ate the wee rat.
Insertion The fat ata tet hew eer at.

Image shows original DNA sequence with matching mRNA and amino acids. Second image shows a deletion mutation with the changed DNA, mRNA and Amino Acids.

Insertions and deletions are also called frameshift mutation. Because our cells read DNA in three letter "words," adding or removing one letter changes each subsequent word. This type of mutation can make the DNA meaningless and often results in a shortened protein.

Self-Assessment

See if you can identify each of these changes in DNA and explain its effect on an organism.

mRNA Codon Chart

Second Base
U C A G
U Phenylalanine Serine Tyrosine Cysteine U
Phenylalanine Serine Tyrosine Cysteine C
Leucine Serine Stop Stop A
Leucine Serine Stop Tryptophan G
C Leucine Proline Histidine Arginine U
Leucine Proline Histidine Arginine C
Leucine Proline Glutamine Arginine A
Leucine Proline Glutamine Arginine G
A Isoleucine Threonine Asparagine Serine U
Isoleucine Threonine Asparagine Serine C
Isoleucine Threonine Lysine Arginine A
Methionine Threonine Lysine Arginine G
G Valine Alanine Aspartic Acid Glycine U
Valine Alanine Aspartic Acid Glycine C
Valine Alanine Glutamic Acid Glycine A
Valine Alanine Glutamic Acid Glycine G

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