The word “mutation” usually conjures up all of its most terrifying consequences, from cancer to ninja turtles. But the vast majority of mutations are actually unlikely to make much of a difference in our lives! To understand why, we must first understand what mutations are. The DNA in each of our cells is divided into long chains of molecules called nucleotides. There are 4 kinds of nucleotides, and the sequence in which they appear on a chain of DNA is of paramount importance. This is because the nucleotide sequence is a blueprint for the creation of proteins, which are responsible for just about everything in cells. A mutation is defined as a change in the sequence of DNA, and when this change leads to a change in proteins, the results can be catastrophic. But why does the DNA sequence change in the first place?
When cells prepare for division they copy their DNA so that after splitting into two, both cells contain identical copies of the DNA. Mutations occur every time the DNA is copied due to the difficulty of copying long sequences perfectly and environmental factors (such as radiation and exposure to various chemicals). However, the vast majority of mutations are corrected by cellular mechanisms before they get the chance to do any harm. Of the tiny percentage of remaining mutations, most will not affect us simply because they are not in “coding” sequences. Roughly 99% of our DNA is not a blueprint for any protein, meaning that the chances of a mutation occurring in a protein-coding sequence, or gene, are vanishingly small.
Proteins are long chains of another kind of molecules – amino acids. In genes, every triplet of nucleotides, or codon, encodes a certain amino acid, thus determining the sequence of amino acids in proteins. Since there are more codons than amino acids, each amino acid can be encoded by several different codons, making these codons synonymous. This redundancy means that if a mutation turns one codon into a different codon that happens to be its synonym, the sequence of amino acids will remain unaltered.
But what if the mutation does change the amino acid sequence? Surprisingly, even this may prove to be insignificant. Proteins are not just chains of amino acids, they are folded chains of amino acids – each protein has a unique 3D fold upon which its functionality depends. Some segments of the amino acid chain will fold in a certain way to serve a certain purpose, such as binding to other molecules, cleaving molecules, or joining molecules. These functionally invaluable areas in the protein’s structure are known as protein domains. The areas between protein domains aren’t nearly as significant, meaning that if a mutation occurs between domains, it is unlikely to affect protein function.
At this point you might be wondering how many more caveats I can come up with now that we’ve narrowed in on protein domains as the sequences that really count, but there’s more! There are 20 different kinds of amino acids, which can be divided into 4 groups of amino acids with similar properties. So if a mutation in a protein domain turns an amino acid into a similar amino acid, the effect on the protein’s function may still be negligible.
You can probably guess where this is going – does it matter if a mutation in a protein domain turns an amino acid into a radically different amino acid? Not necessarily. Many of our genes encode proteins with roles in embryonic development, and these genes cease to be expressed once embryogenesis is completed. Consequently, if a mutation happens to drastically alter one of these genes later in life, it will go unnoticed.
Moreover, many genes are tissue-specific, meaning that different tissues express different subsets of genes. So if a mutation in a cell in one tissue were to wreak havoc on a gene that is only expressed in a different tissue, we wouldn’t even notice. In conclusion, ninja turtle-inducing mutations are probably rarer than they seem.
Comments