Why pseudogenes matter

Genes are what code for what we are: what we look like, how our bodies maintain themselves, and in some cases they even influence our behavior. This is relatively common knowledge, though many people don’t necessarily understand how genes accomplish this.

Genes are stretches of DNA or deoxyribonucleic acid. The important part of DNA is composed of four different molecules that can be described with four letters: A, C, T and G, which stand for adenine, cytosine, thymine and guanine, respectively. Every three letters of DNA codes for a specific amino acid, and these chains of amino acids make proteins. Proteins perform practically every function in our body, ranging from absorbing light in our eyes, fighting off infections, allowing for electrical impulses to travel in our nervous system and carrying oxygen in our blood. This is why the code imbedded in our DNA is so important.

transcription_2

A diagram illustrating how DNA makes proteins (=amino acid chain).

Every triplet of DNA letters that codes for an amino acid is called a codon. There are two special cases of codons: start codons and stop codons. Start codons (ATG) tell the cell where the protein begins. Stop codons (TAA, TAG, TGA) tell the cell where the protein ends. When you compare the DNA of a gene in various animals, quite often the stop codon is in the same place. Occasionally, certain mutations can arise that cause the stop codon to appear earlier than it should. For example, TAT codes for the amino acid tyrosine, but if the 2nd T changes to an A or a G, it will become a stop codon instead. The result is that the protein becomes truncated, which almost always yields a nonfunctional protein. Genes that yield nonfunctional proteins are called pseudogenes.

frameshift

Genes showing various types of inactivating mutations, rendering them pseudogenes.

As an example, CNGB3 (cyclic nucleotide gated channel beta 3) is a protein that is necessary for both color vision and vision under bright light conditions. An early stop codon in the gene encoding this protein will cause the individuals with this mutation to have their vision so severely impaired that they have daytime blindness and cannot discriminate colors, a condition called achromatopsia.

Now imagine that a mutation like this was beneficial to the individual who has it. Or perhaps it’s not necessarily beneficial but can be characterized as neutral, neither helping nor hindering the individual. We can imagine that when this person has children, and their children have children, etc. then it might spread through the population. This is especially likely to occur if it’s beneficial (=natural selection). If it spreads enough, then an entire species might have a pseudogene version of CNGB3 instead of a functional version.

It might seem strange to think that daytime and color blindness could be beneficial or even neutral, but let’s imagine that this mutation occurred in a subterranean mammal, like a mole. Many moles live in almost complete darkness, spending nearly all of their time underground, so why would they need color vision or the ability to see in the bright conditions of the day? Having the proper cells for this type of vision (cone cells) might be energetically costly and therefore have a negative effect on the organism to maintain, so it’s not too difficult to imagine why such a gene might be lost in a species.

In fact, this is what scientists think has happened numerous times in different organisms, and we ultimately think this represents evidence of evolution. Therefore, whenever we find pseudogenes, we think that they reflect the evolutionary past of a species or a common ancestor it shares with other species, which lost the function of a gene due to adaptive or neutral processes.

While this is consistent with evolutionary theory, it does raise questions as to why God might have created species that have nonfunctional genes with functional counterparts in other species.

Questions for Creationists

Can you think of why pseudogenes may become fixed in a species via evolution? Can you think of a reason why God might have created organisms with pseudogenes? Would it not have been easier to create organisms without them?

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