What we can learn from "Mutation"

What do you think when you hear the word 'mutation'? Fear? X-Men? Cancer? This word, which felt something negative, was the source of innovation in the history of evolution. And it can have a similar meaning to the change and development of information and intelligence.

In biology, a mutation usually means a permanent change in the sequence of a gene. DNA can be mutated for a variety of reasons, sometimes by external stimuli, by DNA replication process, or by repairing errors in DNA. There are also cases where mobile genetic elements (transposons) are inserted or deleted in existing DNA. Mutation does not always affect organism characteristics or phenotypes, but it also crosses generations with latent states. These may accumulate and cause explosive changes. Mutations are a cause of diseases such as cancer, hereditary diseases, and immune system abnormalities, but they are also the driving forces that make life adapt to the external environment and evolutions.

When a mutation occurs at a site that plays a vital role in life, the mutation can lead to death, but in some cases it may not bring about any change, and there may be only a few changes that are hard to notice. In a study using fruit flies, about 70% of the results of mutation had negative effects on them due to amino acid changes, but in the remaining cases there was no effect or in some cases it was beneficial to live.


DNA Paradox

DNA, or deoxyribonucleic acid, is a hereditary material that most organisms possess. Nearly every cell in a living organism has the same DNA. Human DNA consists of about 3 billion bases, and over 99 percent is the same for all human.

An important property of DNA is that it can be replicated. Each strand in the double helix of DNA can be used as a template for replicating the base sequences. So that new cell can have exactly the same copy of the DNA in the existing cell before it divides. This property makes information of life handed over generations.

DNA has a structure that can produce fewer errors when replicated than RNA. For example, DNA is not easily hydrolyzed and the outer skeleton naturally protects the inner base sequences, and its double helix has complementary double strands, one strand can be complementary even if the other strand is damaged. I think this is really surprising machinery for the information stability.

There are four types of mutation. Spontaneous mutations, mutations related with error-prone transposon synthesis, errors introduced during DNA repair, mutations due to mutagens. It is interesting to note that the mutation for stably designed DNA is highly related to the DNA repair systems. It's somewhat paradoxical.

Among these types, it is noteworthy that there is a growing evidence that majority of mutations are due to error-prone replication past DNA damage in the template strand. It is known that oxidative DNA damage occurs at least 10,000 times per cell per day in human cells. The damaged DNA is then repaired by the DNA repair system (translesion synthesis). In mice, most mutations occur during this repair process. Furthermore, the DNA double-stranded structure occasionally breaks down, and the mutation mainly occurs during the repair of this structure not from broken strands. Non-Homologous End joining (NHEJ) is the main method to recover double-strand breaks. NHEJ inserts nucleotides to fill the gaps in the broken DNA, removing some nucleotides and allowing incomplete alignments in advance. As a result, NHEJ occasionally causes mutations.

When a cell divides, sometimes the copy is not perfect. from Berkley.edu

In order to keep the genetic information stable, various DNA repair systems are working. But, they actually causing most mutations. What a paradox it is! However, these DNA's characteristics can not be said to be wrong. The structure and repair system of DNA actually works relatively well so that most genetic information does not change too much even though out environment continuously damaging cells. Some of these mutations open up the possibility of introducing changes in the next generation, and are playing a role in securing diversity for future evolution. DNA is doing great jobs balancing inheritance and evolution.

Another implication I want to mention from "mutation" for information and intelligence is related with neutral mutation. A neutral mutation is a mutation that does not affect the fitness of each individual organism, but if it is applied to the finite population, there is very important phenomenon called 'genetic drift' will appear. I will cover this issue in the next post.


References

DNA Damage, DNA Repair and Cancer