Biological Defense I: Protection from Foreign Nucleic Acids

Abstract

All organisms have to protect themselves from the invasion of other harmful species. There are very different systems of defense in various organisms. Here we consider defense from nucleic acids of viruses and bacteria. The simplest defense system, widespread in bacteria, is based on the chemical labeling of own DNA and the destruction of all unlabeled DNA that penetrates inside the cell. Of course, newly synthesized own DNA has to be labeled as soon as possible during the cell division to avoid destruction. Another, more complex system, found in prokaryotes, is based on recording short segments of DNA sequences of potential invaders in the cell’s DNA. If in these records is a DNA segment of an invader, the invader’s DNA is destroyed. This system, called CRISPR-Cas, received enormous attention in recent years due to its potential for DNA editing inside the cells. The third system of defense, which was found in all domains of life, is based on a library of short DNA segments incorporated into the cell genome. RNA molecules, transcribed from these segments, can bind RNA molecules with complementary segments. The binding results in the destruction of this RNA or inhibition of its functioning.