Tertiary structures in the nucleic acids may have dramatic effect on their interaction with the proteins. These structures (i.e. pseudoknots, hairpins, loops, triplexes, etc.) are more common in RNA molecules . DNA is a more stable polymer and is commonly found in one of the three linear double helix forms where two nucleotide chains interact following the rules of complementarity (adenine binds to thymine and guanine binds to cytosine). Normally individual DNA chains do not produce tertiary structures. One of the exceptions are DNA quadruplexes — the complexes of loops in one of the DNA chains that are stabilized by the magnesium or potassium ions. Such structures may occur in the regions that are rich in guanine repeats . Sometimes the quadruplexes are useful for the cell. They may for example protect the chromosome ends . But the formation of the quadruplexes inside the genes can turn the gene function down because polymerases that copy information from the gene could not pass through this tertiary structure.
In the last several years the connection between DNA quadruplexes and the amyotrophic lateral sclerosis (ALS) was demonstrated. This is a yet incurable neurodegenerative disease which is characterized by the damage of the motor neurons in the central nervous system that leads to the paralysis. In ALS the DNA quadruplexes are formed in the GGGGCC repeats inside the C9orf72 gene that normally protects from neurodegenerative pathology [5, 6]. ALS affected famous physicist Stephen Hawking.
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