Chromatin is the ensemble of genomic DNA and a large number of proteins. Despite its fundamental role in biology of eukaryotic cells, scientists lack a comprehensive understanding of chromatin evolution.
In almost every human cell, two meters-long DNA has to fit within a nucleus that is just 8 millionths of a meter wide. Like wool around a spool, the extreme space challenge requires DNA to wrap around structural proteins called histones. This coiled genetic architecture, known as chromatin, protects DNA from damage and has a key role in gene regulation.
Histones are present in both eukaryotes, living organisms that have specialized cellular machinery such as nuclei and microtubules, and archaea, another branch of the tree of life consisting of single-celled microbes that are prokaryotic, meaning they lack a nucleus.
In eukaryotic cells, histones are modified by enzymes, continuously shapeshifting the genomic landscape to regulate gene expression and other genomic processes. Despite this fundamental role, the exact origin of chromatin has been shrouded in mystery.
“Our results underscore that the structural and regulatory roles of chromatin are as old as eukaryotes themselves,” Dr. Grau-Bové said. “These functions are essential for eukaryotic life — since chromatin first appeared, it’s never been lost again in any life form.”
Chromatin, with its structural proteins (histones) apparently came into existence along with eukaryotic cells. Since the probability of forming even a single moderate protein by natural processes is too remote to be expected in our universe, the apparently simultaneous appearance of chromatin and eukaryotes is not consistent with an evolutionary explanation.
The scientists hypothesize that chromatin evolved in this microbe as a result of selective pressures in the primordial environment of Earth.
“Viruses and transposable elements are genome parasites that regularly attack DNA of single-celled organisms,” said Dr. Arnau Sebe-Pedrós, a researcher in the Centre for Genomic Regulation at the Barcelona Institute of Science and Technology. “This could have led to an evolutionary arms-race to protect the genome, resulting in the development of chromatin as a defensive mechanism in the cell that gave rise to all known eukaryotic life on Earth.”
“Later on, these mechanisms were co-opted into elaborate gene regulation, as we observe in modern eukaryotes, particularly multicellular organisms.”