How do you pack your DNA?

Human cells each contain approximately two metres of DNA on which our genetic blueprint, the recipe of life, is encoded. The challenge is how to store this huge volume of information for quick access when the instructions are needed to make new proteins for growth, repair and reproduction.

This challenge is not unique to humans. All non-bacterial organisms, including plants, fungi and algae, must achieve this task. But some organisms do this differently, so researchers in the University of Melbourne’s School of Botany are looking at how this may have evolved.

In human cells, DNA is packaged into the cell’s control centre which is called the nucleus and is only six millionths of a metre across. 

Ross Waller from the School of Botany puts this into perspective, saying if the nucleus was the size of a basketball, the DNA packed within it would be 100km long. 

“During the early evolution of life, as cells became more complex and their quantity of DNA proportionally larger, a brilliant solution to organising DNA evolved. Molecular spools, like bobbins on a sewing machine, made of proteins called ‘histones’ evolved,” says Dr Waller.

DNA is wrapped around each spool twice, then twice around the next spool, and so on. The spools are neatly organised in ordered arrays that enable access to each section of DNA when needed. The spools themselves can also be labeled to tell the cell what genetic information occurs where, and at what time in the cell’s life this information will be needed. 

“The invention of this genetic cataloguing system was such a powerful innovation that nearly all organisms continue to use it,” Dr Waller says.

Key exceptions to this rule are dinoflagellates – single-celled algae that are crucial components of marine and aquatic environments.

Many are photosynthetic, using sunlight to produce energy, and so provide a food source for many other species and form essential partnerships for reef-building corals (coral bleaching is the loss of the dinoflagellate). They are also infamous for ‘red tides’ which occur when dinoflagellates get to such large numbers that the water may appear golden or red, impacting other marine creatures.

Sebastian Gornik studies dinoflagellates in Dr Waller’s laboratory and has recently discovered that their nuclei are radically different. Their DNA quantity is greatly increased – using the basketball size analogy, up to 10,000km long – and they have stopped making the molecular histone spools. 

Dr Gornik has shown that dinoflagellates pack their vast lengths of DNA using a new protein that, amazingly, they stole from a virus. This act of molecular theft has enabled dinoflagellates to invent a new system of genetic organisation.

He says this new insight into dinoflagellates tells us the system for organising nuclear DNA might not always be as simple and widespread as the histone-spool model leads us to believe. 

“We are now investigating how dinoflagellates were weaned from their dependence on histones, and also what new capabilities might have been gained by dinoflagellates switching to a different DNA packing protein.

“But the million dollar question is why dinoflagellates have so much more DNA than other organisms, even humans,” Dr Waller says.

“In the past it has been speculated that the ’excess DNA’ is involved in packaging itself, in place of the usual DNA-packing proteins. To explore this we are currently working on sequencing one of these dinoflagellate genomes which no one had previously done because of their enormous size.

“So we hope to gain insights into how the DNA is organised, and perhaps also why dinoflagellates have so much of it. But it also might just be simply because they can!”

Dr Waller and Dr Gornik’s recent discoveries will be published in the 18 December issue of Current Biology.

www.botany.unimelb.edu.au