#histones

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Chromosomes are a feat of tidy packaging, ensuring all our DNA fits inside a cell nucleus, while keeping it accessible for duplication or transcription as needed. Our 23 pairs of chromosomes contain over 6.4 billion base pairs (the G-C and A-T ‘building blocks’) of DNA in total, representing approximately 2 metres of DNA per chromosome. Interactions with proteins package this into chromatin: DNA is first wrapped around histone proteins, forming nucleosomes, then condensed further, into higher-order structures that are less well understood. To start unravelling their mysteries, we need to know how much protein is involved, so scientists used a powerful X-ray to measure the number of electrons in a set of human chromosomes (pictured), and from that calculated their mass. Weighing in at 242 picograms (or a trillionth of a gram) in total, they may seem diminutive, but are actually far heavier than expected, suggesting there is still much to learn about the structure of chromosomes.

Written by Emmanuelle Briolat

Image from work by Archana Bhartiya and colleagues

London Centre for Nanotechnology, University College, London, UK

Image copyright held by the original authors

Research published in Chromosome Research, March 2021

How does the 2 m DNA fit into a nucleus of 10-15 μm diameter? The answer lies in DNA packaging.

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How are long strands of DNA packed into tiny cells?

Scientists are a step closer to understanding how DNA, the molecules that carry all of our genetic information, is squeezed into every cell in the body. How DNA is "packaged" in cells influences the activity of our genes and our risk for disease. 

DNA is a long, floppy molecule, and there's more than three feet of it in every cell. Our DNA is housed in structures called chromosomes, which condense the DNA to fit into the cell's tight quarters.

Scientists from the department of Biochemistry and Biophysics at the University of Rochester School of Medicine and Dentistry worked with colleagues in France and Japan to describe the first step of DNA packing in a cell. They provided the first-ever detailed picture of the most basic building block of chromosomes, known as the nucleosome, and found that a protein known as H1 (for linker histone H1) helps DNA become more compact and rigid within the nucleosome. In contrast, when H1 isn't present, the DNA is loose and flexible.

The tight structure that H1 creates helps shield our DNA from various factors that can activate or "turn on" certain genes. Without H1, DNA is more accessible to factors that could trigger disease-causing genes.