BLOOD


The secrets of human chromosomes have not yet been cracked by scientists, study suggests

Scientists have finally weighed a full set of human chromosomes and discovered they are 20 times heavier than expected - declaring there could be "missing components". Researchers told the Sunday Telegraph they have no idea as to what that may be. Chromosomes are bundles of genetic material which exist inside almost every cell of all complex lifeforms, from bacteria to humans and everything in between. Most humans have 46 chromosomes — 23 pairs — all of different size and shape, but other species have varying numbers. For example, possums have just 22, foxes have 34 and a great white shark has 82. But Atlas blue butterflies have around 450 and the Adder’s tongue fern has a staggering 1,440. But regardless of number or organism, all chromosomes follow the same basic structure. Individual bases of DNA, called A, G, C and T, pair up and form short, double helix-shaped chains which wrap around a ball of eight proteins to create bundles called nucleosomes. These little packages of genetic material are joined to one another by a thin piece of connective material, and experts refer to them as ‘beads on a string’. But while we know all this, and that the complete copy of a human genome contains more than 6.4billion base pairs of DNA, the exact and total mass of our chromosomes has never been known. Scientists from UCL used a powerful X-ray beam in Didcot, Oxfordshire, called Diamond, to weigh a complete set of human chromosomes for the first time. The researchers bombarded individual chromosomes with X-rays and assessed how much the beam scattered. This diffraction pattern was used to produce a 3D reconstruction of the chromosome’s structure. The brightness of the Diamond machinery, which outshines the Sun by billions of times, allowed for a highly detailed image. Professor Robinson and colleagues published their paper in the journal Chromosome Research and found the mass of all 46 human chromosomes to be 242 picograms. The heaviest is chromosome 1, which is also the largest, and it weighs 10.9 picograms. One picogram is a trillionth of a gram and a grain of sand weighs approximately 0.000000004 picograms. A red blood cell, which does not have a nucleus and therefore is devoid of genetic material, weighs around 27 picograms. “There may be quite a lot of missing components to our chromosomes that are yet to be discovered,” Professor Ian Robinson, senior author of the new study from UCL, told The Sunday Telegraph: “Chromosomes have been investigated by scientists for 130 years but there are still parts of these complex structures that are poorly understood.” He went on: "The mass of DNA we know from the Human Genome Project, but this is the first time we have been able to precisely measure the masses of chromosomes that include this DNA. “Our measurement suggests the 46 chromosomes in each of our cells weigh 242 picograms. “This is heavier than we would expect, and, if replicated, points to unexplained excess mass in chromosomes.” In order to accurately measure the chromosomal mass, the researchers blasted them with X-rays when the cells were in metaphase, before they underwent the splitting process. Scientists are constantly trying to learn more about the human body, and the mapping of the genome was a key step in that. However, this study lays bare the fact there is still a long way to go before we fully understand the nuances of our own body. Archana Bhartiya, a PhD student at the London Centre for Nanotechnology at UCL and lead author of the paper, said: “A better understanding of chromosomes may have important implications for human health. “A vast amount of study of chromosomes is undertaken in medical labs to diagnose cancer from patient samples. “Any improvements in our abilities to image chromosomes would therefore be highly valuable.”

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