Sperm are masters of DNA packing

During sperm production, an enormous amount of DNA has to be packed into a very small space without breaking anything. A central role is played by certain proteins around which the DNA thread is wrapped, called the protamines. A recent study by the University of Bonn provides new insights into this important mechanism. The results have been published in the journal PLoS Genetics.

During their production, sperm are faced with an almost insoluble task. They have to pack 23 DNA threads with a total length of one meter into a head with a diameter of just three thousandths of a millimeter. And in the process, the delicate threads must not become entangled in an inextricable knot, nor must they tear.

We often sit on an overpacked suitcase to close it. The body resorts to a similar trick during spermatogenesis. Normally, DNA forms a comparatively loose tangle. In sperm cells, however, it is enormously compressed. Biologists call this process hypercondensation. During hypercondensation, chemicals called protamines exert a very strong attraction on DNA. The thread therefore wraps itself in very firm and tightly loops around the protamine

Most mammals seem to produce only one type of protamine, PRM1. Humans, and also rodents like mice, produce a second type, PRM2. Exactly what this second protamine is needed for was not known until now. However, it was known that some parts of it are successively cut off during sperm development. These cut-off parts that appear to be immensely important, according to the new study. When mice produce only a truncated PRM2 molecule that lacks the cut-off snippets, they are infertile.

It is possible that a defective protamine 2 can also lead to infertility in human males. The team now plans to investigate this hypothesis further. This could also lead to new therapies against male infertility, the researchers hope.

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