Researchers at the University of Bristol have discovered how the microbes responsible for human African sleeping sickness produce sex cells.
In these one-celled parasites, called trypanosomes, each reproductive cell in turn separates from the parental germ cell, which is responsible for transmitting genes. Conventional germ cells divide twice to produce all four sex cells – or gametes – simultaneously. In humans, four sperm are produced from a single germ cell. So, these strange parasitic cells do their own job rather than sticking to the rules of biology.
The cell biology of trypanosomes has already revealed several curious features. They have two unique intracellular structures – the kinetoplast, a circular DNA network, and the glycosome, a membrane organelle that contains glycolytic enzymes. They don’t follow the central dogma that DNA is faithfully transcribed into RNA, but will go back and edit some of the RNA transcripts once they’ve been done.
Professor Wendy Gibson of the School of Biological Sciences at the University of Bristol led the study. She said: “We’ve gotten used to trypanosomes doing things their own way, but of course what we think of as normal cell biology is based on very few so-called model organisms like yeast and mice. There is a whole world of weird and wonderful. One-celled organisms – protozoa – out there that we don’t know much about! Trypanosomes have gained more attention because they are such important pathogens – both humans and their livestock. “
Biologists believe that sexual reproduction evolved very early, after the appearance of the first complex cells a few billion years ago. Sex cells are produced by a special form of cell division called meiosis which halves the number of chromosomes, so gametes only have one complete set of chromosomes instead of two. The sets of chromosomes of two gametes combine during sexual reproduction, producing new combinations of genes in the offspring. In the case of pathogens like the trypanosome, sex can potentially lead to the combination of many harmful genes in a single strain. Thus, research on sexual reproduction helps scientists understand how new strains of pathogens appear and how characteristics such as drug resistance spread between different strains.
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