Tiny ‘microchromosomes’ (<0.5 µm) are the building blocks of all animal genomes, but they underwent ‘dizzying rearrangement’ in mammals, including humans, according to a paper published in the Proceedings of the National Academy of Sciences.
La Trobe University’s Professor Jenny Graves and colleagues made the discovery by lining up the DNA sequence of microchromosomes that huddle together in the cells of birds and reptiles.
When these microchromosomes were first seen under the microscope, scientists thought they were just specks of dust among the larger bird chromosomes, but are actually proper chromosomes.
“Using advanced DNA sequencing technology, scientists can at last sequence microchomosomes end-to-end,” Professor Graves said.
“We lined up these sequences from birds, turtles, snakes and lizards, platypus and humans and compared them.”
“Astonishingly, the microchromosomes were the same across all bird and reptile species.”
“Even more astonishingly, they were the same as the tiny chromosomes of Amphioxus, a little fish-like animal with no backbone that last shared a common ancestor with vertebrates 684 million years ago.”
In marsupial and placental mammals, these ancient genetic remnants are split up into little patches on our big, supposedly normal, chromosomes.
“The exception is the platypus genome, in which the microchomosomes have all fused together into a few large blocks that reflect our oldest mammal ancestor,” Professor Graves said.
“The findings highlight the need to rethink how we view the human genome.”
“Rather than being normal, chromosomes of humans and other mammals were puffed up with lots of ‘junk DNA’ and scrambled in many different ways.”
“The new knowledge helps explain why there is such a large range of mammals with vastly different genomes inhabiting every corner of our planet.”
Paul D. Waters et al. 2021. Microchromosomes are building blocks of bird, reptile, and mammal chromosomes. PNAS 118 (45): e2112494118; doi: 10.1073/pnas.2112494118
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