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Tiny versions of chromosomes show up in things like birds, reptiles, and amphibians. These mysterious lint-like flecks may be the building blocks for our entire genomes.

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Go to Brilliant.orgSciShow to take  your STEM skills to the next level! [♪ INTRO] Imagine you’re a scientist peering  down the microscope at some bird cells. But your view, unfortunately, is  blocked by little tiny flecks of lint.

Well that is exactly what happened  when scientists first discovered the little genetic mysteries that we’re  going to be talking about today. Except with some careful cleaning,  scientists realised it was not dust at all, they were actually minuscule chromosomes. And as they looked more and more  into it, and picked up better genetic sequencing techniques over the years,  scientists would come to discover just how crucial these tiny chromosomes are.

In fact, they might just be the  building blocks for entire genomes. So, chromosomes, right. The little  X’s we learn about in high school bio.

All our DNA is contained in those long  stretches, which can condense down to those convenient X shapes when the  cell is duplicating them to divide. They’ve got caps on the ends, called  telomeres, and joining points somewhere near the middle of the X, called centromeres. And that’s just how cells store  and handle their DNA, right?

No! Wrong! Because the instant you look at the cells  of any vertebrate that’s not a mammal, you see much smaller lengths  of

DNA: microchromosomes. Typically, microchromosomes are around  20 million rungs of the DNA ladder. That might sound like a lot, but it’s less  than one and a half micrometers in size. Compare that to regular chromosomes, which can be 20 micrometers long, or longer.

Which might explain why scientists  initially thought they were lint. See, early cell biology was all about  grabbing a sample of cells, staining them and carefully looking at them under the microscope. So a little hard to tell what was what.

But the more researchers looked, the  more they saw these so-called specks in all sorts of vertebrates: birds,  fishes, amphibians, turtles. Even in some insects, like water bugs. Just not mammals.

Then, as methods got better, scientists  noticed that the specks not only looked like regular, big, chromosomes.  They acted like them, too. They replicate when the cell divides, and they’re transcribed to make proteins.  It’s just very small. But the researchers still  couldn’t figure out how these microchromosomes got there,  or what they actually did.

One of the first ideas was that  they weren’t really chromosomes, but instead a dense jumble of junk  DNA that didn’t really do anything. Which did make sense as a  theory, if you consider how mammal-centric a lot of biology can be. I mean, if mammals don’t have them, they  couldn’t possibly be real chromosomes.

But in recent years, scientists  have been able to sequence smaller chunks of DNA in more detail,  and that’s helped us understand exactly what’s going on with microchromosomes. Early studies in chickens  found that microchromosomes actually contain a lot of genes. And not just any old genes, but  about half the chicken genome.

This told researchers that microchromosomes are important parts of the bird genome. Then, in a 2021 study, researchers lined  up and compared the DNA sequences of both microchromosomes and  macrochromosomes for different animals. The study included 7 species of bird, 10  snakes and lizards, three mammals, and a modern relative of early  vertebrates: the lancelet.

By comparing the DNA of different groups  of animals, scientists could look at where microchromosomes came  from, evolutionarily speaking. And they could map how the sequences  of microchromosomes changed as different groups branched off  and evolved into new ones. When they compared the microchromosomes  of birds, turtles, snakes, and lizards, researchers saw that they were  all pretty similar overall.

But different reptile or bird groups shared different sections of their microchromosomes. Like, even though emu  microchromosomes had 21 regions that were similar to either some lizards or some  turtles, it only shared eight with both. And some of those microchromosomes were found in different bird or reptile macrochromosomes too.

What this tells scientists is that  microchromosomes probably joined up at some point along their evolutionary journey to  form the big chromosomes we know today. And how they joined up and shuffled around was slightly different in different birds or reptiles. The kicker here is where the lancelet comes in.

The researchers found that almost  all bird and reptile microchromosomes lined up with one or two of the  lancelet’s macrochromosomes. Finding that, in an early  offshoot of all vertebrates, means that these microchromosomes  must have been present in an ancient ancestor of vertebrates that lived  more than 684 million years ago. And then those microchromosomes were passed down to birds, lancelets,  and everything in between.

So it’s not microchromosomes that are weird. It’s us mammals, with our  giant, chunky mega-chromosomes. And the researchers proposed  a reason for that, as well.

Although our chromosomes  don’t look like they’ve been built by fusing together smaller ones,  that’s just because their DNA has become so jumbled up that you can’t even  recognize the microchromosomes anymore. How do we know this? Why,  from looking at the world’s weirdest mammal, of course: the platypus.

The platypus represents one of the  oldest branches of the mammal group. Platypuses don’t have microchromosomes,  but their macrochromosomes are made up entirely of microchromosomes  that have stuck together. Scientists think other  mammals might have started out with chromosomes that look like the platypus’s.

But over time, their DNA  got shuffled around enough that the microchromosomes  aren’t recognizable any more. And all that adding in and jumbling might explain why our chromosomes today have  so many repeats and junk DNA. It’s like continuously buying new  clothes, and never donating the old ones.

Your wardrobe gets bigger and bigger and you end up with a couple of the same shirt in there. But aside from building the big  chromosomes we’re most familiar with, microchromosomes might have some other function. It’s not clear exactly why, but  scientists have noticed that microchromosomes will tend to huddle  together in the middle of the cell.

It could be that bits of genes from one  microchromosome somehow regulate bits from another microchromosome,  so they need to work together. Or that the microchromosomes form a compartment in the cell that helps genes to stay active. These are all things geneticists desperately  want to understand better in the future.

Needless to say though,  microchromosomes are doing a lot more than just dusting up microscope slides. If you want to learn more about how  the way we study DNA is changing, you might enjoy Brilliant’s  course Computational Biology. The field of genomics generates so  much data that the people who study DNA increasingly have to learn computer  science too.

This course can show you how. And Brilliant has tons of other courses in  math, science, engineering, and computer science, so no matter what you’re  interested in, there’s something to learn. Courses are becoming more interactive every day, so keep checking back to see what’s new.

And if you happen to be  seeing this the day we upload, you might like to know that you  can gift Brilliant to someone else. Just in case you need some last minute gift ideas. The first 200 of you to go to the link  below will get 20 percent off Brilliant's annual premium subscription, for  yourself or for someone else! [♪ OUTRO]