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23 pairs.
With just those two words, you probably already know what I’m talking about: chromosomes,
the tightly-wound packets of DNA and protein that together make up our genomes.
And not just any chromosomes, but the number of chromosomes in most humans.
But have you ever thought about why it’s 23?
There’s a whole range of numbers of chromosomes different species can have.
And scientists have been puzzling over the possible advantages of having more of them for a while.
Weirdly, this number that most of us have memorized may not be as integral as we thought.
Because in papers published in the journal Nature this week, two separate groups of biologists
reported fusing entire sets of chromosomes together, reducing the count down to just one or two.
And, kind of shockingly, things mostly went okay.
Before you start freaking out, this experiment wasn’t done in humans.
Instead, the researchers worked with Saccharomyces cerevisiae, a fungus better known as Baker’s
or Brewer’s yeast, since it’s the species we use to make bread and beer.
This yeast normally has 16 chromosomes.
But thanks to genetic engineering technologies, including CRISPR, the scientists were able
to progressively stitch together more and more chromosomes.
That required taking out two structural components of the chromosomes:
centromeres, short DNA sequences roughly in the middle of each chromosome where they attach
to each other during cell division;
and telomeres, the repetitive sequences at the ends of chromosomes that help keep them intact.
You only want one centromere and two telomeres per chromosome, so both groups hacked away
at all the extras, and took slightly different approaches to the fusion part.
One team from China was able to pack all 16 into one single, giant chromosome, while an
American team got it down to just two big chromosomes.
Importantly, the researchers made very few changes to the overall DNA sequence, so any
observed differences are almost certainly a result of messing with the chromosomal structure.
It’s actually the first time biologists have ever been able to whittle the chromosome
number down to one in any eukaryote which are organisms like us, whose chromosomes are
stored in a special nucleus compartment in their cells.
So, what happened after this radical change?
At first glance, not much.
No word on what they tasted like compared to the strains we use for bread and beer,
but the chromosome-tinkered strains of yeast looked very much like their normal counterpart in size and shape.
They also went through cell division normally, and it could grow under a variety of conditions.
Yeast geneticists like to test out different nutrients and conditions to see if strains are lacking in certain areas.
And except for slightly slower growth and higher susceptibility to an antifungal compound,
the fewer-chromosome strains did pretty well.
Now, let’s pause here for a second, because this alone is kind of astounding.
Scientists can totally rearrange how an organism packages its DNA at a fundamental level, and
at least for yeast, it’s totally viable.
Even more shocking to the scientists was that the modified yeasts hadn’t really changed
the types of genes that get turned on or off.
Both groups found very few changes compared to regular yeast.
In fact, in the single-chromosome yeast, just 28 genes had substantially changed expression.
That’s less than half a percent of all the protein-coding genes!
This is pretty surprising because biologists have long known that the 3D structure of chromosomes
can affect the types of genes that are expressed, and ultimately which proteins are made.
In this case, basically all of the interactions between chromosomes had been obliterated.
But a good chunk of the interactions within the chromosomes remained intact, and those
at the gene level were largely preserved.
This seems to have been enough to keep the yeasts doing their regular thing.
Still, it’s not all rainbows and butterflies for the giant chromosome yeasts.
Both teams observed reproductive and fitness deficits.
When grown together with 16-chromosome yeast, the altered yeasts grew more slowly, and in
the case of the single chromosome, was quickly out-competed.
The single-chromosome yeast could successfully mate with itself, but those daughter cells
were slower-growing and occasionally had trouble maintaining the correct chromosome number.
The daughter cells also produced fewer sex cells, and had trouble reproducing.
These differences begin to explain why having a larger number of smaller chromosomes might be a good thing.
For one thing, it might be hard for cells to properly replicate such abnormally large
chromosomes, and then pull the two copies apart correctly.
More chromosomes might also offer more flexibility that could prove useful in the course of evolution.
If you have more chromosomes, it’s a little easier to drop one or gain one, which might
give you a better ability to adapt to things like changes in your environment.
Of course, a major change to your genome like that might also be really bad.
It's a bit of a trade-off.
But in the long run, this versatility could provide an evolutionary advantage.
We don’t know the exact reasons, but this species of yeast has had 16 chromosomes for
some 10-20 million years.
So something is definitely working.
At the same time, it’s now clear that while 16 is the magic number, it’s not absolutely critical.
And the same might even be true for us.
Although, it would probably be harder to fuse chromosomes this way in more complex organisms.
That's because they tend to have more complicated DNA sequences around centromeres and telomeres.
So for all we know, we might not be very different if all our chromosomes were fused into one giant one.
But we probably have a little bit more research to do before we can make that happen.
Thanks for watching this episode of SciShow News, and thanks to all of our patrons on
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