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Chromosomes are fascinating little things, and today, Hank explains why more of them doesn't mean more complex, and why different organisms have different numbers of chromosomes. The short answer: mistakes happen.

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Hank Green: Domestic cats have 38, dogs have 78, earthworms have 36, goldfish have 104, and we have 46, the same number as the sable antelope, but two fewer than potatoes. I'm talking about chromosomes. (SciShow intro) You might think that the more chromosomes an organism has, the more complex it is, because chromosomes contain genes and more genes means more fancy, right? Not right. Unless you can convince me that potatoes and goldfish are more complex than we are, though, certain members of Congress, kind of feeling that way right now. But the question is, why do different organisms have different, and seemingly unrelated, numbers of chromosomes? Like how did a species of ant end up with only two chromosomes while a species of fern has more than 1200, and why do we have 46 while chimps, our closest genetic relatives, have 48? The short answer is mistakes happen, and in this case, to DNA, and different kinds of organisms have responded to those mistakes in different ways. Chromosomes are basically spools of DNA. Each chromosome consists of proteins called histones, with a single very long molecule of DNA wrapped around each one. Without the organizing structure of chromosomes, DNA would be way too unruly to replicate or even fit inside a cell. It'd be like a tangle of extension cords in your junk drawer, just a hot mess. But while chromosomes do a good job of keeping DNA safe in the rough and tumble life of a cell, DNA does sometimes get damaged. This damage often leaves little strands of DNA dangling from their chromosomes. When this happens, special repair proteins in the cell recognize the damage and industriously reconnects those strands. But sometimes, they connect the wrong ones together, and two chromosomes become fused. This is simply known as random fusion. This decreases the total number of chromosomes in the cell, but it doesn't necessarily mean that any genetic information is lost, it's just spooled up in fewer packages. For example, the chromosomes of humans and chimps are nearly the same, except for human chromosome number 2, which looks remarkably like 2 chimp chromosomes that got stuck together. So that explains how over time, organisms could end up with fewer chromosomes, but how come other living things wind up with so incredibly many? Scientists think that this is because of another kind of mistake which can happen during the type of cell division that makes sex cells, called meiosis. Typically during meiosis, each chromosome in the parent cell is copied, and the original and its copy are pulled apart into separate sperm or egg cells. But sometimes, just because, I guess, the chromosomes and their copies don't pull apart and end up both getting drawn into the sex cell. Again, this doesn't mean that any additional genetic information is there, just an extra set of copies. Now, you take a couple of these sperm or eggs with extra chromosomes and make a baby with them, 'cause usually they're still viable, and then pow, you've got an offspring with more chromosomes than its parents had. Some organisms, like plants that can self-fertilize, wind up with extra chromosomes because of this, others have had this happen a lot because they've just been around for a really long. That's probably the case with Adder's Tongue, an ancient fern-like plant that boasts more chromosomes than any other plant, 1260. Scientists think this chromosome sticking might have happened to it at least 10 times in its multi-million year history. And it's probably also happened to lots of other organisms, too, but over time, many of them have lost those extra chromosomes, while others, like the Adder's Tongue, have just hung on to them. So, does more chromosomes mean fancier? No. But weirder and fascinating? Definitely. Thank you for watching this episode of SciShow, if you have any questions or comments or suggestions, you can find us on Facebook and Twitter and down in the comments below, and if you want to keep getting smarter with us here at SciShow, you can go to YouTube.com/SciShow and subscribe. (endscreen plays)