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tanarillToday, I shall explain about the mysterious proteins known as histones. It bothers me that histones are not in high-school biology textbooks, so I make this small endeavor to correct it.
As I said, histones are proteins. In fact, each histone is made of two copies each of four different protein chains, for a total of eight. Each set of four makes a lobe; when you snap them together, you get something like the world's tiniest yo-yo. The "string" that wraps around the groove of this tiny yo-yo is, of course, DNA.
Not all the DNA, of course. Only 147 bases of DNA wrap each histone. Then there's a short linker sequence, followed by another 147 bases of DNA wrapped on a histone. In this way, a vast majority of DNA is wrapped around histones all the time.
Why bother? Volume.
Say you have some string. It's a single length, and it's all unwound. You can haul it around by showing it into a bag. Alternatively, you could wind it up into a nice, neat ball, and fit lots of string in the same bag. We humans have five meters of DNA in every cell in our bodies*. You do the math.
Of course, we cannot wind DNA up so incredibly tightly. It must be loose enough that the proteins that "read" it can get to all of it, which it couldn't if the DNA were balled up. The histones are really clever here. While it is true that most of the time they float around with DNA in a loose jumble, during mitosis the histones coil with each other, and then the coils supercoil, and in the end all that jumbled DNA is pushed down into a tiny chromosome. Once mitosis is over, the histones unwind and the DNA springs back out, just fine, and can all be read once again.
There are all kinds of other things you can do with histones, but I think I will stop here for today. It is enough to understand that histones exist and are useful and necessary proteins, without getting into the really interesting stuff.
*Except the ones that don't. Like blood cells.
As I said, histones are proteins. In fact, each histone is made of two copies each of four different protein chains, for a total of eight. Each set of four makes a lobe; when you snap them together, you get something like the world's tiniest yo-yo. The "string" that wraps around the groove of this tiny yo-yo is, of course, DNA.
Not all the DNA, of course. Only 147 bases of DNA wrap each histone. Then there's a short linker sequence, followed by another 147 bases of DNA wrapped on a histone. In this way, a vast majority of DNA is wrapped around histones all the time.
Why bother? Volume.
Say you have some string. It's a single length, and it's all unwound. You can haul it around by showing it into a bag. Alternatively, you could wind it up into a nice, neat ball, and fit lots of string in the same bag. We humans have five meters of DNA in every cell in our bodies*. You do the math.
Of course, we cannot wind DNA up so incredibly tightly. It must be loose enough that the proteins that "read" it can get to all of it, which it couldn't if the DNA were balled up. The histones are really clever here. While it is true that most of the time they float around with DNA in a loose jumble, during mitosis the histones coil with each other, and then the coils supercoil, and in the end all that jumbled DNA is pushed down into a tiny chromosome. Once mitosis is over, the histones unwind and the DNA springs back out, just fine, and can all be read once again.
There are all kinds of other things you can do with histones, but I think I will stop here for today. It is enough to understand that histones exist and are useful and necessary proteins, without getting into the really interesting stuff.
*Except the ones that don't. Like blood cells.
no subject
Date: 2012-01-20 07:29 am (UTC)