Advent Science: Day Four
DNA storage
So far, we have a very very long ladder of DNA. In humans, it's a few billion base pairs (rungs, to horribly overuse the ladder analogy) long. So, it must be stored in some sort of order, right? Or it'd all just get tangled up...
Well, yes and no.
You probably already know that the vast, vast majority of humans have 46 chromosomes per cell in 23 pairs, one chromosome of each pair inherited from each parent, and you probably know that two of those are the X and Y chromosomes, which determine biological sex. While we now know on which chromosome a lot of genes reside, we don't know why they are in the order they are, and many scientists suspect it is in fact random.
So, how is DNA folded up? First, it coils around support proteins called histones, which can be viewed as molecular cotton reels, each of which holds about 200 base pairs of DNA. This string of DNA-around-histones then coils up again into the characteristic X shape of a chromosome.
Now, remember where I said the vast majority of humans have 46 chromosomes per cell? There are several medical conditions where this is not the case, due to a fault in separation between the daughter cells in a cell division. The most well known of these is Down's Syndrome, in which affected people have three copies of chromosome 21 instead of the usual two, known technically as trisomy-21. Most other trisomies result in non-viable foetuses.
The other exception to this is the sex chromosomes. Women with XXX make up about 0.1% of female births, and are pretty much indistinguishable from XX women. XXY has a similar incidence in males, though this can come with reduced fertility and underdeveloped secondary sexual characteristics. Higher numbers of sex chromosomes are occasionally seen, accompanied by developmental disorders of varying severity.
Linked below is a diagram of a karyotype, a way of visualising the chromosomes of an individual. You can also see that the X and Y chromosomes aren't actually X and Y shaped at all, although the Y chromosome is noticeably smaller than the X, and pretty much only contains the genes required to make a foetus male. The relatively small size of the sex chromosomes goes some way to explaining why a non-standard number of them is less damaging than the others.
http://upload.wikimedia.org/wikipedia/commons/4/4f/Human_male_karyotpe.gif
Tomorrow, we will start to look at how the genetic code actually governs phenotype, ie what an organism actually looks like.
So far, we have a very very long ladder of DNA. In humans, it's a few billion base pairs (rungs, to horribly overuse the ladder analogy) long. So, it must be stored in some sort of order, right? Or it'd all just get tangled up...
Well, yes and no.
You probably already know that the vast, vast majority of humans have 46 chromosomes per cell in 23 pairs, one chromosome of each pair inherited from each parent, and you probably know that two of those are the X and Y chromosomes, which determine biological sex. While we now know on which chromosome a lot of genes reside, we don't know why they are in the order they are, and many scientists suspect it is in fact random.
So, how is DNA folded up? First, it coils around support proteins called histones, which can be viewed as molecular cotton reels, each of which holds about 200 base pairs of DNA. This string of DNA-around-histones then coils up again into the characteristic X shape of a chromosome.
Now, remember where I said the vast majority of humans have 46 chromosomes per cell? There are several medical conditions where this is not the case, due to a fault in separation between the daughter cells in a cell division. The most well known of these is Down's Syndrome, in which affected people have three copies of chromosome 21 instead of the usual two, known technically as trisomy-21. Most other trisomies result in non-viable foetuses.
The other exception to this is the sex chromosomes. Women with XXX make up about 0.1% of female births, and are pretty much indistinguishable from XX women. XXY has a similar incidence in males, though this can come with reduced fertility and underdeveloped secondary sexual characteristics. Higher numbers of sex chromosomes are occasionally seen, accompanied by developmental disorders of varying severity.
Linked below is a diagram of a karyotype, a way of visualising the chromosomes of an individual. You can also see that the X and Y chromosomes aren't actually X and Y shaped at all, although the Y chromosome is noticeably smaller than the X, and pretty much only contains the genes required to make a foetus male. The relatively small size of the sex chromosomes goes some way to explaining why a non-standard number of them is less damaging than the others.
http://upload.wikimedia.org/wikipedia/commons/4/4f/Human_male_karyotpe.gif
Tomorrow, we will start to look at how the genetic code actually governs phenotype, ie what an organism actually looks like.