DNA I Ribose


Uploaded by mrphysh on 28.08.2012

Transcript:
DNA is all about base pairing. A goes to T and G goes to C and C goes to G
and all about the double helix, the spiral staircase of DNA
but I think that the key to the structure of DNA lies as much with the ribose
as it does with base pairs
and I think that if you are going to study the structure of DNA
you really need to start with the ribose and I think that if you are going to look at ribose
we could look at a general carbohydrate, first and that carbohydrate is glucose, of course
glucose may be perhaps the most common molecule in the biosphere
perhaps we could start with a 'little housekeeping'
the labels that they use for carbohydrates are the 1' carbon, 2' 3' ... ("1 prime")
carbohydrates are defined as poly hydroxyl aldehydes
the poly hydroxyls are the large number of OH groups OH groups, of course, being characteristic of alcohols
but if you put a lot of them on it with an aldehyde or ketone
it becomes a carbohydrate.... so there are lots of hydroxyl groups
and there is a carbinol group (oxygen) which on the and which makes it an aldehyde
so the 1' has an aldehyde carbinol group
the 1' oxygen forms a hemiacetyl group with the 5' carbon of the same molecule
and it makes a ring structure
The ring structure forms a boat and a chair configuration
so it's stable but there's variations in what kind of shape it can take having six sides, like it does
and all the isomers are nailed down based upon this ring structure
one corner .... the ring structure has six sides
one corner has an oxygen there is a tail, notice, that sticks out, which is the 6'
so the ring is from the 1' to the 5' leaving the 6' sticking out
glucose is the most common carbohydrate and, to repeat,
perhaps the most common molecule in the biosphere
paper and cellulose .. trees and things are made of glucose for the most part
Ribose is pretty much the same
it's a poly hydroxyl aldehyde
it has 1', 2', 3', 4', 5' only
so there is only five carbons in ribose
the aldehyde unit on the 1' forms a hemiacetyl with the 4'
so it makes a ring, very similar to the glucose
but the ring only has five sides
notice that it still has this little tail coming off of it
the one prime goes to the 4'
the ring is similar to glucose
the ribose is more stable than the glucose
or .. it's... it's configuration is more predictable
there is no variation in its structure it doesn't have a boat and a chair
it's a five carbon ring and I think intuitively you can.... it's easy to see that
a five-member ring is going to be more stable than a six-member ring
I (sort of) point all this out to emphasize that ribose is not some exotic sugar
ribose is a very ordinary sugar
we look at this image we have the 1' 2' 3' 4' 5'
the base goes on the 1'
the phosphates go on the 3' and 5' forming the rails of the DNA
so the sides of the ladder are made out of phosphate and ribose
notice how there's no red color on this spot
that's because the ribose in DNA is deoxy ribose
which is to say that one of the hydroxyl groups is missing which goes here
the rails of the DNA are ribose and phosphate
you can see here the ribose is in orange and the phosphate is in yellow
the 5' attaches to the 3' and
it's an extremely important idea in the structure and behavior of DNA
and we want to talk about that in another video