Lesson 4 – Video 1
1312 [00:00:01.21] PROFESSOR: Hi. In this lecture, we'll talk in greater detail about DNA
1313 about the arrangement of DNA into a double helix, the organization of DNA into
1314 and the way that genes are arranged on chromosomes.
1315 [00:00:17.67] In DNA, there are 3 basic parts, as we've seen before. We have
1316 a sugar group-- these two will combine to form the backbone-- and a base, which will
1317 information in the center of the strand. Together, these 3 pieces are called a nucleotide.
1318 nucleotides will join together to form a linear sequence of bases which can be read out
1319 enzymes or other nucleic acid molecules.
1320 [00:00:49.34] It's called a double helix because there are two strands in DNA that base
1321 each other. You have one strand on this side and a second strand going this side.
1322 are anti-parallel That means that the directionality is different. You have a 5 ' end
running to a 3 '
1323 end for one strand, anti-parallel with a 5 ' end running to a 3 ' end over here, with the
1324 in the center pairing with their partners.
1325 [00:01:27.96] I don't want to give the impression that this is a linear configuration.
Again, it's a
1326 double helix. And here we can see a right-handed helix going with and the axis of the
1327 helix running up and down and the bases interacting with each other in the center of
37 [00:01:55.25] Now, of the four bases in DNA, adenine will always 1329 hydrogen bond with
1330 with two hydrogen bonds. And guanine will interact with cytosine with 3 hydrogen
bonds. And a
1331 historically famous way of stating that is Chargaff's rule, which is to say that the
1332 adenine and thymine will always be equal in molar terms. And the G molar content will
1333 equal to C molar content.
1334 [00:02:35.59] To draw in a little greater detail, again we have the axis running up and
1335 the page. And the bases, as they're interacting, are normal both to the axis that we've
1336 to the plane of the picture, so that the normals, the normal vectors, for each of these
1337 these heterocycles are parallel to the axis of the DNA.
1338 [00:03:13.68] The sugar and phosphates form the backbone. That's the backbone from
1339 strand. This is the backbone from the other. Remember that these phosphate groups
1340 negatively charged, so we have polyanions. This has a great deal of electrostatic
1341 between this strand and that strand. In solution, there would be lots of positively
1342 magnesium ions counteracting or binding and screening the negative charges so that
1343 have this electrostatic repulsion between them. Without a divalent cation present, the
1344 will dissociate.
1345 [00:04:01.96] OK, in this slide, we're seeing the double helix with space-filling
1346 can see the bases here in the center that we're seeing them on the edge. And you can
1347 phosphate sugar backbones, the red and the yellow colors here. You can also notice
that between 1348 phosphate backbones there is a smaller section and a larger section. This is the major
1349 is the minor groove. There is greater accessibility of the edges of the bases in the
1350 less so in the minor groove.
1351 [00:04:38.39] But when proteins try to read DNA, they're not unfolding it and gaining
1352 to it. They have to read the projections of hydrogens, oxygens, carbons, and nitrogens
1353 edges here. And that's the way in which the code is read.
1354 [00:05:00.73] There are about 10 base pairs between a full turn of-- 10 or 11-- between
1355 turn of the helix. And it's about 3.4 angstroms between each successive stack of
spaces in the
1356 double helix. Because you have a double helix with base pairs that are complementary
1357 other, this enables you to have two templates for further synthesis, so that when DNA
1358 these two strands will dissociate and serve as a template for the synthesis of a new
strand. This is
1359 termed semiconservative replication. And that just indicates that each new strand has
one of the
1360 original strands of DNA and a newly made one, original and nearly made.
1361 [00:05:56.98] Watson and Crick's original paper, which was only a few pages-- two, I
1362 which they're primarily reporting the structure, at the very end, they have a single
1363 saying that, "It has not escaped our notice that the specific pairing we have postulated
1364 immediately suggests a possible copying mechanism for the genetic material." So they
1365 remembered not only as having identified the first structure of DNA, but also the first to
1366 in print, the copying mechanism.
38 [00:06:41.64] Your DNA is organized into chromosomes. You ha 1367 ve about three billion
1368 pairs. That's an important number for you to remember, three billion base pairs. They
1369 organized in 23 sets of chromosomes-- 22 autosomes, and one pair of sex
chromosomes. Men are
1370 XY, females are XX.
1371 [00:07:10.69] You have two sets of each chromosome. Let's say in chromosome one,
1372 one copy of the chromosome from your mother and one copy of the chromosome from
1373 father. The way they're drawn here and the way they're numbered, they are aligned all
but at a
1374 position in the chromosome call the centromere. They are arranged from largest to
1375 if it doesn't look like that so much here on the screen.
1376 [00:07:39.35] And in each case, the smaller arm, or the p arm, is above the
centromere. And the
1377 longer arm, or the q arm, is below the centromere. The p stands for petite in French.
And I'm not
1378 sure what the q stands for. I think it's probably just the next letter in the alphabet.
1379 [00:08:02.62] The last thing you notice here are the black and white banding patterns.
1380 talk more about that. However, this is what is actually visualized under a light
1381 the DNA is in a metaphase preparation and has been stained with a fluorescent dye.
1382 regions, which are looser and can accept the dye into its structure, will appear whiter or
1383 And those regions which are very compact, and exclude the dye from entering in its
1384 be dark.
1385 [00:08:45.58] These banding patterns are almost identical in all of us. They're highly
similar in 1386 all of us. And when people are referring to particular positions in the chromosome
1387 only had microscopy to go by, they have a numbering system for the different bands.
1388 be so obvious for me to show you here in this slide. But two people could talk about a
1389 as, for example, 9 p 1-- chromosome 9, arm p, band 1. And then they can go into
1390 [00:09:34.34] The number of chromosomes is essentially the same in everybody. But
it's not, in a