Lesson 4 – Video 2
[00:00:01.19] SPEAKER: Hi. In this video, we'll continue 1476 our discussion about
1477 talking about their structure, the three essential parts of the chromosome. We'll talk
1478 chromatin, which is the DNAchromosomes and the bound protein. We'll talk a lot about
1479 proteins, which are the primary proteins which package DNAinside the cell.And we'll
1480 specifically, about the "tails" of the histone proteins, the very ends of them which play a
1481 important role in regulating gene expression. In this video and the next several, we'll be
1482 primarily about events that are happening during interphase that is the part of the cell
1483 is not directly involved with mitosis, which we'll be speaking about at a later time.
1484 [00:00:57.40] During the cell cycle, the chromosomes exist in different configurations.
1485 interphase, which is most of the time in which the cell is doing work, the chromosomes
1486 decondensed. They're uncondensed. They're spread out as far as they go. It's during
1487 the chromosomes will compact incredibly and will form the structures that you can see
1488 [00:01:28.75] There are three essential components to any chromosome-- the telomere,
1489 replications of origin, and the centromere. We've already met the centromere when we
1490 pictures of karyotype and the way that the chromosomes were aligned. The centromeres
1491 center part of the chromosome. And it'll be to these features which the mitotic spindle
1492 and will help pull and separate the two chromosomes from each other during cell
1493 [00:02:07.97] The telomere is the ends of the chromosome.And they have a specific
1494 that helps protect the linear ends of the chromosome from degradation.Additionally,
1495 an important role in regulating the lifespan of normal cells.
1496 [00:02:27.17] Third important part is the replication of origin. Each chromosome has
1497 replications of origin. It's essential that there is, at least, one replication of origin on
either side of
1498 the centromere.As the name suggests, this is the place where DNAbegins replicating
1499 We'll discuss each of these pieces in further slides.
1500 [00:03:00.58] On this slide, you can see a detailed image of an interphase nucleus
where the 1501 chromosomes are at their most unwound. Even in this state, you can see there is some
1502 with the way that the chromosomes are organized in the nucleus. Different colors
1503 patches where a particular chromosome dominates.
1504 [00:03:25.09] We don't yet understand the importance of the relationships of the
locations of the
1505 different chromosomes and whether it's unchanging varies from person to person and
1506 time where it's a fundamental property of the organization of the chromosomes. But it
1507 clear that this organization is regulated in some manner. So there must be a reason for it.
1508 [00:03:55.01] At the bottom here, you can see the electron micrograph of a nucleus. The
1509 of the nucleus are at this barrier right here.And within the nucleus, there's an area of
1510 which is the nucleolus.
1511 [00:04:12.41] Anucleolus is an area of intense activity. It's here where the ribosomal
1512 the RNAfor ribosomal proteins are synthesized. Ribosomes are some of the most
1513 proteins.And so they need to be made in great quantity throughout the cell's activity.
[00:04:35.75] On this slide, you can see the various levels 1514 of organization that the
1515 undergoes as it's packaged from its freeform, which extends a few meters, to its most
1516 form, which is about 10 microns in the longest dimension. I have that at about 100,000
1517 But the point is that it's incredibly condensed.And the way that the cell achieves this is
1519 [00:05:12.95] We'll focus mostly on the top levels of organization.And in particular,
1520 talking about this one, the so-called "bead-on-a-string" level. In this view, you can see
1521 DNAis wrapped around yellow proteins.
1522 [00:05:28.63] Those yellow proteins are the histone proteins that we were talking about.
1523 picture is reasonably to scale. The DNAwinds around histone proteins about twice and
1524 separated from other histones often by about the same characteristic length as the
histone is wide.
1525 [00:05:50.11] Anucleosome refers to one set of histone proteins where the DNAwraps
1526 approximately 1.8 times. This represents one of the beads on the "bead-on-the-string"
1527 The protein core is made up of about four different histone proteins colored in green,
1528 and orange. 1529 [00:06:16.51] And there's two copies of each. So there's actually eight protein subunits
1530 inside of a protein core.Afifth histone protein, H1, binds to the outside of the
nucleosome. But it
1531 is not shown in this picture.
1532 [00:06:36.97] Each of the eight protein subunits has this tail region which can wrap
1533 bind the DNAvery tightly. It does so by possessing a lot of positive charges, a lot of
1534 arginines on the cysteine tail that will interact with the negatively charged DNAvery