النص 🔗
WEBVTT
1
00:00:01.199 --> 00:00:05.240
Today we're going to be looking at DNA replication. Hopefully
2
00:00:05.259 --> 00:00:08.759
try to make it simple for you to do better
3
00:00:08.839 --> 00:00:10.640
on the test. If you know it pretty well, then
4
00:00:10.660 --> 00:00:14.919
this is just a refresher. Every time a cell divides,
5
00:00:14.960 --> 00:00:17.719
it faces an incredible challenge. It must copy billions of
6
00:00:17.780 --> 00:00:21.440
DNA base pairs while making remarkably few mistakes. How does
7
00:00:21.480 --> 00:00:23.940
the cell accomplish this? Why are the two strands of
8
00:00:23.980 --> 00:00:28.679
DNA copied differently? The answer begins with DNA replication. DNA
9
00:00:28.739 --> 00:00:35.880
replication is described as a semi-conservative, meaning each newly formed
10
00:00:35.920 --> 00:00:39.649
DNA molecule contains one original strand and one newly synthesized strand.
11
00:00:40.409 --> 00:00:44.189
In eukaryotic cells, replication occurs during the S-phase of the
12
00:00:44.329 --> 00:00:49.590
cell cycle. The process begins at specific locations called origins
13
00:00:49.630 --> 00:00:54.369
of replication. An enzyme called a helicase separates the DNA
14
00:00:54.390 --> 00:00:58.549
strands by disrupting the hydrogen bonds between complementary bases. This
15
00:00:58.670 --> 00:01:01.189
creates what they call a replication fork where the DNA
16
00:01:01.229 --> 00:01:06.780
strands separate. The single-strand binding proteins help stabilize the exposed DNA,
17
00:01:07.480 --> 00:01:14.920
while topoisomerase relieves twisting stress ahead of the fork. DNA
18
00:01:14.980 --> 00:01:19.739
polymerase can synthesize DNA only in the 5' to 3' direction,
19
00:01:19.760 --> 00:01:22.469
and this is important for the MCAT. This means new
20
00:01:22.530 --> 00:01:24.730
nuclear ties are added to the 3' end of a
21
00:01:24.769 --> 00:01:28.590
growing DNA strand. But because the two original DNA strands
22
00:01:28.650 --> 00:01:32.609
run in opposite directions, replication cannot occur identically on both strands.
23
00:01:33.519 --> 00:01:36.739
One strand is called the leading strand, and that's synthesized
24
00:01:36.760 --> 00:01:40.180
continuously in the same direction that the replication fork advances.
25
00:01:41.599 --> 00:01:46.299
The other is called a lagging strand. Because DNA polymerase
26
00:01:46.359 --> 00:01:50.280
cannot synthesize DNA in the 3' to 5' direction, the
27
00:01:50.340 --> 00:01:54.680
lagging strand must be constructed in short segments. These segments
28
00:01:54.719 --> 00:01:59.170
are called Okazaki fragments, named after Japanese molecular biologist Okazaki.
29
00:02:00.549 --> 00:02:04.709
Each fragment is synthesized in the 5' to 3' direction,
30
00:02:04.810 --> 00:02:10.449
even though overall lagging strand synthesis proceeds opposite the movement
31
00:02:10.469 --> 00:02:15.979
of the replication fork. Well, now let's examine the enzymes involved.
32
00:02:16.360 --> 00:02:20.039
Primase synthesizes short RNA primers that provide the starting point
33
00:02:20.080 --> 00:02:24.710
for DNA synthesis. DNA polymerase extends these primers by adding
34
00:02:24.770 --> 00:02:31.969
complementary nucleotides. In eukaryotes, DNA polymerase epsilon primarily synthesizes the
35
00:02:32.030 --> 00:02:37.120
leading strand, while DNA polymerase delta primarily synthesizes the lagging strand.
36
00:02:38.039 --> 00:02:42.789
I'll repeat that. DNA polymerase epsilon synthesizes the leading strand,
37
00:02:42.849 --> 00:02:48.030
while DNA polymerase delta synthesizes the lagging one. The RNA
38
00:02:48.110 --> 00:02:51.939
primers are eventually removed and replaced with DNA. Finally, DNA
39
00:02:51.979 --> 00:02:56.000
ligase seals remaining breaks in the sugar phosphate backbone, joining
40
00:02:56.020 --> 00:03:00.620
the fragments into a continuous strand. Another important concept is proofreading.
41
00:03:00.699 --> 00:03:08.500
Many replicative DNA polymerases possess 3' to 5' DNA. exonuclease activity.
42
00:03:08.719 --> 00:03:13.389
This allows them to remove incorrectly incorporated nucleotides and improve
43
00:03:13.610 --> 00:03:20.120
replication accuracy. And that's it for our episode today.