WEBVTT 00:00:00.000 --> 00:00:03.680 Captioning is on. Click the CC button at bottom right to turn off. 00:00:03.680 --> 00:00:08.300 Follow us on Twitter (@amoebasisters) and Facebook! 00:00:08.340 --> 00:00:14.000 DNA. We talk about it so much---it is the ultimate director for cells and it codes for 00:00:14.000 --> 00:00:19.930 your traits. It’s a major component of what makes you, you. When you have a really important 00:00:19.930 --> 00:00:25.190 molecule like DNA that is ultimately responsible for controlling the cell…it would make sense 00:00:25.190 --> 00:00:32.189 that when you make another cell (like in mitosis), you would also have to get more DNA into that 00:00:32.500 --> 00:00:39.460 cell. And that introduces our topic of DNA replication, which means, making more DNA. 00:00:39.460 --> 00:00:46.460 First let’s talk about where and when.First where---it occurs in the nucleus. If the cell 00:00:48.079 --> 00:00:54.070 has a nucleus. Remember, not all cells have a nucleus. This video clip is actually going 00:00:54.070 --> 00:01:00.969 to focus on the types of cells that do have a nucleus though known as eukaryote cells. 00:01:00.969 --> 00:01:07.969 Prokaryotes, which are cells that lack a nucleus, do things a little differently. Next When 00:01:13.819 --> 00:01:19.860 does this happen---this typically happens during a stage known as interphase. Interphase 00:01:19.860 --> 00:01:25.119 is when a cell’s growing, it’s carrying out cell processes, and it’s replicating 00:01:25.119 --> 00:01:31.119 its DNA. You know what it’s not doing at the exact same time? Dividing. You don’t 00:01:31.119 --> 00:01:36.030 want a cell to be replicating DNA and dividing at the exact same time. That’s a little 00:01:36.030 --> 00:01:43.020 bit too much multitasking. So DNA replication does not happen during cell division (aka 00:01:43.020 --> 00:01:50.020 mitosis). In fact, the cell replicates its DNA before division processes like mitosis 00:01:50.099 --> 00:01:56.090 and meiosis. Because once you make that new cell, you better have DNA to put in there. 00:01:56.090 --> 00:02:01.399 I think DNA replication would actually make a great video game. It’s actually quite 00:02:01.399 --> 00:02:05.940 exciting. I’m going to introduce the key players in DNA replication so that you can 00:02:05.940 --> 00:02:11.810 get some background information. The majority of these key players that I’m going to introduce 00:02:11.810 --> 00:02:18.319 are enzymes. In biology, when you see something end in –ase, you might want to check as 00:02:18.319 --> 00:02:24.060 it is very possible that it’s an enzyme. Enzymes have the ability to speed up reactions 00:02:24.060 --> 00:02:31.060 and build up or break down the items that they act on. So here we go with the key players. 00:02:32.319 --> 00:02:39.230 Helicase- the unzipping enzyme. If you recall that DNA has 2 strands, you can think of helicase 00:02:39.230 --> 00:02:44.090 unzipping the two strands of DNA. Helicase doesn’t have a hard time doing that. The 00:02:44.090 --> 00:02:46.370 hydrogen bonds that hold the DNA strands together is pretty weak compared to other kinds of 00:02:46.370 --> 00:02:49.879 bonds. DNA Polymerase- the builder. This enzyme replicates DNA molecules to actually build 00:02:49.879 --> 00:02:56.879 a new strand of DNA. Primase- The initializer. With as great as DNA polymerase is, poor DNA 00:02:57.379 --> 00:03:02.510 polymerase can’t figure out where to get started without something called a primer. 00:03:02.510 --> 00:03:07.030 Primase makes the primer so that DNA polymerase can figure out where to go to start to work. 00:03:07.030 --> 00:03:12.359 You know what’s kind of interesting about the primer it makes? It’s actually a piece 00:03:12.359 --> 00:03:19.359 of RNA. Ligase- the gluer. It helps glue DNA fragments together. More about why you would 00:03:20.709 --> 00:03:26.829 need that later. Don’t feel overwhelmed. We’ll go over the sequence in order. Please 00:03:26.829 --> 00:03:32.099 keep in mind, that like all of our videos, we tend to give the big picture but there 00:03:32.099 --> 00:03:38.409 are always more details to every biological process. There is more involved than what 00:03:38.409 --> 00:03:44.319 we cover. DNA replication starts at a certain part called the origin. Usually this part 00:03:44.319 --> 00:03:50.310 is identified by certain DNA sequences. There can be multiple origins within the DNA strand. 00:03:50.310 --> 00:03:57.310 At the origin, helicase (the unzipping enzyme) comes in and unwinds the DNA. 00:03:57.420 --> 00:04:05.160 SSB proteins (which stands for single stranded binding proteins) bind to the DNA strands to keep 00:04:05.170 --> 00:04:11.790 them separated. Primase comes in and makes RNA primers on both strands. This is really 00:04:11.790 --> 00:04:17.290 important because otherwise DNA polymerase won’t know where to start. 00:04:17.399 --> 00:04:24.780 Now comes DNA Polymerase. Remember, it’s the important enzyme that adds DNA bases. 00:04:24.789 --> 00:04:30.180 Now you have 2 strands right? But they’re not identical.Remember they complement each other. They 00:04:30.180 --> 00:04:37.180 also are anti-parallel so they don’t really go in the same direction. 00:04:37.180 --> 00:04:44.100 With DNA, we don't say it goes North or South. The directions for the DNA strands are a little different. 00:04:44.110 --> 00:04:51.110 We say that DNA either goes 5’ to 3’ or 3’ to 5’. What in the world does that 00:04:51.340 --> 00:05:01.000 mean? Well the sugar of DNA is part of the backbone of DNA. It has carbons. The carbons 00:05:01.000 --> 00:05:08.000 on the sugar are numbered right after the oxygen in a clockwise direction. 1’, 2’ 00:05:08.180 --> 00:05:17.020 3’, 4’ and 5.’ The 5’ carbon is actually outside of this ring structure. Now you do 00:05:17.220 --> 00:05:24.379 the same thing for the other side but keep in mind this strand is flipped just because 00:05:24.379 --> 00:05:30.270 DNA strands are anti-parallel to each other. So let’s count these---again, clockwise 00:05:30.270 --> 00:05:37.270 after the oxygen. 1’, 2’ 3’, 4’ 5’. And the 5’ is out of this ring. This strand 00:05:37.340 --> 00:05:46.380 on the left runs 5’ to 3’ and the strand on the right here runs 3’ to 5’. Well, 00:05:48.949 --> 00:05:55.039 it turns out that DNA polymerase can only works in the 5’ to 3’ direction. So…the 00:05:55.039 --> 00:06:02.000 strand that runs 5’ to 3’ is fine. It is called the leading strand. But the other 00:06:02.000 --> 00:06:07.729 strand will make it a little tricky. DNA polymerase can only go in the 5’ to 3’ direction. 00:06:07.729 --> 00:06:13.720 (NOTE: Reads in 3' to 5' direction). Primase has to set a lot of extra primers down to 00:06:13.720 --> 00:06:20.720 do that as shown here. It takes longer too. This strand is called the lagging strand which 00:06:21.400 --> 00:06:27.159 is pretty fitting.On the lagging strand, you tend to get little fragments of synthesized 00:06:27.159 --> 00:06:33.020 DNA. These are called Okazaki fragments. Okazaki. What an amazing name. The primers have to 00:06:33.020 --> 00:06:40.020 get replaced with DNA bases since the primers were made of RNA. Ligase, the gluing enzyme 00:06:40.620 --> 00:06:46.159 as I like to nickname it, has to take care of the gaps in the Okazaki fragments.Now at 00:06:46.159 --> 00:06:52.020 the end, you have two identical double helix DNA molecules from your one original double 00:06:52.020 --> 00:06:59.020 helix DNA molecule. We call it semi-conservative because the two copies each contain one old 00:07:01.050 --> 00:07:08.050 original strand and one newly made one. One last thing. Surely you have had to proofread 00:07:08.830 --> 00:07:14.050 your work before to catch errors? Well, you definitely don’t want DNA polymerase to 00:07:14.050 --> 00:07:21.050 make errors. If it matches the wrong DNA bases, then you could have an incorrectly coded gene…which 00:07:21.110 --> 00:07:28.099 could ultimately end up in an incorrect protein---or no protein. DNA polymerase is just awesome…it 00:07:28.099 --> 00:07:35.099 has proofreading ability. Meaning, it so rarely makes a mistake. Which is very good. That’s 00:07:35.550 --> 00:07:38.410 it for the amoeba sisters and we remind you to stay curious! 00:07:38.410 --> 00:07:47.570 Follow us on Twitter (@amoebasisters) and Facebook!