1 00:00:00,000 --> 00:00:03,680 Captioning is on. Click the CC button at bottom right to turn off. 2 00:00:03,680 --> 00:00:08,300 Follow us on Twitter (@amoebasisters) and Facebook! 3 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 4 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 5 00:00:19,930 --> 00:00:25,190 molecule like DNA that is ultimately responsible for controlling the cell…it would make sense 6 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 7 00:00:32,500 --> 00:00:39,460 cell. And that introduces our topic of DNA replication, which means, making more DNA. 8 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 9 00:00:48,079 --> 00:00:54,070 has a nucleus. Remember, not all cells have a nucleus. This video clip is actually going 10 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. 11 00:01:00,969 --> 00:01:07,969 Prokaryotes, which are cells that lack a nucleus, do things a little differently. Next When 12 00:01:13,819 --> 00:01:19,860 does this happen---this typically happens during a stage known as interphase. Interphase 13 00:01:19,860 --> 00:01:25,119 is when a cell’s growing, it’s carrying out cell processes, and it’s replicating 14 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 15 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 16 00:01:36,030 --> 00:01:43,020 bit too much multitasking. So DNA replication does not happen during cell division (aka 17 00:01:43,020 --> 00:01:50,020 mitosis). In fact, the cell replicates its DNA before division processes like mitosis 18 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. 19 00:01:56,090 --> 00:02:01,399 I think DNA replication would actually make a great video game. It’s actually quite 20 00:02:01,399 --> 00:02:05,940 exciting. I’m going to introduce the key players in DNA replication so that you can 21 00:02:05,940 --> 00:02:11,810 get some background information. The majority of these key players that I’m going to introduce 22 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 23 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 24 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. 25 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 26 00:02:39,230 --> 00:02:44,090 unzipping the two strands of DNA. Helicase doesn’t have a hard time doing that. The 27 00:02:44,090 --> 00:02:46,370 hydrogen bonds that hold the DNA strands together is pretty weak compared to other kinds of 28 00:02:46,370 --> 00:02:49,879 bonds. DNA Polymerase- the builder. This enzyme replicates DNA molecules to actually build 29 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 30 00:02:57,379 --> 00:03:02,510 polymerase can’t figure out where to get started without something called a primer. 31 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. 32 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 33 00:03:12,359 --> 00:03:19,359 of RNA. Ligase- the gluer. It helps glue DNA fragments together. More about why you would 34 00:03:20,709 --> 00:03:26,829 need that later. Don’t feel overwhelmed. We’ll go over the sequence in order. Please 35 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 36 00:03:32,099 --> 00:03:38,409 are always more details to every biological process. There is more involved than what 37 00:03:38,409 --> 00:03:44,319 we cover. DNA replication starts at a certain part called the origin. Usually this part 38 00:03:44,319 --> 00:03:50,310 is identified by certain DNA sequences. There can be multiple origins within the DNA strand. 39 00:03:50,310 --> 00:03:57,310 At the origin, helicase (the unzipping enzyme) comes in and unwinds the DNA. 40 00:03:57,420 --> 00:04:05,160 SSB proteins (which stands for single stranded binding proteins) bind to the DNA strands to keep 41 00:04:05,170 --> 00:04:11,790 them separated. Primase comes in and makes RNA primers on both strands. This is really 42 00:04:11,790 --> 00:04:17,290 important because otherwise DNA polymerase won’t know where to start. 43 00:04:17,399 --> 00:04:24,780 Now comes DNA Polymerase. Remember, it’s the important enzyme that adds DNA bases. 44 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 45 00:04:30,180 --> 00:04:37,180 also are anti-parallel so they don’t really go in the same direction. 46 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. 47 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 48 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 49 00:05:01,000 --> 00:05:08,000 on the sugar are numbered right after the oxygen in a clockwise direction. 1’, 2’ 50 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 51 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 52 00:05:24,379 --> 00:05:30,270 DNA strands are anti-parallel to each other. So let’s count these---again, clockwise 53 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 54 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, 55 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 56 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 57 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. 58 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 59 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 60 00:06:21,400 --> 00:06:27,159 is pretty fitting.On the lagging strand, you tend to get little fragments of synthesized 61 00:06:27,159 --> 00:06:33,020 DNA. These are called Okazaki fragments. Okazaki. What an amazing name. The primers have to 62 00:06:33,020 --> 00:06:40,020 get replaced with DNA bases since the primers were made of RNA. Ligase, the gluing enzyme 63 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 64 00:06:46,159 --> 00:06:52,020 the end, you have two identical double helix DNA molecules from your one original double 65 00:06:52,020 --> 00:06:59,020 helix DNA molecule. We call it semi-conservative because the two copies each contain one old 66 00:07:01,050 --> 00:07:08,050 original strand and one newly made one. One last thing. Surely you have had to proofread 67 00:07:08,830 --> 00:07:14,050 your work before to catch errors? Well, you definitely don’t want DNA polymerase to 68 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 69 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 70 00:07:28,099 --> 00:07:35,099 has proofreading ability. Meaning, it so rarely makes a mistake. Which is very good. That’s 71 00:07:35,550 --> 00:07:38,410 it for the amoeba sisters and we remind you to stay curious! 72 00:07:38,410 --> 00:07:47,570 Follow us on Twitter (@amoebasisters) and Facebook!