Mitosis (UPDATED 2026) — Transcript

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1. 0:00[Captions on, music intro]
2. 0:10Have you ever looked down at your leg or  your arm to find some cut and you have no
3. 0:14idea where it came from or how you got it? So  you put a bandage on it and a few days later,
4. 0:19it’s gone. It’s all healed, and you  don’t think anything of it, of this
5. 0:23amazing process that causes this to happen. The amazing process that involves mitosis:
6. 0:29how your body cells make new body cells. After  a cut, your cells have to repair the damage and
7. 0:35mitosis is what allows them to make more cells to  do that. Mitosis is great for repairing damage.
8. 0:41Another observation: you may notice you’re  a lot bigger than you were when you were a
9. 0:45little kid.In humans, cell division plays a  major role in growing from child to adult.
10. 0:50It’s not that each and every cell in your body  expanded to some massive size. But… that's not
11. 0:57to say individual cells themselves can't grow  in size. Muscle tissue for example can grow from
12. 1:02the muscle cells themselves increasing in size.  But mitosis, which helps cells make more cells,
13. 1:08is important for growth. And not just of  your general body size compared to young
14. 1:13childhood - that mitosis is also involved  with the growth of your hair or your nails.
15. 1:18Mitosis specifically is the division  that happens in eukaryotic cells and
16. 1:23technically it is the division  of the nucleus. Cytokinesis,
17. 1:28which starts up while mitosis is happening but  won’t complete until after mitosis is finished,
18. 1:33specifically splits the cytoplasm so that  you ultimately will end up with two cells.
19. 1:38Why this focus on the nucleus?  Because in eukaryotic cells,
20. 1:42dividing what is inside the nucleus is critical  for making a new cell. The nucleus of most human
21. 1:47body cells hosts DNA. DNA is really important  because it’s your genetic information. And if
22. 1:53you’re going to make more cells, you  need to have the same DNA in those new
23. 1:57cells as you did in your original cells.  You want it to be identical, no mistakes.
24. 2:01The problem is you’ve got a lot of DNA.  And we’ve got to get that DNA into the
25. 2:05new cells using mitosis. So, there has to  be a better way to organize that DNA. Well,
26. 2:12what actually happens is that DNA can be organized  into these condensed units called chromosomes.
27. 2:18Chromosomes are made of DNA and protein. In most  cases, humans have 46 chromosomes in their body
28. 2:24cells. You can find them in most body cell nuclei.  Nuclei is the plural of nucleus. Not…nucleuses.
29. 2:32So visualize 46 chromosomes in a nucleus  of a body cell; in most cases, this is the
30. 2:38number of chromosomes present in body cells.  Organizing DNA into condensed chromosomes also
31. 2:44makes it a lot easier to move over when  you’re making new cells. Before mitosis,
32. 2:48there’s this very long stage called interphase.  Interphase is where DNA replication happens. Those
33. 2:55chromosomes get replicated. But - let me just  say because I still think after all these years,
34. 3:00the math here is a little odd - but if you  replicate 46 chromosomes that look like this,
35. 3:06you still have 46 chromosomes but each of  them now are made of 2 sister chromatids.
36. 3:13These sister chromatids are identical.  They are copies. So before interphase:
37. 3:1846 chromosomes but each one is a single  unit. After interphase: 46 chromosomes
38. 3:25but each one has two sister chromatids, so if  I counted the chromatids, there would be 92.
39. 3:33So, these are replicated chromosomes and  I know they are replicated because they
40. 3:36each have two sister chromatids. And as  long as the sister chromatids are still
41. 3:40attached here at the centromere region, as  a unit, this is one chromosome. For now,
42. 3:46while it stays that way. It’s really important  to explain that because otherwise the mitosis
43. 3:50math doesn’t make sense. So let’s get  into the mitosis phases, shall we?
44. 3:55I like to tell students to remember PMAT. It’s  a little acronym that helps you remember. The P
45. 3:59is for prophase. The M is for metaphase. The A is  for anaphase. The T is for telophase. So remember:
46. 4:06PMAT. The stages in order. Now I will  mention that depending on the level
47. 4:10of biology you may be studying, you might  learn about intermediates within that - like
48. 4:15prometaphase - which is in between prophase  and metaphase. Learn more in our description,
49. 4:20but PMAT has the four major  phases we’ll be focusing on.
50. 4:23So, very first step. Prophase.  It’s the beginning step,
51. 4:27the nucleus is still there, it’ll later  start to disassemble and it will be gone
52. 4:32before metaphase starts. During prophase,  the chromosomes are visible; in fact,
53. 4:37we say they’re condensing which means the  chromosomes are thickening and visible.
54. 4:43The next stage is metaphase. Remember the nucleus  had already broken down before metaphase started;
55. 4:48you won’t see it here. With metaphase, I  remember M for metaphase, M for middle,
56. 4:54because in this stage the chromosomes  line up in the middle of the cell,
57. 4:57waiting where they can be  pulled apart for the next stage.
58. 5:00Next the A is for anaphase. In anaphase, I like  to think of the A for “away.” With the help of
59. 5:06spindle fibers, the chromatids will move away from  each other to opposite sides of the cell. Now,
60. 5:12to really point out, once a chromatid is  no longer attached to its sister chromatid,
61. 5:17you can call it a chromosome. So notice here, the  sister chromatids were separated and 46 of the
62. 5:22sister chromatids - now called chromosomes - were  pulled away to one side of the cell. And 46 of
63. 5:29the sister chromatids - now called chromosomes -  were pulled away to the other side of the cell.
64. 5:34The last stage of mitosis---think  T is for telophase. In telophase,
65. 5:38the chromosomes are actually at the opposite  ends and new nuclei are forming on each side
66. 5:42to make these two new cells. And the nuclei  are starting to surround the chromosomes on
67. 5:47both sides. So I like to remember the T can  stand for “two” because you can really see
68. 5:51in this step that the end goal is going to  be two cells that each have 46 chromosomes.
69. 5:56Cytokinesis is responsible for the final  separation into two cells by splitting
70. 6:01the cytoplasm; that splitting of the cytoplasm  will be complete after the PMAT mitosis stages.
71. 6:07Ok so four last things to  keep in mind with mitosis:
72. 6:10#1) It’s really common for mitosis to be equated  with cell division and there’s a little bit of
73. 6:16simplification there because if you look at  these steps, mitosis is specifically of the
74. 6:20nucleus - mitosis is the nucleus division. Now  it’s a major part of the cell and critical for
75. 6:25cell division but you don’t want to forget  that cytokinesis - that part that actually
76. 6:29splits the cytoplasm - is really important or  otherwise you wouldn’t end up with two cells.
77. 6:34#2) The body cell thing. We said that, in humans,  mitosis is for body cells. I’m talking skin cells,
78. 6:41stomach cells - these are body cells that do  mitosis. There are exceptions; mature red blood
79. 6:46cells are body cells and they don’t do mitosis.  And if we go beyond body cells to gametes,
80. 6:51which are sperm and egg cells - they don’t  do mitosis and they aren’t directly made by
81. 6:56it either. Human gametes, sperm and egg cells,  are directly made by something called meiosis,
82. 7:02which unfortunately has a name that sounds  kind of similar, another video on that.
83. 7:06#3 When you have a skin cell and it does mitosis,  the final result will be two genetically identical
84. 7:11skin cells. So what happens if they’re not  genetically identical, maybe there are changes
85. 7:16in the cell? Well those changes could influence  the control of its cell division. Uncontrolled
86. 7:21cell division - which would mean the mitosis is  not controlled - is often a feature of cancer
87. 7:26cells. In our cell cycle video, we talk about  how, most of the time, cells are interphase.
88. 7:33Most of the time, a cell is not dividing and  therefore not doing mitosis and cytokinesis. But
89. 7:39in cancer cells, cell division is typically not  being controlled as it should. As we continue to
90. 7:45gain understanding of all of this, we can better  develop treatments for conditions like cancer.
91. 7:51#4 Diagrams and illustrations. They’re trying  to represent concepts but they have some
92. 7:56limitations. In most cases, human body cells  have 46 chromosomes. But most textbooks won’t
93. 8:02illustrate that in cell division pictures  because that’s…a lot of chromosomes.
94. 8:06They’ll show 2 or 4 chromosomes typically or  6 or 7. You’ll notice with our cell division
95. 8:13illustrations, we don’t show all 46 either.  Petunia’s pseudopods would not survive the video.
96. 8:19Well, that’s it for the Amoeba Sisters,  and we remind you to stay curious.