Cell cycle phases | Cells | MCAT | Khan Academy
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Cell cycle phases | Cells | MCAT | Khan Academy

So here I’ve got a picture of an average person. They’re a little more than 5 feet tall. So if we were to convert that into meters, the average person is a little less than about two meters tall. Now, a person is the largest unit of life we consider. When we talk about the smallest unit of life, we’re talking about a cell that relatively looks like that. And while a human being is about two meters tall, a cell has a diameter of about 100 micrometers, which, to put into perspective relative to a human being is about one one-millionth of the size. And yet there’s so much that goes on here. Just as human beings grow and maybe will have babies, cells do the exact same thing. They grow and have babies, as well, or they undergo cell division. So, in the next couple of videos, we’re going to talk about how a cell grows and divides. So let’s zoom in on the cell right here and spend the next couple of videos talking about how a cell grows and divides. The life span of a cell can be described by what’s called the cell cycle. The cell cycle can be thought of as seasons in a year. Just as we have seasons such as the spring or the summer where things grow, versus fall and winter where they don’t, the cell has times when it grows and divides and other times when it doesn’t divide. There are two main overarching seasons, or types of seasons, that we can talk about here. There’s this period here that’s more like the fall or the winter, where you don’t have as much cell division, but you have more growth of the cell. This period is where the cell spends most of its time, and it’s called interphase. Interphase: where we primarily have cell growth occur, but not cell division. Interphase is where cells spend most of the time, so most cells live here. But there is one key exception. What do you think that one exception might be? I think I heard you correctly if you said, “Cancer.” You are absolutely right. Cancer cells have some defect in them that causes them to want to divide more so than grow, and we’ll talk more in detail about how that occurs in a minute. The other main phase of the cell cycle here is where you have active cell division, and it’s called mitosis. Mitosis, or sometimes it’s abbreviated with just an “M.” Mitosis is the time where you have active cell division. Now, there are a few other phases that occur within interphase. The first part of interphase is a growth phase. It’s usually abbreviated, “G1.” As you can see here, G1 is the longest phase of the cell cycle, so most of a cell’s life is spent here, and it’s in this phase that we produce extra organelles, such as ribosomes and proteins. So we make proteins that will be useful when we get to the point of cell division.>From here, the cell has a choice. If it wants to continue growing and move towards the direction of cell division, it will move forward this way to the next phase that’s called the, “S phase.” The S phase just stands for synthesis, more specifically DNA synthesis, because here we’re going to have DNA replication. That’s where we take 23 pairs of chromosomes, so 23 pairs. We call them pairs because half of them are from your mom, half of them are from your dad, and we duplicate them, we replicate them, and we end up with 46 pairs. and most cells go in this direction as I mentioned over here. Some cells however, instead of going forward, from G1 they’ll go in another direction here to a phase that’s called, “G not,” or G 0,” where you have no more division. No more cell division. Because there are certain cells in the body that don’t like to divide or don’t tend to divide, and you can think of a quick example like neurons in the brain. Once your brain is formed it doesn’t necessarily need to divide any more. You just have cells grow. So that means it’s sort of the end point for these types of cells. They won’t usually come back and enter the cell cycle in this way. But let’s continue as if it had. The next step, or the next phase of the cell cycle, is called, “G 2,” which is another growth phase where we are more directly preparing for mitosis. So, we prepare for mitosis in a couple of ways. Perhaps a good example is we make microtubules, which will be used to pull our chromatids apart when it comes time for anaphase, and we’ll talk about what that means in a separate video. And finally, to be complete, we have our last phase right here, which is just mitosis, which is our final season of the cell cycle where our cell will divide. And once it’s divided and turned into two cells, each of the cells will next enter the G1 phase, where they will grow and produce extra organelles and proteins that will eventually allow them to divide again. So, as you can see, just like seasons in the year, the cell cycle goes around and around as we divide cells further and further, unless it’s a cell that goes off to this G not or G0 phase where we don’t need any more cell division.

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100 thoughts on “Cell cycle phases | Cells | MCAT | Khan Academy

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  3. love the video and explination..how long is the process…does this happen in seconds..minutes..hours..days…weeks ect. ? @khanacademymedicine

    As the first active phase of mitosis, prophase is when structures in the cell’s nucleus begin to disappear, including the nuclear membrane (or envelope), nucleoplasm, and nucleoli. The two centrosomes, duplicated in the synthesis process during interphase and each containing two centrioles, push apart to opposite ends of the nucleus, forming poles.

    The centrioles produce protein filaments that form mitotic spindles between the poles as well as asters (or astral rays) that radiate from the poles into the cytoplasm.

    At the same time, the chromatin threads (or chromonemata) shorten and coil, forming visible chromosomes. The chromosomes divide into chromatids that remain attached at an area called the centromere, which produces microtubules called kinetochore fibers. These interact with the mitotic spindles to assure that each daughter cell ultimately has a full set of chromosomes. The chromatids start to migrate toward the equatorial plane, an imaginary line between the poles.

    After the chromosomes are lined up and attached along the cell’s newly formed equator, metaphase officially debuts. The nucleus itself is gone. The chromatids line up exactly along the centerline of the cell (or the equatorial plane), attaching to the mitotic spindles by the centromeres. The centromere also is attached by microtubules (spindles) to opposite poles in the cell.

    In anaphase, the centromeres split, separating the duplicate chromatids and forming two chromosomes. The spindles attached to the divided centromeres shorten, pulling the chromosomes toward the opposite poles. The cell begins to elongate. In late anaphase, as the chromosomes approach the poles, a slight furrow develops in the cytoplasm, showing where cytokinesis will eventually take place.

    Telophase occurs as the chromosomes reach the poles and the cell nears the end of division. The spindles and asters of early mitosis disappear, and each newly forming cell begins to synthesize its own structure. New nuclear membranes enclose the separated chromosomes. The coiled chromosomes unwind, becoming chromonemata once again. There’s a more pronounced pinching, or furrowing, of the cytoplasm into two separate bodies, but there continues to be only one cell.

    Cytokinesis means it’s time for the big breakup. The furrow, formed by a contractile ring that will divide the newly formed sister nuclei, migrates inward until it cleaves the single, altered cell into two new cells. Each new cell is smaller and contains less cytoplasm than the mother cell, but the daughter cells are genetically identical to each other and to the original mother cell, and will grow to normal size during interphase.

  5. My lecturer went over this for a whole hour and I didn’t get it, you spent 5 minutes and it was perfect! Thanks

  6. Dude, 5 feet = 2 meters hahaha
    For reference, 2 meters are about 6 ft 6 or 6 ft 7.
    I got a heart attack when you said the average person is 2 meters tall 😀

  7. since a cell's diameter measures 100 micrometers and a micrometer is 10^-6 of a meter… at 1:40 shouldn't it be 1/10 000 of the human size?

  8. correct me if I am wrong… I am still a student, but 0:25 100µm is not 1/1 000 000 of an average human (which you claim to be >2m tall) however 1µm is 1 000 000 of a meter.

  9. I NEED TO MAKE SUMMARY OF THE CELL CYCLE. Someone plz say all of that in 3 simple sentences at least 5-8 i just rlly need all of that summarized so i know what it is. I can memorize it easier

  10. how does the cell know that it needs to go into the next phase? why does the cycle not move backwards? how does the cell know?

  11. Medicine students do not have much time to listen unnecessary discussion please get quickly to the topic in your next videos.

  12. It is illogical to believe cell division causing mitosis and structural inheritance could have evolved. Since the internal structures of the cell and of it's membrane are reproduced not by genetic information and development, but by division, we have profound evidence that a chicken has always been a chicken, a human has always been a human, and that cells never evolved. This is profound empirical evidence that organisms have always reproduced without coming into being from precursors, and that life was Intelligently Designed and Specially Created in the beginning. In other words, evolution theory cannot be true because of Cortical Inheritance, also known as Structural Inheritance, Cytotaxis, or Directed Assembly. This is only one of thousands of discoveries which have verified creation and disproved evolutionism. Evolutionists refuse to accept the scientific discoveries for what they are and dogmatically hold to their philosophical constructs because they refuse to accept that they are a product of Special Creation.

    Other such discoveries are
    1. that polymerase is a product of it's own translation
    2. that the rhibose sugar backbone molecule of RNA and DNA are not produced by natural chemistry but only by the workings of cellular machinery
    3. that DNA is a medium encoded with information which is organized linguistically and operates algorithmically
    4. that cytozine is not produced by natural chemistry at a level which can account for the evolution of the cell against natural entropy
    5. that mutations are slowly replacing the nucleotides of DNA with thymine
    6. that mutations do not design anything biological but instead cause weakness, deformities, and death
    7. that the load of slightly deleterious is ever-increasing and at a rate far too high for mankind to have existed for 250,000 yrs, much less for the evolution of any family of organisms from a antecedent family
    8. that everything in a cell is irreducibly complex
    9. that abiogenesis has been discovered to be chemically impossible (see Dr. James Tour)
    10. that there are no series of fossils which show clear evidence of the transition from one family of life into another (see stasis and Steven J. Gould)
    …and countless others. Evolutiomism is a philosophy, not science, nor is it scientific. It is a philosophy which existed before the advent of modern science, which has thoroughly discredited it and provided a n astonishing wealth of evidence of Special Creation.

  13. Does the S phase only happen to sex cells (gametes) or all cells experience it? The video explains "many" cells go in that direction but I thought it's only about sex cells. (Thanks superb clear explanation video!)

  14. Neurons do divide, if you don't drink alcohol and if you don't eat carbohydrates. Read Dr David Perlmutter's Grain Brain

  15. most cells go through the Go phase. Some cells stay there permanently and others continue to the S phase but it's part of the cycle for all cells.

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