Every living thing — every plant, animal, fungus and microbe — is built from cells. You are a community of around 37 trillion of them, each one a microscopic, self-sustaining machine more sophisticated than any factory humans have built. This course goes inside the cell, the fundamental unit of all life. (It complements our courses on DNA and the microscopic world, focusing here on the cell itself.)You'll start with a grand tour of what a cell actually is, then explore the bustling world inside it — the organelles that each do a vital job. You'll discover how cells power everything you do by turning food into energy (the molecule called ATP), and finish with the elegant process of cell division that lets a single fertilised egg become a whole body. Honest note: cell biology is well-established science, but it's also a vast field — this is an introductory tour of the essentials, not the whole of molecular biology.
Cells are the smallest living units of an organism. All cells have three things in common no matter what type of cell they are. All cells have a cell membrane which separates the inside the cell from its environment, cytoplasm, which is a jelly-like fluid, and DNA which is the cell's genetic material. There are two broad categories of cells. The first category is eukaryotic cells. They have organelles which include the nucleus and other special parts. Eukaryotic cells are more advanced, complex cells such as those found in plants and animals. The second category is prokaryotic cells. They don't have a nucleus or membrane enclosed organelles. They do have genetic material but it's not contained within a nucleus. Prokaryotic cells are always one celled, or unicellular organisms, such as bacteria. So what are organelles? Organelle means "little organ." Organelles are the specialized parts of a cell that have unique jobs to perform. Let's start with the nucleus, the control center of the cell. The nucleus contains DNA or genetic material. DNA dictates what the cell is going to do and how it's going to do it. Chromatin the tangled, spread out form of DNA found inside the nuclear membrane. When a cell is ready to divide DNA condenses into structures known as chromosomes. The nucleus also contains a nucleolus, which is a structure where ribosomes are made. After ribosomes leave the nucleus they will have the important job of "synthesizing", or making, proteins. Outside the nucleus the ribosomes and the rest of the organelles float around in cytoplasm, which is the jelly-like substance. Ribosomes may wander freely within the cytoplasm or attach to the endoplasmic reticulum, sometimes abbreviated as ER. There are two types of ER: rough ER has ribosomes attached to it and smooth ER doesn't have ribosomes attached to it. The endoplasmic reticulum is a membrane enclosed passageway for transporting materials such as the proteins synthesized by ribosomes. Proteins and other materials emerge from the endoplasmic reticulum in small vesicles where the Golgi apparatus, sometimes called the Golgi body receives them. As proteins move through the Golgi body they're customized into forms that the cell can use. The Golgi body does this by folding the proteins into usable shapes. or adding other materials on to them such as lipids or carbohydrates Vacuoles are sac-like structures that store different materials. Here, in this plant cell, the central vacuole stores water. Going back to the animal cell, you'll see an organelle called a lysosome. Lysosomes are the garbage collectors that take in damaged or worn out cell parts. They are filled with enzymes that break down this cellular debris. The mitochondrion in is an organelle that is the powerhouse for both animal and plant cells. During a process called cellular respiration the mitochondria make ATP molecules that provide the energy for all the cells activities. Cells that need more energy have more mitochondria. Meanwhile the cell maintains its shape through a cytoskeleton. The cytoskeleton includes the thread-like microfilaments which are made of protein and microtubules which are thin hollow tubes Some organisms such as plans that are photoautotrophic meaning they capture sunlight for energy have cells with an organelle called a chloroplast. The chloroplast is where photosynthesis happens It's green because it has a green pigment called chlorophyll. Plant cells also have a cell wall outside of their cell membranes that shape, support, and protect the plant cell. Animal cells never have a cell wall There are many other unique structures that only some cells have. Here are just a few. In humans, for example, the respiratory tract is lined with cells that have cilia. These are microscopic hair-like projections that can move in waves. This feature helps trap inhaled particles in the air and expels them when you cough. Another unique feature in some cells is flagella. Some bacteria have flagella. A flagellum is like a little tail that can help a cell move or propel itself. The only human cell that has a flagellum is a sperm cell. In summary remember: eukaryotic cells are plant and animal cells with a nucleus and membrane-enclosed organelles While prokaryotic cells are unicellular organisms without these things. All cells have a cell membrane, cytoplasm and genetic material. And even though only plant cells have chloroplasts both plant and animal cells have mitochondria.
Have you ever looked down at your leg or your arm to find some cut and you have no idea where it came from or how you got it? So you put a bandage on it and a few days later, it’s gone. It’s all healed, and you don’t even think anything of it of this amazing process that causes this to happen. Or let’s say you're looking at your nails and you notice they're a lot longer than the last time you cut them. Or let’s say you're looking in the mirror and you notice that you are a lot bigger than you were when you were five years old. What do all of these things have in common? One major thing they both have in common is mitosis. Mitosis is a type of cell division done by most of your body cells and it’s really important for your cells to divide. If they didn’t divide, you wouldn’t grow. I mean, how do you grow if you can’t make more cells, right? So one reason why you’re bigger than you were when you were 5 is mitosis. Mitosis also is great for repair of damage. If you have some kind of accident like when we were talking about that cut on your arm or leg, well you want to make sure it can get repaired well so you have to make more cells to do that. Mitosis is great for that. Now it’s really important to understand what it is not---mitosis is not a process that makes sperm or eggs cells, because that's something different called meiosis, which sounds like mitosis…unfortunately, but it is a different process. Mitosis is done to produce body cells. Mitosis makes identical cells, that is the goal, identical cells. So if you’re trying to make more skin cells, to replace worn out or damaged skin cells, you don’t want to start suddenly making stomach cells there. That would be ridiculous! You want to make sure you have identical cells replacing what was lost, so mitosis makes identical cells. It’s a really important thing. Now, it’s also important to understand that your cells are not dividing all the time. If all they did was divide, it would just be rapid crazy growth. In fact, this is kind of what cancer is. Cancer is uncontrolled cell growth. We have a clip on the cell cycle and what the cell is usually doing most of its daily life, which is actually a phase called interphase where it’s growing and replicating its dna and carrying out its daily cell functions. That’s where cells spends most of their time in respect to the whole cell cycle. Mitosis is a very short amount of time in respect to the whole cell cycle. But mitosis is a critical process because this is where it is going to divide and make more cells. Before we get into the steps of division, it’s really important to understand that your cells have something inside them – an organelle called the nucleus. And the nucleus holds your DNA. DNA is really important because it’s your genetic information. And if you’re going to make more cells, you need to have the same DNA in those new cells as you did in your original cells. You want it to be identical, no mistakes. Very important. The problem is you’ve got a LOT of DNA. And we’ve got to get that DNA into the new cells using mitosis. So there has to be a better way to organize that DNA. Well, what actually happens is that DNA can be organized into these condensed units called chromosomes. Chromosomes are made of DNA and protein. You’ve probably heard before that humans have 46 chromosomes. That means 46 chromosomes are found in most human body cell nuclei. What are nuclei? Well it’s the plural of nucleus. You don’t say nucleuses; you say nuclei. Well in the nuclei, there are 46 chromosomes. Organizing DNA into condensed chromosomes makes it a lot easier to move over when you’re making new cells. So if you have 46 chromosomes in a human body cell, you have to duplicate those chromosomes in interphase before mitosis starts. That basically means you’re duplicating your DNA, since chromosomes are made of DNA and protein. You have to do this before mitosis starts, because if you’re going to make an identical cell that has 46 chromosomes just like the original, well it makes sense you have to duplicate the genetic material before splitting. So if you look at our cell cycle video clip we talk about interphase. That's a stage where most of the time, cells are spending their time. They're actually duplicating their DNA during that time. So ready for the tricky part? Because we tend to count chromosomes by the number of centromeres present, when the 46 chromosomes duplicate, we still say there are 46 chromosomes as the sister chromatids are still attached and we’re counting by centromeres. So 46 chromosomes here, they replicate in interphase, and you still have 46 chromosomes in this picture. But you went from 46 to 92 chromatids. We have a video explaining that in more depth and how that factors in for mitosis. Ok so now we can get right into mitosis. I like to tell students to remember PMAT. It’s a little acronym that helps you remember. The P is for prophase. The M is for metaphase. The A is for anaphase. The T is for telophase. So remember: PMAT. The stages in order. The very first step is prophase. Prophase because it’s the beginning step, the nucleus is still there and it’s going to go away later on but this is a stage where it's actually still there. The chromosomes are visible; in fact, we say they’re condensing which means they are thickening and visible. The next stage is metaphase. M for metaphase, but I also like to remember M for middle because in this stage the chromosomes line up in the middle of the cell. The nucleus has been disassembled, it’s no longer there so we’ve got the chromosomes in the middle waiting there. Next the A is for anaphase. In anaphase, I like to think as the A for “away.” The chromosomes move away, they are moving to opposite sides of the cell, so they are moving towards the poles of the cells. Now one thing to point out, these chromosomes...they're not moving by themselves, they actually have something called spindles. These spindles are fibers that help move the chromosomes to the ends. Kind of helps them move along. The last stage of mitosis---think T is for telophase. In telophase, the chromosomes are actually at the complete opposite ends and new nuclei are forming on each side to make these two new cells. The nuclei are starting to surround the chromosomes on both sides. I like to think the T is for “two” because you can really see in this step that the end goal is going to be two cells and in the human body, they're each going to have 46 chromosomes. And, again, remember, they are identical. Cytokinesis is responsible for the final separation into two cells by splitting the cytoplasm, which completes after the PMAT mitosis stages. So why did all of matter? Without understanding cell division, we wouldn’t understand how growth and repair happens---because they both require more cells to be made. Understanding mitosis is also very important for cancer research too. Cancer itself is uncontrolled cell growth - so in other words, uncontrolled mitosis. Well, that’s it for the amoeba sisters and we remind you to stay curious.