BIOLOGY explained in 17 Minutes

Hi! You're on a rock, floating in space. Have you ever wondered how we got here? Well, about 4.5 billion years ago, the Earth was a big ball of flame and rocks, constantly bombarded by even more rocks from space. Fun fact! Those rocks probably had some water inside them, which has now turned into steam. Breaking news! The Earth is cooling down. Oh yeah, did I mention that... Whoops! Everything's flooded. But hey, at least there's some cool stuff at the bottom, like hydrothermal vents, which are piping hot and filled with a bunch of chemicals that can make some very interesting stuff. Wait a minute! What the heck is going on here? Biology is the study of life, but really, it's just chemistry in disguise. I mean, you and I are basically just a big ball of molecules that can make funny sounds. Carbohydrates, lipids, proteins, and nucleic acids are some of the molecules that are fundamental to life. Carbohydrates give you quick energy, lipids store long-term energy and make membranes, proteins make up tissues, and nucleic acids make DNA. Also, to make all the chemical reactions possible, living beings have inside of them a bunch of enzymes. They're special proteins that act as catalysts, which just means they help chemical reactions speed up by either breaking down or combining one specific thing. For example, lactase breaks down lactose, the sugar found in milk. Okay, so enzymes make life possible by speeding up chemical reactions, but what even is life? Scientists don't really seem to agree, but obviously a cat is different from a rock. The cat can produce energy by metabolizing food, it can grow and develop, reproduce, and it responds to the environment, whereas the rock does not. Also, unlike rocks, every living thing on Earth is made of cells, of which there's two main categories, eukaryotes and prokaryotes. Eukaryotes have fancy organelles which are bound by membranes, like the nucleus inside of which is DNA. Prokaryotes have none of those organelles, and the DNA is just kinda chillin' there, like, freely floating around. This is why prokaryotes are just single-cell organisms like bacteria and archaea, whereas eukaryotes can form complex organisms like protists, fungi, plants, and animals. These are what's known as kingdoms, which is a taxonomic rank, so basically how we classify different living things and how they're related to one another. Because there are quite a few species of life on this planet, and naming them cat, dangerous cat, and water cat wouldn't really be all that helpful, we also give every species a unique and unambiguous scientific name consisting of the genus and the species. One thing every species has in common is homeostasis, a.k.a. keeping certain conditions in check so you don't die. If you feel warm, your body will sweat. If you're cold, your body will shiver. A cell does kinda the same thing, just that it balances out concentrations of certain chemicals. You see, enzymes, for example, only work in a very specific environment, let's say some specific pH value. If this changes too much, the enzymes will denature and won't work anymore. To counter this, the cell needs to constantly keep up this specific pH value, Okay, but like, how does the cell do that? The secret lies in the cell membrane. You see, it's a semi-permeable phospholipid bilayer- Okay, that's way too many words. All it is is two layers of these funky looking molecules with a polar head and a non-polar tail. This allows small molecules like water and oxygen to slip right through, whereas larger particles like ions need special channels that can be opened or closed, which gives the cell control of what goes in and out. Naturally, particles move with the gradients, so from a place of high concentration to a place of low concentration. Or, in the case of water, it can also move to a place of high solute concentration. So, for example, salt. Welcome to Biology Pro Tips Season 1. Tip of the day, do not drink too much salt water. There's a bunch of salt in salt water. In fact, more salt than inside of a cell, which means it will draw water from your cells and dehydrate you. Yeah, that's it. Have a great day. Anyway, the process of balancing out gradients is known as diffusion and happens automatically. But, by using a little bit of energy, particles can actively be moved against the gradient. The energy comes from adenosine triphosphate, or ATP. To be exact, the highly energetic chemical bonds between the phosphate groups can be broken to obtain energy. This is kind of important, as in, every organism and every cell needs to make ATP. For example, through cellular respiration, which happens in the mitochondria. Together with oxygen, glucose, so sugar, is turned into water, carbon dioxide, and ATP. This is nice, but it only works if you already have glucose. Humans are heterotrophs. They eat food, inside of which is sugar, which is then broken down into glucose. Plants, on the other hand, are autotrophs. Simply put, they said, Screw food, I'll just make my own glucose by staring at the sun. You see, plant cells have smaller ganels called chloroplasts, inside of which is chlorophyll, which absorbs red and blue light, but reflects green light, which is why most plants look green. The absorbed energy from light is used to split water and make a special form of carbon dioxide, which can then be turned into glucose and oxygen. Okay, quick recap. Once you have glucose, either from food or photosynthesis, you can do cellular respiration to get energy in the form of ATP. Chemically, ATP is what's known as a nucleotide. It has a phosphate group, a 5-carbon sugar, and a nitrogenous base. You know what else is made of nucleotides? Deoxyribonucleic acid, or DNA. It consists of two strands of nucleotides, with the sugar and phosphate groups, but the actually important part is the nitrogenous base, which comes in four flavors. Adenine, thymine, cytosine, and guanine. These bases can form base pairs through hydrogen bonds, where adenine goes with thymine and cytosine goes with guanine. These bonds are what holds the two strands of DNA together. Okay, that's cool, I guess, but like, how the heck does that store genetic information? I'm glad you asked. A gene is a section of this DNA that codes for a special trait by carrying a certain sequence of base pairs, which is like a recipe for making a protein. Why proteins? Because they're like, really important. They transport molecules, act as enzymes, and determine the way you look. For example, the difference between brown and blue eyes is the amount of a pigment called melanin in the cells of the iris. The OCA2 gene codes for P protein, which we believe controls the amount of melanin in cells, meaning that the proteins made from this gene could be what determines your eye color. Pretty cool. There's just one issue. Your DNA and its information is in the nucleus, but proteins are made in organelles called the ribosomes. How do we get the information from A to B? The answer is RNA. It's kind of like DNA, just that it's most often a single strand. It uses ribose instead of deoxyribose, and instead of thymine, it uses uracil, which makes it less stable. But that's besides the point. Here's what RNA actually does. Let's say you want to make the protein coded for by this gene. An enzyme called RNA polymerase will split the DNA and make a strand of RNA with a complementary basis, essentially copying the information from the DNA to the RNA. This is called transcription. The new strand is called messenger RNA, or mRNA, because it carries this message out of the nucleus to a ribosome. Remember how I said that a gene is like a recipe for a protein? Well, on the mRNA, which carries the same base sequence as that gene, every group of three bases, which is called a codon, codes for a specific amino acid, which are the building blocks for proteins. These amino acids are carried by special molecules called transfer RNA, or tRNA, which have a unique anticodon that can only attach to its matching codon on the mRNA. The job of the ribosome is to read over codons on the mRNA and attach the matching tRNA molecules, which then leave behind their amino acid. As the ribosome moves along the mRNA and attaches more tRNA, which happens a couple thousand times, the amino acids combine into a polypeptide chain, which is just a really long chain of amino acids that can be bunched up, creased, smacked, and folded into a protein. Okay, let's recap. A gene is copied onto mRNA, which is then used to build proteins by assembling a chain of amino acids, aka transcription and translation. Welcome to Biology Pro Tips Season 2! If you want to decode a sequence of RNA, there is actually a chart for that. Yeah, that's all. Have a great day. Oh yeah, did I mention that you have, like, a bunch of DNA? You have about 20,000 protein-coding genes, each thousands to millions of bases long, and that only makes up around 1% of your entire DNA. The rest is just non-coding. Plus, almost every cell in your body contains your entire genetic code, but certain genes can be turned on and off depending on the cell, which is good, because otherwise your brain cells might just start making stomach acid, which would not be good. Fun fact! If you were to stretch out the DNA of just one single cell, it would be about 2 meters long. Wait a minute, how does that fit into a microscopic cell? Well, if you were to look inside the nucleus, you wouldn't find the DNA just floating around like this, or even this. No, you would actually find lots of these worm-looking things. To be exact, DNA is coiled up around proteins called histones, which are then condensed into strands of chromatin, which are then coiled up even more to make tightly packed units of DNA called chromosomes, which kind of look like worms. Different sections on a chromosome carry different genes, and the entire human genome is split amongst 23 different chromosomes, although every body cell has two copies of every chromosome, one from the mother and one from the father. For most chromosomes, the two copies are said to be homologous, meaning that they carry the same genes in the same locations. However, the two versions of a gene can be different, so the mother's gene could code for brown eyes, while the father's gene codes for blue eyes. These different versions of a gene are called alleles. For most of your genes, you have two alleles, one on each chromosome from either parent. These alleles can be dominant or recessive, which determines which of them is expressed. For example, brown eye color is a dominant trait, which is shown by an uppercase B, whereas blue is recessive, which is shown by a lowercase b. All this means is that if you have the dominant brown allele, you will have brown eyes, no matter what the second allele is. Only when there are two recessive alleles will it be expressed. With this knowledge, we can predict the future. Let's look at how this trait is inherited from parents to children. Both of these parents have brown eyes, but also have a recessive blue allele in their genotype. Every child receives one allele from each parent randomly, so these are the possible combinations for the children. Most combinations contain the dominant brown allele, so the child will have brown eyes. But there is a small chance that a child gets two recessive alleles and has blue eyes, even though both parents had brown eyes. You see, it's what's on the inside that counts. Alright, that's cool, but reality is not always so simple. Some genes are not fully dominant, but not fully recessive either, which means that the phenotype, so the appearance, appears to mix. Crossing a red and a white snapdragon, where red is dominant and white is recessive, gives you a pink phenotype, which is somewhere in between, a.k.a. intermediate inheritance. Or, crossing a brown and a white cow, where both colors are dominant, could give you a spotted cow, so both phenotypes are expressed equally, a.k.a. codominance. Hey, remember how I said that almost all chromosomes are homologous? Well, there's one exception, the sex chromosomes. Females have two big X chromosomes, whereas males have one X and one smaller Y chromosome. These are partially homologous at the top, but since the Y chromosome is so small, it's missing genes that are present on the lower part of the X chromosome. These genes are called X-linked genes. If one of these genes is a recessive trait, like colorblindness, males are stuck with that trait, whereas females probably have another dominant allele to override it. This is why most colorblind people are male. Now, for genes to even be passed on, the body has to make new cells which can inherit the genes. There's two main mechanisms. Mitosis, which is how the body makes identical copies of body cells to grow in a pair of tissues, and meiosis, which is how the body makes gametes, so sperm and egg cells. Mitosis starts with a diploid cell, so a cell with two sets of chromosomes. These chromosomes consist of one chromatid, which has to be replicated for the new cell. After replication is when you see the familiar X shape consisting of two identical sister chromatids. These are split into two identical diploid cells with two sets of chromosomes consisting of one chromatid. Meiosis also starts with a diploid cell, but after replication the chromosomes co-mingle and exchange genetic information in a process called crossing over. The cells then split into two non-identical haploid cells. These have one set of chromosomes, but they still consist of two sister chromatids. These cells split again into four genetically different haploid cells, where each chromosome has one chromatid. Meiosis produces haploid cells, so that when two gametes combine into a fertilized egg or zygote, it again has the correct number of chromosomes. This is cool, but cell division is only a tiny part of a cell's entire life cycle. Most of its time is actually spent in interphase, a.k.a. just chilling. All it does here is grow and replicate all of its DNA so that it actually has enough genetic material and size to divide in M phase. There's multiple checkpoints in the cell cycle which are controlled by proteins like p53 or cyclin to check if the cell is healthy and ready to reproduce. If a cell is not quite right, it's either fixed or it destroys itself, which is called apoptosis. Or at least, that's what it should do. Normal cells replicate until there's no need to, but some cells just keep going. This is because they don't respond correctly to these checkpoints and end up replicating out of control and functioning wrong, which is also known as cancer. This damaging behavior is often the result of a gene mutation, which is a change somewhere in the base sequence of a gene. This can happen during DNA replication when a single base is changed, left out, or inserted into the original sequence. This often changes the protein coded for by that gene, and let's just say that change is often not optimal. Another type of mutation happens in chromosomes, where entire sections of a chromosome could be duplicated, deleted, flipped around, or transferred between chromosomes. The most famous chromosomal mutation is probably when the 21st pair of chromosomes has an additional copy so that there's three instead of two. The result? Down syndrome. Mutations might seem like a bad thing, but actually, they can also be neutral or even beneficial. For example, a species of yellow grasshoppers might mutate and become green, which makes them blend in with the grass and get eaten less. Over time, you can expect to see more and more green grasshoppers as their fitness has increased. Not that kind of fitness, fitness as in they can have more offspring because they get eaten less. This is natural selection and the driving factor behind evolution, as the poorly adapted species gets selected against, and the fittest species, which has adapted to the environment, survives and has the most offspring, passing down the trait that made them survive. But sometimes, random species get lucky and survive big extinction events or maybe find new land with no competition. They're gonna survive, even if their genes are actually, like, kinda bad. This is called genetic drift. If you think adaptation is cool, yeah, but it also kinda sucks. You see, humans can get sick from bacteria or viruses, but nowadays we have medicine that works. Good. However, what if the bacteria mutates and suddenly the medicine doesn't work anymore? Well, that's kind of exactly what's happening and we have no clue how to fix it. So... yeah. Oh yeah, by the way, one thing many people confuse is bacteria and viruses, and no, they're not the same. Bacteria are prokaryotes, they consist of a single cell which can reproduce on its own, and we treat bacterial infections such as strep throat and tetanus with antibiotics. Viruses are not made of cells. In fact, we're not even sure they're alive. They share some signs of life, but they can only reproduce inside a host. They don't grow, so it's not really alive, but it's not dead either. It's more like a non-living kind of thing. Also, you cannot treat viral infections with antibiotics. Most of the time, you just have to chill out and let your immune system do its thing. Now you might think bacteria are a bad thing, but actually you have millions of good bacteria inside your gut. They live in symbiosis with you. You give them food, and they help you digest it. Speaking of digestion, your body is made of many complex organ systems that work together to make sure you don't die. And I know what you're thinking. Actually, I don't, but I know how you're thinking. The nervous system, consisting of nerves which connect to the spinal cord and lead to your brain, is made of cells called neurons, which can conduct electricity along this long tube called the axon. Anything you see, think, and feel, it's all just electrical signals going to your brain, and your brain telling your body how to respond. To be exact, the signals are called action potentials and happen at the same strength and the same speed every time. So the only difference between, hey, I'm a little cold, and oh my god, I'm on fire, is where it happens and how frequent the signals are. When a neuron is just chilling, the axon is more negative on the inside than on the outside because there's an unbalanced amount of ions. This causes an electric potential of about negative 70 millivolts. When there's a stimulus, signaling molecules called neurotransmitters dock onto ion channels on the axon and open them, letting ions flow and changing the electric potential around that area. Now, action potentials are all or nothing. A small stimulus won't really do anything, but if the potential exceeds about negative 55 millivolts, the neuron gets excited. Ion channels around the stimulus open and ions rush into the cell. This causes the charge distribution in that section of the axon to reverse for a split second, which is called depolarization. Ion channels that are next to this area are influenced by this and open as well, which causes a chain reaction that sends the signal all the way down the axon. Some neurons have a myelin sheath made of Schwann cells, which insulate the axon and only leave tiny gaps called nodes of Ranvier. If there's a stimulus, the charges can jump across the nodes, which transmits the signal way faster than a normal neuron. But either way, at the bottom, the electric signal reaches a terminal button, which connects the current neuron to the dendrites of the next. If you were to zoom in, you'd notice that the two cells don't even touch. There's actually a small gap. This is once again where neurotransmitters come in. Once the button is depolarized, tiny packages of neurotransmitters get released and bind to receptors of the following dendrite, either blocking it from doing anything or causing another action potential, which repeats the cycle. Hmm, something in my brain's telling me that you should definitely subscribe, and also, if you want to stimulate your neurons and find out how math and differential equations are used in biology, a resource I can't recommend enough is Brilliant, which has thousands of interactive lessons for everything from basic math to advanced data analysis and programming. They use a hands-on approach so that instead of memorizing formulas for hours on end, you actually understand and remember what you're even learning. Not only that, but they have plenty of real-life applications that you can immediately apply the knowledge to, building your problem-solving skills along the way. For example, their scientific thinking course lets you interact with scientific principles and theories, from simple machines like Gears and the physics behind playing snooker, all the way to Einstein's special theory of relativity. Sounds cool if you ask me. The best part? You can try everything they have to offer, for free, for a full 30 days by visiting brilliant.org slash wackyscience. You'll also get 20% off an annual premium subscription. Thanks to Brilliant for sponsoring this video.