In this video, let's review the whole year worth of AP Biology content in about 50 [Music] minutes. Hey guys, this is Mikey from Avil Prep Academy and on this channel we cover AP Bio content. And before we begin today's massive review, I wanted to let you all know that there's no reason to say goodbye here today. I will be expanding this channel to cover more courses like chemistry, math, and even SAT content. So don't forget to subscribe and keep us in your feed. But today in this video, we're going to be reviewing the entire AP bio course from units 1 through 8 in one go. In this first unit called the chemistry of life, we deal with three major topics. Water, carbon chemistry, and macroolelecules. Water is the solution in which life evolved and continues to operate. Our cells are composed of some 80 to 90% water, and all of the biological reactions take place in the context of water. So it goes without saying that water deserves some attention. First, let's take a look at the structure of water. Water is made of one part oxygen and two parts hydrogen built in a bend shape that lends to its very important property which is polarity. The reason that polarity forms is that the electron shared between the oxygen and hydrogen atoms are more attracted to the oxygen atom due to its greater electro negativity. This results in water having slightly positive ends where the hydrogen atoms are and a slightly negative end on oxygen side. This results in the ability of water to form hydrogen bonds. An intermolecular attractive force formed from these partial charges. The four major properties of water to know are as follows. Cohesion and adhesion or water's ability to stick to each other through those very hydrogen bonds or even its ability to stick to other things. Water's ability to moderate temperature as it would take a lot of energy to begin breaking those numerous hydrogen bonds between the water molecules. Reduce density upon freezing that allows ice to float insulating organisms in bodies of water over winter. And lastly, its ability to act as a universal solvent, dissolving many different types of substances, ranging from other polar and charged substances. This last point is especially important because it lays down the groundwork for how hydrophilic and hydrophobic substances interact with the cell and its membranes. Moving on to carbon. Well, carbon forms the backbone of almost all biologically relevant molecules. And the reason for this has again a lot to do with carbon structure. Looking at the bore model of the carbon atom, we see that there are four veence electrons, each of which could form a covealent bond with another atom next to it. This is sort of the maximum number of bonds a single atom could form, allowing carbons to form larger chains, branching patterns, single, double or even triple bonds as well as cyclical rings. In this part two, we are first introduced to the importance of the relationship between structure and function. The course demonstrates this by introducing isomers and inantimer, which are substances that have the same chemical formula but slightly different structures. Specifically, we learned that right-handed and left-handed sides of the molecules could exist with only one of the two being biologically relevant due to its specific structure. Imagine putting your right foot into your left shoe. Structure matters. Moving on to macroolelecules, we built from that study of carbon and learn about four major categories of carbon-based macro or very large molecules that form cells. Carbohydrates, lipids, proteins, and nucleic acids are these four. But before we get there, we need to first understand that these macroolelecules are typically polymers with the exception being lipids. Polymers are substances made from simpler building blocks that are called monomers. These monomers combine with one another through dehydration synthesis, releasing water in the process of forming coalent bonds. The reverse reaction is also possible and it's known as hydraysis, a term that you'll run into quite a bit in this course. Starting with carbohydrates, we observe that elemental composition of C, H, and O. These are typically formed in a 1:21 ratio in its monomeic form that we call monossaccharides. Glucose is the most important monossaccharide that you'll need to know and it has a formula of C6 H1206. When glucose molecules combine together, we get polysaccharides. And we need to know about three. First, starch is a polymer of glucose that is primarily used as a storage polysaccharide found in plants. Secondly, we have glycogen that serves the same storage function but in animals. Lastly, we have cellulose, a structural polysaccharide found in plants to create that tough fibrous quality. Keep in mind that while animals have the capacity to digest starch through enzymes like amalayise, we don't have the ability to digest cellulose. We're introduced to the idea of symbiosis in herbivores wherein bacteria or proise that can digest cellos form a mutualistic relationship with cows and termites and so on. Next, we have lipids. First, we acknowledge that lipids are non-polar for the most part, meaning that they are largely hydrophobic. Here we learn that there are three major classes of lipids to know, each of which serving a slightly different and a unique function. Fats and oils are made from one part glycerol with each of the hydroxide groups attached to a fatty acid chain. Saturated fats sees a straight hydrocarbon chain and are typically solid at room temperature, while unsaturated fats see kinks in the chain that lends to its liquid state at room temperature. These are both used for long-term storage by animals and plants respectively. Next, we have phosphoippids which are incredibly important for biology because they are made from one part glycerol but two fatty acid chains and a phosphate group which incidentally is charged and that forms the heads of the molecule. This charged head makes the phosphoipid hydrophilic on one side while hydrophobic on the lipid tail side. This means that phosphoippids in aquous solutions can form bilayers that constitute the foundations of plasma membranes. Lastly, we have steroids, which have a rather unique shape with these carbon rings. They show up on this course as either hormones like testosterone or estrogen that can slide right through the membrane during signal transduction, or they can be cholesterol, which regulates cell membrane fluidity in animal cells. Moving on to proteins, we have perhaps the most important macroolelecule for this course. Proteins do everything. And with such great degree of function comes great degree of structures. The reason for protein's ability to be structurally diverse stems from not one, not two, but 20 amino acids that act as monomers. When amino acids combine, we call them polyeptides formed through, well, you guessed it, peptide bonds. All amino acids share the same template structure with each amino acid having a unique R groupoup. These R groupoups are chemicals, of course, and as such contain unique chemical properties, but they can largely be categorized as polar, non-polar, and charged amino acids. And when many amino acids form a polyeptide chain, these R groupoups can interact, forcing the chain of amino acids to twist and fold into a unique three-dimensional shape. These interactions can include hydrophobic interactions, ionic bonding, and even covealent bonding. The shape of course can confer protein its function. With that being said, we should also note that we discuss different levels of protein structures here too. The primary structure is the sequence of amino acids while secondary structures are commonly found shapes within proteins such as alpha helyses and beta pleetus sheets. Tertiary structures refer to that unique shape due to the R groupoup interactions discussed earlier. And finally, coordinary structures form when multiple polyeptides combine to form a single functional protein. And while nucleic acids do show up here in unit one, I'm going to hold off on them until we reach unit 6 a bit later in this video because we're going to have to discuss the DNA and RNA structure at that point again. Anyways, let's move on to unit two on the topic of cell biology. The first part of this unit pertains to the very important idea that all living things are made from cells. As such, we're introduced to the concept of the last universal common ancestor, which presumably passed down its cellular structure to all of its descendants, including us. But here we do see that at one point the lineage bifurcated with procarotic cells retaining a simpler cell structure and ukarotic cells developing additional features. Let's first talk about the features of all cells. All cells are surrounded by a plasma membrane which we'll go into in much greater detail in just a few moments. They have a slime-like cytoplasm on the inside and they also contain genetic material in the form of DNA that are eventually expressed as proteins via ribosomes and all cells are rather small. Now this limitation on cell size seems to stem from the decreasing surface area to volume ratio as the dimensions of the cells become larger and larger. As cells utilize the membrane not only for exchange but also for many other functions, the growing demand of a growing volume cannot be met by the limited relative surface area. Now, while this part of the course does delve into different organels and what they do, keep in mind that the course investigates the details of very specific organels as we move on to topics like photosynthesis and cell respiration. So, the main focus is on a select set of facts that connect to some of the larger ideas in the course. First, we learn about the endomembrane system. The endomemembrane system is a collection of membrane bound organels starting from the nucleus continuing onto the endopplasmic reticulum both smooth and rough the gogi apparatus and the plasma membrane and of course all the vesicles that connect all of these components together. Let's include lysosome here too. The reason for its importance is that the proteins that are excreted from the cell must utilize much of the endommembrane system to be released to the outside. These proteins are produced on the ER surface and then placed into the lumen after which they are packaged into vesicles to be sent through the remainder of the system. Next, we learned that organels such as mitochondria and chloroplast originated as freeliving procaryotic organisms that moved into the evolving ukarotic cells through a process we call endo symbiosis. There are two major pieces of evidence that you need to note here. First, these organels have a double membrane indicating that the endoccyic event must have occurred. And second, they have their own circular DNA, a remnant of procotic origins, as procotic cells have that circular genome, they also have their own ribosomes too. Also note that while cytokeletons don't play a huge role in the course, microtubules in particular should be noted because they are the very spindle fibers that we see during motic and myotic divisions. Any disruptions to the microtubial formation, such as done by chemicals like taxol, could limit cell division, which is a great solution to cancerous cell division. And sure, animal cells and plant cells differ due to the lack of cell walls and chloroplast in the former, but these differences were most likely fleshed out during general biology. So I won't go into too much detail here. Now for unit 2, one specific feature of cells takes the lion share of attention and that is the plasma membrane. And as seen earlier, the foundational building blocks of plasma membranes are phospholippids. However, we call our plasma membrane model the fluid mosaic model. The mosaicism is explained by the presence of proteins either embedded or at least associated with the membrane. While the fluidity stems from their ability to move around laterally. That being said, the fluidity can be regulated with greater degrees of saturated fatty acids in that phosphoipids that can pack membranes together while decreasing the fluidity or more unsaturated fatty acids that can create greater gaps that make the membrane a little bit more fluid. Cholesterol can also play a role in keeping a moderate fluidity by acting against both ends of this fluidity spectrum. In this segment on cell membranes though, the main focus is on the transport of materials in and out of the cell. So before we do any of that, let's first review some fundamental concepts regarding diffusion and osmosis. Diffusion is the movement of materials from an area of high concentration to low concentration. This is simply how our universe operates. Although a more detailed analysis would reveal its relationship to entropy. On the other hand, osmosis is the movement of water through that semi-permeable membrane from an area of lower solid concentration to an area of higher solid concentration. Both of these phenomena play slightly different roles in understanding how things move in and out of the cell. First, diffusion can be used to understand the movement of things other than water. Let's begin with the simple diffusion. In simple diffusion, we have non-polar gases such as carbon dioxide and oxygen that almost just ignores the membrane and goes wherever there is a lower concentration of the said gas. But for charged ions and polar substances, we require facilitated diffusion where channel proteins can help move things in and out of the cell again following that concentration gradient. Keep in mind that these channels can be regulated by closing or opening dependent on the situation. Now, if you want to move things against their concentration gradients, this is where we need energy. And like how a soccer ball does not simply roll up a hill, we need to use pumps that can utilize ATP to move things toward an area of greater concentration, like the proton pump that we see here. Before we move on to osmosis, however, let's just remember one thing. When energy was used to pump substances and concentrate them on one side of the membrane, that energy is now held within the concentration difference or the cheosic potential. That is to say that molecules wanting to move back across to the other side presents a potential energy that can be leveraged for other energetic purposes later. We'll see this a lot in the next unit. Okay, so that's a lot of information about movement of substances across the membrane. But what about the movement of water? Well, in theory, water should not be able to pass through the membrane due to its polarity. But in almost every cell, there are specialized channels called aquaporins that simply allow water to move in and out. So the question then becomes which direction would water move? Here is where we can use our previous understanding of osmosis. So let's take a look at three potential situations. A cell in an isotonic solution would have a cytoplasmic concentration of the solutes that's equal to the outside environment and as such the net movement of water would be zero. In a hypertonic solution, the outer environment has a greater solute concentration and as per osmosis, water would leave the cell. This could be bad as cells could shrivel up or plasmalize in the case of plant cells. In a hypotonic solution though, the external solute concentration is lower than the cytoplasm resulting in water rushing into the cells. Now in animal cells, this will cause lis, but in plant cells, well, they're actually okay due to their cell wall. Lastly, there's endoccytosis, which constitutes the method through which bulk transport occurs, and there's also exocytosis going the other way. Here we see the actual membrane forming vesicles that could be brought in or out through the merger and dissociation of phospholipids. Now that our discussion of cells is complete, we can now discuss unit 3 which primarily focuses on energy, photosynthesis and cell respiration. We'll also discuss enzymes at the end a bit. Let's begin with energy. Energy is the capacity to do work and living things primarily do three types of work. One transport work which we've discussed before such as in the case of active transport that requires ATP. Another is mechanical work which involves anything moving because as Newton stated that force is required to move anything move or to make it stop. And last but not least, cells do a ton of chemical work. Here we're mostly dealing with reactions that require energy, particularly those that involve creating complex structures from simple ones. And to elaborate on that point, our universe likes to take complex things and break them down into simpler things. This is set to increase entropy or the randomness in a system. But because we're doing something that the universe so prefers to do, this releases energy resulting in an exroonic reaction. But the opposite also applies. When simple things become complex, as mentioned with chemical work, it requires that energy. And to that point, there's a common currency of energy used in a wide range of chemical reactions in biology and it's called a denosine triphosphate or ATP. Here we see that as the complex ATP becomes simpler that is ADP and phosphate energy is released. This energy is used to perform work but ADP and phosphate can be recharged and that is primarily going to be the focus of the next two sections in discussing how we eventually get that ATP in the cell. We can take a broader approach and see how our sun is connected to all of this. The initial source of energy for almost all of biology is the electromagnetic radiation that arrives on our planet as visible light. This light is used to convert those simpler carbon dioxide and water molecules into a more complex glucose molecule while producing oxygen as well. This is of course photosynthesis. Next we see the breakdown of glucose back into carbon dioxide and water which as an exorgonic process releases energy. This is the energy that is used to recharge those ADPs and phosphates back into ATP and that process is cell respiration. So it goes without saying that the whole process is in fact intricately connected together in a single story. But just keep in mind that while plants do photosynthesis and cell respiration as autorophic organisms, heterotroofes like you and me must consume directly or indirectly the sugars that are produced by these plants to charge up our ATPs. Let's explore photosynthesis first. It's divided into two major parts. The light reaction and the Calvin cycle. Both processes take place in a special organal called the chloroplast which has an inner region called the stroma and stacks of membrane inside called phaloids. Along theoids we have the electron transport chain which is responsible for the light reaction. Here we see large proteins called photo systems that contain pigments that respond to the reds and the blues of the visible light spectrum. The absorption of energy by these chlorophyll pigments can excite electrons that are fed through that electron transport chain. This energy is gradually harnessed and put to use in pumping H+ ions from the stroma to the inside of the thyloid space. As we saw in the previous section, the buildup of the chemiosmotic gradient acts as sort of a potential energy as protons leave through the specialized enzymes called ATP synthes. ADP and phosphates are combined creating ATP. Oh, and here we see that water is in fact split into protons and oxygens in photosystem 2 in order to continue supplying fresh batches of electrons in this process. Now the electron arriving at photos system 1 is charged up again and then passed along where this relatively high energy electron is eventually placed into an electron carrier called NADPH. Now both NADPH and ATP are used in the Calvin cycle in the stroma. The Calvin cycle starts with a 5carbon substrate called RUBP which is combined with a carbon dioxide by the enzyme rubiscoco. This substance immediately splits into 3PG where we input those ATP and NADPH that we created earlier to make glyceraldi di3 phosphate which is partially removed from that cycle to create sugar. Much of the substrates in the Calvin cycle are then simply regenerated into RBP to keep everything going through but ultimately as long as there's carbon dioxide water and light plants are very capable of producing sugars for itself and storage. Just keep in mind that sometimes the gas availability can be compromised when the stamata close due to excessive heat. Here we see that C4 and CAM pathways kick in which are alternative methods of initial carbon fixation that can help with this situation but more on that in the description below. Now the full breakdown of these sugars require aerobic cell respiration which is our next major topic of discussion. Here we see three stages glycolysis, pyuvate oxidation and the crep cycle and oxidative phosphorilation. Glycolysis which is an ancient and universal process occurs right in the cytoplasm. Here we see the breakdown of glucose into two identical pyuvate molecules while producing two ATP and two NADH which are electron carriers that carry away high energy electrons from the bonds stripped away from that glucose molecule. Pyuvates are then imported into the mitochondria through pyuvate oxidation where they lose a carbon dioxide from the structure while gaining co-enzyme A. This also produces an NADH. This transformed pyrovate into a COA which gets ready for the KB cycle. Now during that KB cycle in the mitochondrial matrix, a COOA provides that twocarbon substrate to be reacted with a 4carbon oxaloacetate. Now that forms citrate which then undergo several chemical reactions to release the remaining CO2s while simultaneously producing additional NADH, FADH2 and even ATP. Now the ATPs made here and in glycolysis are typically said to have undergone substrate level phosphorilation each requiring enzymes but no oxygen. But the remaining energy that are still held within NADH and FADH2 are processed in the next stage which is oxidative phosphorilation. Now along the inner membrane of the mitochondria we have a familiar site the etc. These proteins aren't really motivated by light energy but the incoming electrons that were once in sugars brought to them by NADH and FADH2. These electrons help to pump protons from the matrix of the mitochondria into the intermembrane space where the concentration gradient once again is leveraged to make ATP through that ATP synthes. Another super important thing happens here. The electrons that arrive at the tail end of that ETC must be removed to allow for a consistent flow. As such, oxygen acts as the final electron acceptor that takes these electrons and combine with protons to form water. In the absence of oxygen molecules, we see anorobic respiration where the only energy harvesting process in action is glycolysis. However, due to the shutdown of the ETC, the NADH produced during glycolysis cannot be regenerated back into NAD+ to keep that glycolysis going. And as such, fermentation, both ethanol and lactic occur in various species in order to dump those electrons back into pyrovate while regenerating that NAD+. The last thing to discuss in unit 3 is the topic of enzymes. Now, enzymes are proteins that have the specific ability to catalyze chemical reactions. Even favorable reactions often do not occur randomly due to excessive activation energy. Here, enzymes can lower that energy by orienting substrates, temporarily binding to them, or even creating micro environments. Most importantly, enzymes can do their job over and over again as long as they're around. Now, there are two types of inhibitors to enzymes, though, typically created as toxins or venoms by other organisms. Competitive inhibitors bind to the active site of the enzyme, therefore competing directly with whatever substrate that was supposed to bind with it. On the other hand, non-competitive inhibitors bind to a different site on the enzyme that alters the shape of the active site, therefore deactivating that enzyme from functioning. We can also regulate enzyatic activity using a similar idea to this non-competitive inhibition by regulating the active site. Let's get to unit four, which primarily deals with cell communications and cell division. We'll begin with cell communication. In this chapter, we discuss how cells are capable of relaying information to one another, which is particularly important for multisellular organisms. Earl W. Sutherland, an early pioneer in this field, discovered that the process that we call cell signal transduction occurs through three major phases. Reception, transduction, and response. In reception, we often see membrane bound receptors that have a signal or lian binding site on the outer region. The binding of such a signal changes the shape of that receptor to relay that message to the inside of the cell. The most common kind is the G-proin couple receptor where the connection of the ligan will eventually result in the activation of a G-proin that go on to activate more things. As we move on to transduction, we see that these receptors generally activate the production of second messengers such as cyclic AM. Now, this is important because these second messengers act as leazones between signal reception and what's going to happen in that target cell cytoplasm. Cyclic AMP for example has the capacity to activate protein kinases which then activate more kinases which activate even more kinases resulting in the amplification of that message within the cell. This of course is transduction during which phosphorilation cascade as described just now occurs. Now this process not only amplifies but can also add additional points of regulation that could be used to fine-tune the final response. And of course that final response completes the model. However there are many different types of responses. Generally, we have cytoplasmic responses, which involves activating certain enzymes to get the cell to do its job. Other times, we have transcription initiation that can occur, which simply means that we're going to be getting these cells to begin producing certain proteins from their genes. Also, just as a quick side note, non-polar signaling molecules do not require a membrane bound receptor as they just simply pass through that membrane while attaching to an intracellular receptor on the inside of the cell. Now moving on to cell division, we pivot from our previous unit and we begin discussing cells and their continuity across time. In single-sellled organisms, a cell dividing would effectively be their reproductive process. But in multisellular organisms, cell division can lead to growth, development, and repair. But what makes cell continuous across time though is the passing on of its genetic information. And in AP biology, we primarily focus on cell division occurring in deployed species. So let's take a look at what that means. First, deploy refers to the idea that sexually reproducing organisms have two copies of each chromosome. And the reason is simple. We have a mom and a dad, each of whom gave us one of the two chromosomes at each position. This carotype of a human shows 23 pairs of chromosomes that we normally have. Not that you need to know this now, but these chromosomes are compacted DNA that use proteins called histones and contain regions called genes that encode for proteins. So when we take a look at the cell cycle, it's important to know when DNA replication occurs and how that DNA is handled such that the two daughter cells receive the exact same genetic data after that separation. First, let's take a look at the whole cycle. We begin with G1. G1 phase known as the gap 1 is typically thought of as the growth and the life of the cell. However, due to some factors either internal or external, the cell may decide to divide. This is when the Sphase kicks in because the Sphase stands for synthesis and it refers to the synthesis of DNA or that DNA replication that we talked about earlier. Each of those chromosomes will be fully and accurately duplicated during this phase. Now during the G2 phase, additional checks are performed to ensure that everything looks good to go and we finally enter the motic phase. The motic phase in AP bio is divided into five phases. First is prophase when the chromosomes and their duplicated copies all condense. Each original and this duplicate form a single chromosome with each playing the role of a sister chromatid held together at the centromeir. We also see those spindle fibers of microtubules beginning to form aers. Promeophase sees the dissolving of the nuclear envelope and the attachment of those spindle fibers to the chromosomes and by metaphase these chromosomes align in a single file line across that metaphase plate with the anaphase splitting the sister chromatids apart which are now called daughter chromosomes. But because of that single file line, the last two phases ensure that the exact same set of DNA is passed down to each of the daughter cells. During telophase, these chromosomes relax while the nucleus reforms. This phase overlaps with cytochinesis, which is essentially the physical separation of the two daughter cells. While these phases are important to know, AP bio is more interested in the regulation of cell division. You could imagine that a cell that loses its ability to control division could become cancerous. Here we learned that internal cytoplasmic signals such as cyclines and their kynise targets could regulate cell division in a periodic manner. We also see external factors such as growth factors and other hormones that could accelerate cell division. But cells can of course inhibit division by requiring a substrate or just recognizing that there are cells already nearby through something called density dependent inhibition. Now majority of these systems are regulated by proteins and enzymes. So be on the lookout for how errors in the operations of cell cycle regulation can cause cancer cells to develop. We are now more than halfway through the course as we move on to unit 5, heredity. This is a jam-packed unit that ranges from meiosis to mandelian genetics to even more advanced concepts like linkage and kaiquare tests. So let's get going. One of the first things that we'll do here is to explore a bit more depth into how chromosomes work. As mentioned prior, chromosomes are long strands of DNA that have regions that we call genes. These genes contain data on building proteins that are expressed at the right place at the right time to make an organism develop into who they are and even facilitate their current responses and their interactions with the environment. But because we have pairs of chromosomes, each of which having the same genes at the same locus, it goes without saying that every diploid organism has two copies of each gene. These copies don't need to be identical and can sometimes encode for slightly varied versions of proteins that can act as the basis for variation that we see in any population. It turns out though that during sexual reproduction we only pass on one of these copies to our offspring because we can only pass on one set of chromosomes from 1 through 23 in the case of humans. Now the process of producing these so-called hloid cells is meiosis and it results in the production of gameamtes. These gametes are more colloally called sperms in males and eggs in females. Meiosis of course occurs in germline cells which are cells designated to become our gameamtes and it happens through two separate division events. While its main goal is to have the number of chromosomes, it's also important to note that it has the effect of creating as much variation as possible in these hloid cells. First, we begin with DNA replication that occurs during the Sphase, no different from what happened in the interphase of mitosis. However, during prophase 1, chromosomes condense and find their homalues, pairing up and undergoing the very important process of crossing over. This recombination event allows for one's paternal and maternal chromosomes to exchange alals or those variations of genes, creating new combinations not seen before. But during metaphase 1, these homologous chromosomes line up as pairs along the equator, allowing for random assortment of paternal and maternal chromosomes along that central line. Anaphase 1 sees the separation of these chromosomes while telophase 1 and cytochinesis occurs with you know predictive outcomes. But in meiosis 2 prophase 2 recondenses the chromosomes while metaphase 2 now allows these chromosomes to align in a single file line. Anaphase 2 is set to separate those chromatids resulting in telophase 2 and cytochinesis which then produces a total of four hloid cells accounting for both daughter cells from the first division. Ultimately, crossing over random assortment of chromosomes and the eventual randomness in fertilization allows for a great degree of variation to arise in sexually reproducing species. At this point, we put these ideas on hold and explore Mendle's study in a slightly greater detail before we doveetail these two concepts together. So, Mendle worked with pea plants and wanted to challenge the commonly held belief that offspring simply were blends of their parents. As such, he chose very specific characters such as seed color to test his hypothesis. After creating purebreeding yellow seeded peas and green seed peas for instance, he crossbred them to discover that all of the resulting offspring produce yellow seeds. Now that of course means that the blended inheritance didn't make any sense. So this allowed Mendle to stipulate the heredity operated with discrete and heritable particullet that we now call alals in genes. And in Mandelian genetics, we are taught three major laws. The law of segregation allows us to relate Mendele's discoveries with what we just discussed in meiosis wherein only one of the two alals are passed on to the next generation from each of the parents. This also allows us to use the punish square as a proxy for both myiotic segregation of the chromosomes as well as the random fertilization event that inevitably goes into the realms of probabilities. We also see the law of dominance which stipulates that certain dominant alals and therefore certain traits of a character can show up 100% even in the presence of its recessive counterparts. And lastly, the law of independent assortment allows us to see how random assortment of chromosomes and crossing over would create randomly shuffled versions of chromosomes such that multiple genes and their alals recombine in every permutation. Now there are also some atypical patterns of inheritance that we should know. Epistasis is a case where in a gene can affect the expression of a different gene as in the case of lab retriever coat colors. Polygenic inheritance where many genes all contribute to a single character and environmental effects on gene expression where we acknowledge that for many characters the environment plays a large role in the regulation and expression patterns of those genes. Adding to this, we're introduced to pedigrees, which if you can do your punish queries really well, shouldn't pose too much of an issue. But here, just keep in mind that pedigrees are typically used to track disorders that follow one of several types of inheritance patterns. Whether it be autoomal recessive, autoomal dominant, X-link recessive, or perhaps mitochondria. Be sure to review how to detect and describe each pattern for this exam. For a detailed look at pedigrees, you can check out the video in the description below. Perhaps what makes this unit most challenging though is how the ideas of chromosomes converge with mandelian genetics. But there's no way around it. So the best way to think about this is by looking directly at linkage. Linkage is a case in which two genes are closely located on a single chromosome. The reason that this matters is that with linked lossi we do not see independent assortment. During crossover events, genes closer along a chromosome have a lower probability of being split up and as such alic combinations of the original parental chromosomes tend to travel together during meiosis. This is clearly seen in test crosses analyzing two genes simultaneously. where we should expect to see a clear 1:1 to1 ratio. We sometimes see skewed ratios that over represent certain genotype combinations over the others. Clearly linkage is occurring which we could even use to calculate the distance between those genes. The rationale is that the degree of recombination is commensurate to the distance between the two genes as the further they are the more recombinations there would be. Therefore, simply calculating the re combination frequency or the number of those recombined offsprings over the total can give us a map unit when multiplied by 100 to allow for the rudimentary mapping of chromosomes. In this brief review, I of course can't get into all the specific examples to help elucidate the intricate relationship between mandalian genetics and our modern understanding of chromosomes. But please do take some time to make these connections clear before you tackle the May exam. Moving on to unit 6, we get to what most people consider to be the meat and potatoes of AP biology. This is where we discuss gene expression starting with DNA replication. As mentioned earlier, genes along our chromosomes encode for genetic data and it is therefore replicated fully during the Sphase. This section takes a closer look at the replication process. But before we do that, let's take a moment to lay out that structure of DNA or deoxxyribboucleic acid that we skipped out on earlier. DNA's monomer is the nucleotide which itself is composed of one of four nitrogenous bases, a fivecarbon deoxyibbal sugar and a phosphate group. The base is set to attach to the sugar at the first prime carbon while the phosphate is at the fivep prime carbon. This will be important in a minute because when we look at the polymeric structure, we see that DNA is an anti parallel double helix. Meaning that we have one DNA strand that runs one way and another paired with it that runs the other way. More specifically, we can define the ends of these DNA strands by looking at those numbered carbons that we just mentioned. A fivep prime end of DNA is where that phosphate is exposed attached to, of course, the fifth carbon. Down below we see a threep prime end where the threep prime carbon is left dangling with its hydroxide. The antip parallel aspect of the DNA molecule means that the other strand running parallel is actually going in the opposite direction simultaneously following the charg's rule of complimentary base pairing. Now this forms hydrogen bonds between adanines and thymines and guanines and cytosines allowing us to know exactly what will be on the other side of the DNA molecule given that you know what's on the one side. And as such, this pairing lends to DNA's incredible ability to replicate itself. And as mentioned, each strand has the complete set of data to recreate the opposite strand. A's against T's and G's against C's. Just keep in mind that all synthesis of DNA must occur in a five to threep prime direction. Now, during DNA replication, a replication bubble forms, allowing the two strands of DNA molecule to become slightly separated. On one side of the replication bubble, the helilicase enzyme sits at the fork, unwinding that DNA during replication. The initial enzyme called primise sees the original strand of DNA on one side and creates a short RNA primer from that five to threep prime direction. Next, the DNA pulsease binds and continues that primer heading towards that replication fork. Newly unwound regions would continue to be copied against this leading strand. Now, this primer from earlier is replaced by DNA with another DNA pymerase. The issue is really the opposite strand because DNA is antiparallel. The opposite strand would require the new synthesis of DNA proceeding in the opposite direction to that of the replication forks movement. And as such, this side of the replication occurs discontinuously with primise and DNA pulase creating disconnected fragments that we call Okazaki fragments. As the overall movement of the replication process keeps with the leading strand, this bottom strand is typically then known as the lagging strand. And each Okazaki fragment is eventually connected by ligase. At the end of the day, this process would perfectly copy that original DNA into two, each of which would have one half of the original DNA double strand. And as such, the model that we've just described is often called the semiconservative replication mechanism. Now that we have DNA replication covered, let's take a closer look at why we're doing any of this. The truth is that DNA carries genetic data that is responsible for the development, the maintenance, and the eventual reproduction of that cell or the organism. In this segment on gene expression, let's focus on just one of the thousands of genes on hand and discuss how proteins can be created from its genetic data. First, we can take a look at the macroscopic of a gene. It has a coding region that contains the real data needed to make proteins, but it also has a region called a promoter that is responsible for initiating this entire gene expression pathway. Looking closer at the coding region, we see that since DNA is double stranded, we actually have two different sets of codes as it were. The one that contains the sensible data is known as the coding strand while its complimentary strand is referred to as the template strand. Gene expression occurs in two phases transcription and translation. Transcription is the process of copying that DNA onto an mRNA template and that's where we're going to start. During transcription, we see three major phases. The first phase is called initiation. And here we see various transcription factors that bind to the promoter to ready it for expression. We see the eventual binding of the RNA pulymerase which then begins the process of making a copy of that DNA as RNA. During elongation, the template strand of DNA is used to create complimentary matches with the incoming RNA monomers, thereby temporarily using that complimentary base pairing to ensure that the information on the coding strand is copied onto an RNA strand. This process finishes with termination, which isn't too well developed in this course. In proaryotic cells, this RNA that is produced from the template strand is directly moved to the translation stage. But in ukarotic cells, we see three major mRNA processing steps taken before the translation actually occurs. The fivep prime end of the RNA is capped with a fivep prime cap. The cap allows for protection from hydraytic enzymes in the cytoplasm and is also known to facilitate ribosomal binding to speed up translation. The polyatail is added to the threep prime end and this also allows for the stability of the mRNA molecule. Lastly, we have regions called introns which are found in the genes as well as the original premrna and they are cut out linking together the remaining exxons to produce the final mRNA. It turns out that by selectively cutting out and keeping in certain exxon regions, you can make multiple variants of a protein from a single gene. Once the mRNA is ready for translation, it's sent to the cytoplasm. In translation, we see that the ribosome and aminoas trnas play a major role in creating that polyeptide from the information present on the messenger RNA. Remember that a polyeptide is really a series of amino acids and nucleotide bases in its triplicate codon form can dictate which amino acids need to be added in what sequence. This is generally referred to as the genetic code and it's universal in all living species. The process of translation begins with the initiator tRNA which corresponds to aug which then binds to that mRNA. This tRNA would contain methane the very first amino acid in many polyeptide chains. The large ribosomal subunit binds allowing additional tRNA and their amino acid counterparts to be matched to the triplet bases on that mRNA. Each amino acid is polymerized as a polyeptide during this process while the entire thing comes to an end when the stop codon is reached. The completed polyeptide would then fold and bend in order to gain that structure and thereby its function. But in biology, we don't want to be just expressing all the genes all the time. And as such, gene expression regulation is very important in allowing organisms to respond to their environment and maintain high resource efficiency. In bacterial cells, we see inducible operons like the lac operon which is only triggered for expression only in the presence of lactose. This is done through lactose sugar binding to a repressor that is typically blocking that promoter from the coding region only to be lifted when lactose binds to that repressor. In yet other cases we see repressible operons such as the trypoperon where the enzymes responsible for the synthesis of tryptophan is blocked by repressors when too much tryptophan in the cytoplasm is detected. Now there are some hints of translational regulation that sometimes comes out on this exam where in ukarotic cells mrna's ability to become translated could be regulated. In these scenarios, we typically see large amounts of mRNA in the cytoplasm without any protein present. And this is a good indication that transcription is occurring but not translation. Now we move on to unit 7 on evolution where those genes that we discussed in the previous two units battle it out on the arena of natural selection. To put simply, we begin with the hypothesis that all living things came from a single common ancestor. And through billions of years, species gradually changed and branched into forming the diversity of life that we see on our planet today. We begin with Darwin's idea. In the early to mid 1800s, Darwin traveled the world, observing adaptations in various species. He wanted to know whether these adaptations could have arisen through natural means and proposed his theory of natural selection. In this theory, he stipulated the following. populations exhibit great variation that could be heritable from one generation to the next. Given that the environment is finite and does not allow for survival and reproduction of all individuals, those with advantageous traits would more likely survive and pass on more of those genes that encode for those advantageous traits over time. From these beginnings, he argued that various finch species with all slightly different looking beak morphologies could have stem from a single ancestral finch species through adaptive radiation. Now, fast forward 200 years. We now know evolution to be as strong a theory as gravity with a tremendous amount of evidence that buttresses it. For instance, we see real-time natural selection as random mutations impart antibiotic resistance in bacterial cells. These variants when exposed to antibiotics later survive and form the majority of the bacterial strains. This of course is worrisome but just remember that antibiotic resistance actually precedes the application of antibiotics as evolution does not operate through a reactionary process. Furthermore, we see homologous structures which are structures that seem similar in varying species due to their common ancestry. The fact that we all have the same type of forearm bone structures as one another demonstrates our divergence from a common tetropod ancestor. Even vestigial structures show up here in this section where we see the remnants of old traits that are simply there due to evolutionary history. Tailbones of humans or the teeth, genes, and birds are some great examples. But diving a bit deeper into our study of evolution, we learned that the topic is divided into micro and macroevolution. Micro evolution is defined as a change in alo frequency over generations. And whether we're looking at a single gene or multiple, we know that evolution must be occurring if the alals that encode for particular traits about a character are not distributing themselves randomly across individuals or time. For instance, if we assume that a particular alil little f makes the rabbits white and that being a white rabbit in a snowy environment is favorable, we should see more and more individuals carrying this little F alil across time. This is because white rabbits will more likely to make more white rabbit babies, therefore increasing that alil's frequency. But it turns out that aside from this example of natural selection, there are additional mechanisms through which alle frequencies can change. Mutations can change alilles in real time and be of evolutionary significance if it occurs in the germline cell. But aside from the obvious, we are also introduced to gene flow where alals can come in and out of metapopuls as immigration and immigration moves individuals who are carrying those alals. Genetic drift also play a large role in micro evvolution when it does occur. It's preceded by a large reduction in a population size. In bottleneck effect, this reduction is caused by a catastrophic event like a volcano or a pandemic. Ultimately, we observe that the original alo frequency is drastically altered through this throttling mostly due to what we call the sampling effect in statistics. These events are generally associated with a reduction in genetic diversity of the population. Now another possibility is the founder effect where we see a small group of individuals from an original population move to a new area to begin a new. Here too we see the same statistical effect with reduction in genetic diversity and a drastic change to alo frequencies. Lastly we see sexual selection. In sexual selection we largely explain how seemingly ill adaptive traits such as the peacock's tail could arise despite harmful effects on the carrier. It seems to be strange until we realize that these features are typically found in males, the sex that generally competes for reproductive opportunities with choosier females. Here we see that traits that females prefer, whether they're beneficial or not, could play a large role in reproductive success with females sort of acting as a gatekeeper of male reproductive success. Now, when we discuss macroevolution, it's important to keep all of these microevolutionary processes in mind. And this is because we employ each and every one of these mechanisms of evolutionary change as a basis for how populations can change over time and speciate. And as such, the primary focus of macroevolution is on that divergence of a single species into daughter species through the process of speciation. First, we establish that a species is a group of organisms that have the capacity to interbreed and produce viable fertile offspring. And as such, a speciation event would split an existing population into two if somehow the two groups no longer were capable of reproducing with one another. Ultimately, there's one major force that results in speciation, and that is reproductive isolation. See, what keeps a species capable of interbreeding is the degree of genetic similarity that allows the hloid sperm to fertilize an hloid egg and develop properly into an individual. But when reproductive isolation occurs, the two separated groups accumulate those microevolutionary changes independently, diverging further and further genetically until they can no longer reproduce. Now this generally occurs through alopatric speciation where a geographical barrier splits a one single population into two. Here the reproductive isolation is guaranteed so long as the geographical barrier inhibits the movement of individuals across it. Different selectional pressures, independent accumulation of mutations, both nucleotide and chromosomal would all lead to large scale changes at the genomic level across long expanses of time independently in these two populations. But in some instances, we do see sympatric speciation where factors such as new habitat availability within a same area could still lead to a separation of a single population into two groups. But here we mostly see very specific cases rather than generalized patterns or rules. But once you have that species diversion, there's still several factors that keep those two species apart. Here we see prezygotic barriers, which are factors that inhibit species from forming that very first cell that we call the zygote. Habitat, temporal, and behavioral isolations all exist to limit interactions between individuals of different species. While mechanical and gimic isolation can limit population or even fertilization. But if the two species are sufficiently similar that they could mate and produce a zygote, we still have reduce hybrid viability and reduce hybrid fertility to keep these species apart. In relating these ideas to how we express these patterns, it's very helpful to know how to use and create cllaytograms. Know that nodes represent common ancestors and branch tips represent daughter taxa. Positions of the branches could be switched given that the nodes simply rotate, indicating that the clatoggram is really all about relationships. It's also helpful to know some very common evolutionary relationships on this exam, such as the larger resolution primate ancestry and some animal fogyny with respect to tetropod evolution. These come out frequently on the exam, so it's just simply easier to know them before going into the test. We finally arrive at unit 8 on ecology. Technically, this should be the most recent unit, so it should all be pretty fresh on your minds. In the larger scale, we see how organisms interact with the abiotic factors such as gases. In the carbon cycle, carbon dioxide is fixed by plants into sugars, releasing oxygen back into the atmosphere. In the nitrogen cycle, we pay close attention to the nitrogen gas, which are fixed into biologically available nitrogen through a lot of these symbiotic relationships between plants and bacteria. But other than those, it's pretty rare to see the phosphorus or the hydraologic cycle on the AP exam. Another abiotic and rather physical factor known as energy is revisited here. Just as we saw in unit 3, we see that plants can use light to create glucose molecules that now contain chemical energy. This production is what gives the plants the name of primary producers. But in tracking this energy, we see it moving through the primary consumers or the herbivores, secondary, tertiary, and even cordinary consumers. Dead organic matter from each level is decomposed by decomposers such as fungi and bacteria. When we focus a bit more specifically on populations, we learn that populations can grow due to births and shrink due to deaths. But no population can truly grow exponentially because we live in a finite environment. So a typical population grows in accordance to the logistics growth model that's shown here. As the population gets larger, density dependent factors such as limitations of resources, increased waste and higher transmission rates of infectious diseases could decrease that growth rate until it reaches the carrying capacity. Just remember to differentiate these factors from density independent factors such as natural disasters that can keep the population size in check. When discussing interactions between multiple populations, we delve into the field of community ecology. Here we're interested in the ways in which different species can interact with one another. In competition, the two species compete for similar resources, typically leading to the competitive exclusion where the less competitive of the two species can go locally extinct. Sometimes though, they can specialize and not step on each other's toes per se, resulting in niche partitioning. And we also see predator and prey relationships where the population dynamics can be tracked along this type of math model. More prey, more food, but more predators, more deaths. And as a result, the oscillation pattern of both populations are observed. In more intimate relationships, we see symbiosis. Here, mutualism serves to benefit both species involved in the relationships. Soybeans and their nitrogen fixing bacteria in their root nodules, or corals and algae are some great examples. In commensalism, we generally see one species benefiting at no cost to the other, while in parasetism, the parasite benefits while the host is generally harmed. Be sure to know some examples of these symbolic relationships, and you should be okay. On the test, we sometimes see specific terms like cryptic coloration, malarian mimicry, and bisian mimicry. But that's just camouflage, warning coloration, and cheaters wearing warning colorations without any bite in that specific order. Now, species diversity of a community plays an important role on the exam, too. The greater number of species with good evenness is going to help that system avoid collapse due to redundant species at each level. And be sure to know your food chain and food webs too as you could be asked to decipher certain interactions of species on this exam. And if you ever see anything like this, know that you're dealing with ecological succession where we describe how systems mature toward a climax community. Now, primary succession begins with the production of soil from bare rock, whereas secondary succession begins with the soil already intact. The order of species noted in this diagram should be understood and noted for the exam. At the end of the day, this is a very big exam with so many chapters that span everything from chemistry to ecology. In this video, my aim was to showcase the major themes and concepts, but be sure to watch all of my other videos that go into much more detail on any of the sections that you find difficult. And as always, if you found this video helpful, be sure to click that like button and subscribe. We are expanding into other courses including chemistry, mathematics, and even SAT into the future. So, be sure to keep us on your feet, even if this is the last video you watch before your AP bio exam. I wish all of you the best of luck, and let's show that college board what you're made of. It's been a great year. Love you all. Mikey out.