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Asexual and sexual reproduction

Reproduction is a process that comes in two main forms: asexual and sexual. Asexual reproduction involves a single organism making a copy of itself, resulting in two organisms with identical genetic material. This can happen through binary fission, budding, or fragmentation. On the other hand, sexual reproduction requires two organisms (or cells from two organisms) to create a third organism. This involves the fusion of an egg cell and a sperm cell, each carrying genetic information from their respective parent. This leads to a new organism with a mix of genetic information from both parents. Created by Sal Khan.

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Video transcript

- [Instructor] Let's talk a little bit about reproduction. Now, on earth, and who knows if we go to other planets we might find new ways that organisms can reproduce. But on earth, there's two primary ways that organisms reproduce. The first is, is that, let's say this is some type of unicellular organism, it essentially just makes a copy of itself. So that's it there, and then that's its copy. And in this situation they have identical genetic material. So let's say this is the genetic material of the first one, and this is the first one to begin with, and now you have the second one. Now this type of reproduction, where one organism by itself can turn into two organisms, this is known as asexual, asexual reproduction. Or another way to interpret asexual is it is not sexual. So the next question is, what is sexual reproduction? This is where two organisms, or at least cells from two organisms, need to get together in order to produce a third organism. Here's a cell from one of the organisms, let's say that cell is from the mother, it could be an egg cell. Here's a cell from the other organism, let's call that organism the father, this would be a sperm cell. And when the egg and the sperm get together, when they fuse together, they have genetic information from both parents. And that new cell can become the offspring. And if sexual reproduction sounds familiar, it's because that's how you came to be. Now, to make this a little bit more tangible, let's look at some cases of asexual reproduction and sexual reproduction. So right over there we see a diagram of a type of asexual reproduction known as binary fission. In general, if you hear the word fission we're talking about splitting something, if people are talking about nuclear fission they're talking about splitting of the atom, but binary fission we're talking about one organism splitting into two organisms. And you can see that you have its genetic material, the genetic material it replicates, and then the organism splits into two identical organisms, or at least identical from a genetic point of view. Now binary fission applies to prokaryotes that we talk about in other videos. These are organisms that don't have organelles, that don't have a nuclear membrane, they don't tend to have multiple chromosomes. When we talk about eukaryotes things that do have nuclear membranes, things that have multiple chromosomes, things that have membrane-bound organelles, you have a similar type of reproduction. This right over here, and we'll study it in depth in biology, this is mitosis. And mitosis you can view it as the analog of binary fission but we're now dealing with eukaryotic cells. Eukaryotic cells are more complex, so this process is more complex. But at a high level, they're both asexual reproduction. In both cases, one cell or one organism in some cases, has turned into two organisms that have the same genetic information. And it's worth calling out when you take one organism and you produce another organism with the same genetic information, it has a word associated with it, and that is a clone. And you might be familiar with the term clone from science fiction movies, and they usually use it in a somewhat related context. In Star Wars when they talk about the clone wars all of those clones they all have the same genetic information as the original bounty hunter from which they were derived. Now mitosis might also seem familiar to you because this is how most of your cells in your body divide. And this is how you grow, this is how you went from being a unicellular organism to being a multicellular organism. Now you might be a little bit confused at this point. I just told you that most of your cells have experienced or are experiencing mitosis, but you're also the outcome of sexual reproduction. Well what we'll see in a second is, as soon as sexual reproduction occurs, then this newly formed cell right over here, through mitosis forms the multicellular organisms, the types that are you and me, but we also as we grow and develop, we also produce what are known as gametes, that each have half of our genetic information. For human beings, this would be the egg produced by a female, and this would be the sperm produced by a male, and when they get together, the sperm fertilizes the egg, well then, they can produce a viable cell that can turn into an offspring. Now along the lines of binary fission, you have another type of asexual reproduction in which case a mother cell produces buds off a smaller daughter cell. So like binary fission, it has to replicate all of its DNA and then that DNA gets inserted into the daughter cell but the end product you have a bigger cell and a smaller cell. And we would say that the smaller cell has budded off of this larger one. And so this we call budding. Another form of asexual reproduction. When we think about multicellular organisms there are some typical types of asexual reproduction. For example, starfish are famous for their ability to form a new organism from a piece of an original organism, so let's say you were to cut that part off of the starfish for many species of starfish that could then grow into a new starfish. The original starfish would grow that back. And so this type of reproduction is known as fragmentation. It's pretty easy to, fragmentation, it's pretty easy to remember, you take a fragment of it and it grows into another organism and that's how it reproduces. Now another fascinating type of reproduction is known as parthenogenesis, that's a fancy word, partheno, parthenogenesis. And what happens here is is you have an organism, and this is a baby komodo dragon right over here, that can develop from an unfertilized egg. Remember in sexual reproduction we talk about how a sperm from a father organism needs to fertilize an egg from a mother organism and then once they fuse together then that fertilized egg can go through mitosis to produce a multicellular organism. But in parthenogenesis, you have an egg on its own that can, that does not need to be fertilized, that can turn into a multicellular organism, like a baby komodo dragon. Now you might ask, well how can this be? Well, sometimes these mother, these eggs, don't have half the genetic material like we're used to with the eggs from sexual reproduction. For parthenogenesis, many times this egg has a complete set of genetic information and then it can just go and start doing mitosis and turn to an organism or there's even situations where the egg does have half the genetic information, we'll talk about this in future videos, this would be a haploid cell, it has half the genetic information of its parent, but the haploid cell in certain cases can develop into a full organism. And there are organisms like komodo dragons that can reproduce both with parthenogenesis which would be asexual, and in certain cases with sexual reproduction. So I'll leave you there, two big categories, I'll draw a dividing line here so we don't get confused. All the stuff that I drew on the left, to the left of this yellow line, these are forms of asexual reproduction. Where for the most part, one organism can make a copy of itself that has the same genetic information. On the right we have sexual reproduction where in order to produce a new organism, an egg cell and a sperm cell need to get together. The sperm cell needs to fertilize the egg cell and each of them would have a sample of half the genetic information from their parents. Now, I'll leave you with a final question, we should always be asking this in biology, is why do we think sexual reproduction emerged? It actually emerged much later in our evolutionary history than asexual reproduction. Well one argument for it is is it produces much more variation in its offspring. And if you have more variation in your offspring, and if you have more variation in a population, you have a higher likelihood that certain variations will be better suited for the environment that you are in. I'll let you think about that a little bit more.