Lesson Video: Asexual Reproduction Science • Third Year of Preparatory School

Video Transcript

In this video, we will review the life process of reproduction. We will then describe the defining features of asexual reproduction. And we’ll discuss its advantages and disadvantages. We will then move on to learning about the major methods of asexual reproduction.

Reproduction is the process through which organisms produce offspring, also called descendants. It is one of the defining processes of all forms of life and is required for the survival of any species. Biological entities that cannot reproduce on their own, for example, viruses, are not considered by most scientists to be alive.

The two major types of reproduction are sexual and asexual. Sexual reproduction involves the union of two gametes, typically from two different parents, a process known as fertilization. Fertilization usually occurs when a sperm cell from a male combines with an egg cell from a female. Because of how genes are rearranged and inherited, the offspring produced during sexual reproduction are different from both biological parents. So we say that these offspring are genetically unique. And if the same two parents have additional offspring, these offspring will also be genetically unique, different from each other and different from their biological parents.

The production of genetically unique offspring leads to populations with higher genetic variation, which in turn leads to populations that are more resilient to environmental changes, such as short- or long-term climate changes or the introduction of new pathogens. Imagine that a novel virus invades a population with high genetic variation. Some individuals in the population will have the genotype that makes them susceptible to the virus. Other individuals will be lucky enough to have the genotype that offers full immunity to the virus. And still others will have the genotype that gives partial immunity.

Although the susceptible and partially immune individuals may become infected with the virus and might even die, the high genetic variation in this population means that at least some of the individuals with the genotypes leading to beneficial adaptations will survive. And the survival of these individuals in the face of a new virus ensures the survival of the species.

In contrast to sexual reproduction, asexual reproduction requires only one parent to produce offspring. These offspring are genetically identical to each other and to the parent and are often referred to as clones. Asexual reproduction is typically faster than sexual reproduction because no time is needed to find a mate for fertilization or for gestation. However, it does not increase genetic variation, so if the environment changes, there may not be individuals with the adaptations needed to cope with the change. This leaves the population vulnerable to size reductions or even extinction.

There are many methods of asexual reproduction. And one of the most common methods is binary fission. You may remember that the prefix bi- means two and the word fission means to split, which describes the process in which a parent cell, such as this amoeba, first copies its DNA and then divides or splits its nucleus and cytoplasm to produce two genetically identical daughter cells. Binary fission is the most common form of asexual reproduction used by prokaryotes, such as bacteria. However, it can also be used by single-celled eukaryotes, such as Euglena or the Amoeba shown in the diagram.

Remember that we stated earlier that one of the advantages of asexual reproduction is that it is faster than sexual reproduction. For example, some types of bacteria can undergo binary fission in as little as 20 minutes. This means in 12 hours a single bacterium could produce over 68 billion offspring. Now that’s fast reproduction.

A second method of asexual reproduction is called budding, in which offspring arise from a bulge or bud attached to a parent organism. Budding is seen in prokaryotes, such as cyanobacteria, as well as eukaryotes, including single-celled fungi and multicellular organisms like hydra. The process of budding begins with the growth of a small outward projection from the parent. This is the bud. The budding offspring develops to maturity while still attached to the parent.

In some species, like hydra, the offspring eventually break off from the parent and settle elsewhere to become independent clones. In other species, like coral, the offspring remain attached to a colony that expands over time. But whether they detach or remain part of a colony, the new individuals produced through budding will be genetically identical to each other and to their parent.

Another way organisms can reproduce asexually is through the use of spores, which are special reproductive cells that, under the right conditions, are able to grow into offspring. They’re usually haploid, which means they have a single set of chromosomes. Spores are used in asexual reproduction by fungi, algae, certain groups of protists, and nonflowering plants like mosses and ferns. They are also used by some organisms for sexual reproduction.

Spores are typically produced within special reproductive cells or structures. One spore-producing structure that you’re probably familiar with is a mushroom, which is generated by the underground mycelium of certain fungi when conditions are right for reproduction. Although the spores found on mushrooms are produced sexually, the dispersal process is similar to that of asexual spores.

Spores are usually released in large numbers by the parent organism, and fungi are no exception. Underneath the cap of the mushroom are billions or even trillions of spores, which are released in clouds to be dispersed by the wind. Other organisms may disperse their spores through water or while attached to animals. Mosses and certain types of algae even have flagellated spores that can actively swim through water. Once the spores settle, they can survive for long periods of time until conditions become ideal, at which point the spore will germinate and begin to grow into a new individual.

A method of asexual reproduction, used by both nonflowering and flowering plants, is called vegetative propagation. It refers to the production of plant clones from parts of the parent, including leaves, stems, and underground food storage structures like tubers and bulbs. Succulent plants, for example, aloe or cacti, are well known for their ability to produce offspring through vegetative propagation. Prickly pear cacti can produce offspring from fruits or stem segments called cladodes that fall to the ground. The fruits and cladodes will grow their own roots, eventually developing as clones next to their parent plant.

If this process is repeated enough times, it leads to large clonal colonies of prickly pear, which can be quite problematic for farmers or ranchers. This is exactly what occurred in Australia, where prickly pear, a nonnative plant, was introduced in the late 19th century. By the 1920s, prickly pear had taken over 240,000 square kilometers, which is roughly the same size as the entire United Kingdom! The Australian government tried many tactics to get rid of the prickly pear, with the most successful being the release of a cactus-feeding caterpillar called Cactoblastis. The Cactoblastis caterpillars were able to eat their way through 80 percent of the prickly pear in just about seven-years time. Those were some hungry caterpillars!

The final method of asexual reproduction we will cover in this video is called regeneration, which is the ability to regrow damaged or missing body parts. Regeneration is often used in ways that are nonreproductive. For example, if a sea star is missing a limb due to predation, it can regenerate a new limb in its place. In this type of regeneration, no new offspring are created, and it is not a form of reproduction.

There are also cases where two or more pieces or fragments of an organism each grow into a new individual. To distinguish it from nonreproductive regeneration, this process is often referred to as fragmentation. And the individuals produced through fragmentation will be genetic clones. Fragmentation is used by fungi, sea stars, sponges, flatworms such as Planaria, and segmented worms.

Now let’s check what we’ve learned about asexual reproduction with a couple of practice questions.

Complete the following sentence. Asexual reproduction produces offspring that are blank identical to the parents, so it reduces genetic blank. (A) Not, expression; (B) genetically, variation; (C) almost, mutations; or (D) partially, similarities.

The two words in each answer choice will be used to fill in each of the two blanks. For example, if we choose the answer choice almost, mutations, then we would be saying that the correct sentence should read “Asexual reproduction produces offspring that are almost identical to the parents, so it reduces genetic mutations.” To fill in the first blank of the sentence, we need to know the relationship between offspring produced by asexual reproduction and their parents.

You may recall that asexual reproduction requires only one parent. So let’s take a look at a single bacterium as it undergoes the common form of asexual reproduction called binary fission. Bacteria are prokaryotes, so their DNA floats freely in the cytoplasm, often in the form of a circular chromosome. As the bacterium enlarges to prepare for division, it makes a copy of its chromosome. The bacterium then splits into two daughter bacteria, each containing a copy of the chromosome. Because there is no second parent to mix genes with, the two daughter bacteria are genetically identical. Or to put it another way, they are clones of each other. So we have determined that the correct term to fill in the first blank is genetically.

Unlike asexual reproduction, sexual reproduction requires two parents, with the biological father typically providing the sperm cell and the biological mother providing an egg cell. After the sperm and egg unite in a process called fertilization, they form a structure called a zygote. And the nucleus of the zygote will have genetic material from both of the biological parents, represented here by green and pink squiggly lines. The zygote will divide and grow, eventually becoming a fully functioning offspring.

Because of how genes are rearranged and inherited during sexual reproduction, this new offspring will be genetically unique from its biological parents and from any additional offspring those same parents have together. Each instance of sexual reproduction produces genetically unique offspring. And over time, this increases the genetic variation within a population, which refers to the differences in the genetic material between individuals in the same population.

In contrast, if we look at asexual reproduction over time, we can see that the same chromosome is copied and passed down to each generation of offspring, leading to a population of clones with reduced genetic variation. This example shows that the correct term to fill in the second blank is variation. So the correct complete sentence is “Asexual reproduction produces offspring that are genetically identical to the parents, so it reduces genetic variation.”

The table below outlines some asexual reproductive methods and the organisms that use them. What should replace X?

To determine what should replace X in the table, let’s first take a look at the information that’s already filled in. The first column gives us the organism, the second, the method of asexual reproduction, and the third column provides a brief description of that method. In the first row, we can see that Salmonella bacteria use the method of binary fission. This is when the parent cell copies or doubles its genetic material before splitting into two separate daughter cells. In the second row, we can see that yeast use budding for asexual reproduction. In this method, the parent cell forms a growth, or bud, that develops into a small individual and eventually detaches.

In the third row, we’re told that flatworms reproduce asexually when the parent splits into two parts and each of these parts grows into a new organism. The question asks us to replace X with the correct term for this method of asexual reproduction. The description of method X is similar to the description of binary fission and that both involve the parent organism splitting into two parts. However, in binary fission, these two parts become two separate cells, while in method X, each of the new parts eventually become new multicellular organisms. This is an important distinction because binary fission is only used by single-celled organisms, and flatworms are multicellular.

Many multicellular organisms, including flatworms, use a method of asexual reproduction called regeneration. Let’s take a look at the process of regeneration in a type of flatworm called a Planaria. If the Planaria is split into two parts or fragments, each fragment will regrow or regenerate the parts of the body it is missing, shown in the shaded pink regions of the diagram. The end result is two fully functioning offspring. The process of regeneration matches the description of method X given in the table. So we can say that the correct term to replace X in the table is regeneration.

Now let’s go through some of the key points from the video. Reproduction is a defining life process. And the two major forms of reproduction are asexual and sexual. Asexual reproduction only requires one parent and does not involve gametes or sex cells. In contrast, sexual reproduction involves the union of two gametes, typically from two different parents. Asexual reproduction produces genetically identical offspring, which lowers the genetic variation of a population. Sexual reproduction on the other hand produces genetically unique offspring, which tends to increase genetic variation. Asexual reproduction is usually faster than sexual reproduction because no time is needed to find a mate or for fertilization or gestation.

There are many methods of asexual reproduction, including binary fission, which is used by single-celled organisms; budding, which can be seen in multicellular or single-celled organisms; vegetative propagation used by plants; the production of spores seen in fungi, algae, and nonflowering plants; and regeneration used by sea stars, sponges, flatworms, and segmented worms.