Video Transcript
In this video, we will learn how to
describe the defining features of asexual and sexual reproduction and some examples
of these two types of reproduction. We will also learn what the term
genetic variation means and how asexual and sexual reproduction differ in the
genetic variation they introduce to their offspring.
There are several characteristics
that all living organisms have in common. From the smallest microscopic
organisms, like bacteria, to the tallest trees, to humans, like you and I, all of us
move, though some of us move faster than others. We must also release energy, which
we do through a process called respiration that occurs in almost all of our
cells. We’re all sensitive to our
surroundings, and we all grow. Though not every individual will
reproduce, as a species, some individuals must reproduce for that species to
continue. We must all excrete waste products
from our cells. And in order to carry out these
energy-demanding processes, we must all obtain food or nutrition. By looking at the first letter of
each of these essential life processes, we find an easy way to remember them: MRS
GREN.
In this video, we’re focusing on
reproduction. But what is reproduction, and why
is it so important? Reproduction is a biological
process by which living organisms are able to produce new individuals, which are
otherwise known as offspring. The new individuals that are
produced will have some of the genetic information passed on from their parents and
so, therefore, will have some of the same characteristics as their parents. For example, this individual might
have had the genetic information that codes for black hair passed on by her parents
and so also has the black hair characteristic. Many individuals in every
generation of a species become parents to a set of offspring. And some individuals will not
become parents.
The offspring that are produced
form the next generation. This new generation will grow and
mature, and they eventually reproduce and have children of their own. And so the process continues. If these stick people did not
reproduce, they will eventually die without producing the next generation. If this happened on a larger scale,
the species of stick people might become extinct. Without reproduction, the species
of living creatures that existed hundreds of years ago wouldn’t still exist
today. In fact, many species that became
so rare that they have failed to reproduce in the past have become extinct.
While the stick people are not a
real species, one real species that was declared extinct in 2020, among many others,
was the splendid poison frog, pictured here. The ultimate goal of every species
is to ensure that this species continues to survive throughout generations without
going extinct. This is exactly why at least some
organisms in every generation of every species must reproduce, to ensure that its
species survives.
Let’s take a look at the different
types of reproduction that we’ll be exploring in this video. There are several different ways in
which organisms can reproduce. But these methods can be broadly
grouped into two main categories: asexual reproduction and sexual reproduction.
Let’s explore asexual reproduction
first. In asexual reproduction, there is
only one parent involved. And this single organism is capable
of producing offspring that are identical to the parent. It involves two identical daughter
cells being produced from one parent cell. Usually, the type of organisms that
undergo asexual reproduction exclusively are single-celled organisms, like bacteria
or the fungus yeast, although there are a few interesting examples of plants and
animals that undergo asexual reproduction, too. As we’ve mentioned, there are
several different ways in which organisms can reproduce asexually.
Let’s learn more about this process
through an example in a bacterium. Bacteria, or a singular bacterium,
are single-celled organisms that can sometimes cause diseases in humans. However, many bacteria are very
useful to us, too. For example, those that live in our
digestive system can help us to digest our food. Many bacteria reproduce asexually
through a process called binary fission. The word binary comes from the
Latin word meaning two together, while the word fission refers to things splitting
or dividing. This is because in binary fission,
the original bacterial cell divides into two new cells, which are called daughter
cells.
Binary fission produces two
identical cells. This means that every time a
bacterium reproduces, the two new bacterial cells produced are completely identical
to each other and to the original parent cell. Bacteria are prokaryotic organisms,
which means they don’t have a nucleus in their cells. Eukaryotic organisms, for example,
the single-celled Amoeba proteus, do have a well-defined nucleus in each of
their cells, however. Although it mainly occurs in
prokaryotic cells, some eukaryotic cells, like those of the amoeba, can carry out
binary fission, too.
Asexual reproduction is a very
rapid process, and it doesn’t require a lot of energy. In fact, binary fission in some
bacteria can happen in a shorter time as 20 minutes. In contrast to binary fission,
which occurs predominantly in prokaryotes, eukaryotes can asexually reproduce using
a different type of cell division called mitosis, which also produces two
genetically identical daughter cells from one parental cell. Because of this, we say that
asexual reproduction in both prokaryotes and in eukaryotes always produces clones of
the original parent cell. However, some differences may arise
between the two daughter cells produced due to their environmental conditions.
Fungi are a large group of
organisms, some of which reproduce asexually. There are some fungi which are
unicellular, such as yeast. One yeast you might’ve heard of,
which is used to help bread rise, is called baker’s yeast. Yeast cells reproduce asexually
through a process called budding, whereby a new organism buds off the original
cell. And both of the cells produced will
be genetically identical to each other.
Other fungi, such as those which
cause bread mold, can use special structures called spores to reproduce
asexually. In fact, when you see mold on a
piece of bread like this, you’re actually seeing colonies of spores and the fungi
trying to reproduce as much as possible to spread all over the bread.
Some plants can also reproduce
asexually through a process called vegetative propagation, in which new plants can
grow from the vegetative parts of the plant, which include the leaves, stems, and
roots. Let’s take a look at this process
occurring in this strawberry plant. As you can see, the strawberry
plant produced extensions of a stem-like structure called a runner. Runners can grow either above or
below the ground surface till they find a spot which is suitable to set down roots
in, establishing a new plant nearby to the parent plant.
Let’s take a look at sexual
reproduction next. Sexual reproduction always involves
two parents of the same species: one male and one female. Since sexual reproduction requires
two parents, an individual organism must find a mate of the opposite sex in order to
reproduce. Each parent produces many cells
called gametes, otherwise known as sex cells or their reproductive cells. These gametes contain half the
genetic material of a normal body cell. So let’s find out why.
There are two main stages involved
in sexual reproduction: the formation of these gametes and the fusion of the
gametes, which occurs through a process called fertilization. The formation of gametes occurs
through a type of cell division called meiosis. When cells divide by meiosis, four
new cells are formed. And as we mentioned earlier, each
of these four gametes will contain half the genetic information of a normal body
cell and of this original parent cell.
While this parent cell had four
chromosomes, each of these daughter cells only has two. This is because in sexual
reproduction, one gamete from each parent fuses together in a process called
fertilization to form a zygote with a full set of genetic material. The zygote that’s produced, unlike
the clones produced in asexual reproduction, is genetically different from its
parents. This zygote grows and changes over
time to eventually become a fully developed organism.
So let’s look at how this can
happen in different groups of organisms. Sexual reproduction can also happen
in several different ways. In animals, the gametes are called
the egg cells in females and sperm cells in males. In most mammals, including humans,
fertilization and development of the zygote occurs within the mother’s body. And the development process is
called a pregnancy. After this period of development,
live offspring or babies are born.
In most birds and reptiles,
internal fertilization also occurs, but these animals lay eggs. In most fish and amphibians,
however, fertilization usually takes place externally to the mother’s body, often in
water, and eggs are also produced. After a period of development in
these eggs, offspring can hatch out of them.
Sexual reproduction in plants
occurs slightly differently. Although many plants can reproduce
asexually, several types of plants also undergo sexual reproduction. The reproductive organ in plants is
called the flower. The male gametes in plants are
found in grains of pollen, which can travel to the female gametes, or the ovule, by
moving through certain parts of the flower. Here, the gametes will fuse by
fertilization, eventually forming seeds, which can grow into new individual
plants. If the pollen grain is produced by
the same flower as the ovule that it eventually fertilizes, this is an example of
asexual reproduction, as both gametes were produced by the same parent. However, if this pollen comes from
a different plant, it is sexual reproduction, as the offspring produced will be
genetically different from each parent.
Now that we’ve seen some of the
basics of both asexual and sexual reproduction, let’s identify some of the
similarities and differences between these two processes using this Venn
diagram. Both processes involve producing
new offspring and passing on genetic material to these offspring to form a new
generation. One clear difference between the
two, as you might have noticed, is that asexual reproduction only requires one
parent, while sexual reproduction requires two.
As we mentioned earlier, asexual
reproduction is a rapid and efficient process that requires very little energy. This also means that organisms that
reproduce asexually can give rise to a large number of offspring in a short period
of time. Sexual reproduction, on the other
hand, is far more time-consuming and demands a great deal of energy. Fewer and larger organisms tend to
be produced through sexual reproduction. For example, you might know that a
human mother undergoes a nine-month period of pregnancy during which the baby grows
and develops. And generally the larger an
organism gets, the longer this period is. For example, African elephants
carry the young for a period of 22 months before giving birth, which is close to two
years.
The offspring produced by asexual
reproduction are genetically identical to their parents so can be referred to as
clones, while sexual reproduction gives rise to offspring that are genetically
different from their parents, as they have a combination of characteristics from
both of the two parents.
We can describe these genetic
similarities and differences between different organisms through a concept called
genetic variation. Genetic variation describes the
differences in the genetics of different living organisms and gives these different
individuals unique combinations of characteristics. Organisms that reproduce asexually
tend to have a very low genetic variation, as their daughter cells are produced
through processes like mitosis, which makes them genetically identical to the parent
cells.
Organisms that reproduce sexually,
however, tend to have a much higher genetic variation due to the production of
genetically different gametes through meiosis. Through genetic variation,
combinations of genes from each parent give rise to a unique set of characteristics
for every new organism. Let’s explore the benefits of
having a high genetic variation that’s afforded by sexual reproduction through an
example.
The peppered moth, or Biston
betularia, is a species of moth with a speckled black-and-white
appearance. There are two types of this moth:
one with a light-colored body and one with a dark-colored body. In England, just before the
Industrial Revolution in the 18th century, dark-bodied peppered moths were very rare
and the light-bodied moths were far more common, as they camouflaged well against
pale trees, like this birch. While this protected the
lighter-colored moths from predators like birds, the darker moths stood out clearly
and were gobbled up quickly.
Due to natural genetic variation,
however, small numbers of darker peppered moths remained. During the early decades of the
Industrial Revolution, however, things started to change for the peppered moths. Dark-bodied moths began to be seen
in larger and larger numbers, eventually even bypassing the number of light-bodied
moths. How and why do you think this shift
happened? Pause the video and have a
think.
During the Industrial Revolution,
several cities in England began to burn coal to power their factories, which
produced lots and lots of smoky black pollution. The air pollution in cities
increased to such an extent that the streets, buildings, and trees were blackened
with soot. These environmental changes meant
that the darker moths now camouflaged better with the darker surfaces, whereas the
lighter ones showed up far more clearly.
Let’s imagine a hypothetical
situation. If peppered moths reproduced
asexually, would they have survived the Industrial Revolution? As asexual reproduction produces
clones of the parent organism, all of the peppered moths would be white. Light-bodied peppered moths would
in this scenario keep giving rise to more light-bodied moths. This means that all of the moths
are now not very well camouflaged and puts the whole species at risk of
extinction. Instead, as these moths were
reproducing sexually, the gene for a darker body became more prevalent in the
Industrial Revolution because it enabled the moths to camouflage against the
blackened surfaces and survive predation. While the light-bodied moths were
preyed upon, the dark-bodied moths could grow in number, effectively saving the
population of peppered moths from going extinct.
This example shows us one of the
benefits of a high genetic variation to increase the resilience of a species against
environmental changes. It provides a tool that gives more
possibilities for a species to adapt to their environment in order to thrive in
spite of unfavorable conditions.
Now that we’ve learnt some more
about the different types of reproduction, let’s summarize the key points that we’ve
covered in this video. We’ve learnt that there are two
types of reproduction: asexual reproduction and sexual reproduction. In asexual reproduction, there is
only one parent involved, while sexual reproduction requires two parents: one male
and one female. Asexual reproduction is rapid,
while sexual reproduction is a lengthier process that requires finding a mate. While asexual reproduction produces
clones of the parent organism with no genetic variation in their offspring, the
offspring produced through sexual reproduction have a combination of characteristics
from both parents and are genetically different from those parents. So they tend to have a high genetic
variation.
