Download Life Science: Session 4

Life Science: Session 4
Teaching Tips: Asexual Reproduction in Plants
A potato eye — an example of an
asexually reproducing structure
Sexual reproduction and the unique process known as alternation of generations
characterize the life cycles of plants. Other strategies for reproducing can also be
observed in plants — strategies that don’t involve the union of sperm and egg. These
strategies involve asexual reproduction, where one parent plant propagates offspring that
are genetically identical to it. While a few animals can reproduce asexually, most do not
have this ability.
Have you ever stored potatoes or onions that began to sprout new parts after a while?
These are great examples of asexual reproduction in plants. Interestingly, asexual
reproduction in plants is most often associated with food storage structures that the plant
uses to sustain itself during times of dormancy. The new plant arises from a bud that
exists on the food storage structure. The following are some examples of asexual
reproduction in plants—from buds in leaves, stems, and roots.
Leaves
Onions, garlic, and lilies are examples of modified leaves, called bulbs that serve as
underground food storage for a dormant plant. After these plants have flowered and the
above ground leaves have fallen away, buds at the base of the bulb represent next season
’s growth.
Stems
Many plants reproduce asexually from stems—either below or above ground. Corms,
rhizomes, and tubers are good examples. Corms are short, rounded stems with buds
located at the top. An example is a gladiola. You can tell the difference between a bulb
and a corm by the presence of leaves in the storage unit of the former. Rhizomes are
underground stems with buds located at the tips of the branches. Irises produce these
types of rhizomes. Most people are surprised to learn that potatoes are actually enlarged
parts of stems—mostly occurring underground. These types of food storage structures are
called tubers. The “eyes” on a tuber consist of a tiny leaf and a bud, which can sprout into
a new plant.
Roots
Sweet potatoes represent storage roots. These structures include tuberous examples, like
the sweet potato, beets, and turnips as well as longer tap roots, like carrots and parsnips.
On the tuberous examples, the buds occur above the root or are scattered upon it. On the
taproots, tiny buds exist at the base of the shortened stem.
Consider bringing some of these examples into your classroom and starting a garden of
plants that reproduce asexually!
© Annenberg Foundation 2013.
All rights reserved.
The two methods of reproduction
Asexual reproduction means reproducing without the interaction of two sexes or
genders, whereas sexual reproduction involves the fusion of two special cells called
gametes, one from a male source and one from a female source.
Asexual reproduction
Before a cell divides, its nucleus divides. Each chromosome is copied, and each nucleus
receives the same genetic material: genes, made of DNA.
As each cell divides into two, the resulting "daughter" cells are therefore exact copies of
one another.
This process is responsible for the increase in number of cells which occurs during
normal growth and development, and when tissues are replaced following injury.
Normal cell division is also the basis for asexual reproduction. Only one type of cell is
involved, with no input from another individual. Because no new genetic material is
introduced, there is no variation in the resulting offspring.
Since the offspring from this process contain the same genetic material as one another
(and the same as the original single parent), they can be described as a clone.
Examples of asexual reproduction
Asexual reproduction in plants
There
are
many
exampl
es of
asexual
reprod
uction
in
plants, e.g. the spider plant Chlorophytum which produces plantlets on stolons branching
from buds in the parent plant.
These are genetically identical and will grow to look alike, provided that they are raised
in the same environment.
What features of the plant's environment would be need to be standardised (for them to
look the same)?
>same illumination (amount of light)
Many plants used for food can be propagated, i.e increased in number, by the method of
asexual reproduction.
Do not confuse asexual reproduction with (sexual) reproduction in flowering plants,
which often combine both male and female parts in the same flower.
Fruits and seeds are produced as a result of sexual reproduction.
Some parts of plants become enlarged as a result of normal cell division and this is called
vegetative growth. This is often linked with surviving adverse weather conditions and
keeping food reserves for the plant in order to grow again the next season. These plants
are called vegetables, and Man often uses these reserves for himself.
Each of the examples of food plants below use asexual and sexual reproduction in
different ways
strawberries
bananas
potatoes
onions
apples
Asexual reproduction in bacteria
Bacteria reproduce asexually by a simpler
process known as binary fission.
More about bacteria
Asexual reproduction in animals
Asexual reproduction is much less common in animals, but it is often seen in simpler
animals e.g. Hydra.
Identical twins are produced by a form of asexual reproduction when the ball of cells
making up the embryo breaks into two, and each implants in the uterus and grows
independently (after the normal sexual form of reproduction, obviously!)
In animal lifecycles, asexual reproduction sometimes
alternates with sexual reproduction.
Sexual reproduction
Both male and female sex cells (sperms and eggs in animals, pollen and ovules in plants)
are produced by a special cell division process which halves the number of chromosomes
in each resulting cell. The chromosome separation process ensures that each sex cell has
a unique combination of genes in its nucleus.
Fertilisation is also a random process and so when the nuclei fuse the resulting fertilised
egg (zygote) has an individual genetic makeup.
In contrast to asexual reproduction, sexual reproduction introduces variation into
offspring. This is an essential feature in order for evolution to take place.
Further development after fertilization
This zygote then divides again and again using the normal process of cell division,
producing cells containing genes which are exact copies of the original. So each cell of
the embryo, and the adult organism into which it develops, contains cells which are
genetically identical. This is fortunate because the body's immune system will target any
"foreign" cells (normally invading microbes) which differ from the others.
However as each organ develops, the cells within it (collectively known as tissues)
become specialized for their particular tasks, e.g. muscle cells, nerve cells, red and white
blood cells, and they "read" and use only part of their genetic information to do this.
As they have differentiated into these different cell types, it appears that they have lost
the ability to divide again into other cell types. A few unspecialized cells (e.g. stem cells)
retain the ability to do this.
Summary of differences
Asexual reproduction
Sexual reproduction
Number of
parents
1 (either male or female)
2 (male and female)
Makeup of
offspring
genetically identical (to
parent and other
offspring)
genetically different
Cell division
process
normal cell division
following nuclear
division (by mitosis)
special cell division following nuclear
division (by meiosis) producing sex cells
(gametes): after fertilization subsequent
divisions: normal
Advantages
quick - good for bulking
up of numbers to
colonize new areas
produces variation - the basis of evolution
Disadvantages
disease may affect all
slower - needs special processes to bring
together gametes and protect zygote,
embryo etc during development
Life cycle
useful when conditions
ideal for growth
may be synchronized with (end of ?)
growing season
Site author Richard Steane
Advantages of Asexual
Reproduction
A short account of advantages of asexual reproduction in plants is provided in the article
below. One should, however, note that this form of reproduction though, offers shortterm benefits, sexual reproduction always scores over it.
A type of reproduction in which fertilization and other processes like ploidy reduction,
meiosis, etc., doesn't take place can be termed as asexual reproduction. This form of
reproduction doesn't require a partner; which means that only a single parent is involved
in the process. A more technical definition of asexual reproduction would be the
regeneration which takes place as a result of fusion of gametes. The single-celled
organisms which reproduce asexually are protists, bacteria and archaea.
The different modes through which asexual reproduction takes place are binary fission,
spore formation, vegetative reproduction and budding. The budding process results into
formation of daughter cells from mother cells. This types/mode of reproduction is
observed in unicellular as well as multicellular organisms. Binary fission is the simple
process in which a single cell divide into two and the process continues.
Advantages of Asexual Reproduction
The advantages of asexual reproduction are very less in comparison to that of sexual
reproduction. Here is a list of these advantages.
Higher Number of Offspring
In asexual reproduction, higher number of offspring are produced. The advantage of
having offspring in greater number is that their survival rate increases greatly.
Spores
Production of spores is one form of asexual reproduction that is highly effective from the
point of dissemination. The spores being light in weight, are carried to long distances by
means of wind and water. Even animals carry out dispersal of spores that are attached to
their bodies.
Partner not Required
For asexual reproduction to take place, there is no need of a partner. It is therefore, a
faster and easier form of reproduction. The energy spent by organisms in finding a
partner during sexual reproduction is saved in this form of regeneration.
Cloning
The individuals produced as a result of asexual reproduction have the same DNA. It
means that daughter cells produced as a result of asexual reproduction are clones. In the
commercial farming, priority is given to the production of greater number of crops/plants
in less amount of time. Cloning would thus, prove to be beneficial in such cases.
Breeding Stock
One of the advantages of asexual reproduction is the production of breeding stock. The
breeding stock obtained from asexual reproduction is used for the propagation of species
in the future. The process is more or less like cloning.
The advantages, limitations of asexual reproduction need to be understood properly to get
into the depths of the subject.
Asexual reproduction doesn't serve any purpose to enrich the genetic diversity. Sexual
reproduction on the other hand leads to the development of best possible offspring.
The offspring produced as a result of asexual reproduction have lesser chances of
survival in comparison to that of sexual reproduction. In sexual reproduction, two
gametes of different organisms come together and share their genetic information. It
causes the development of an individual with a stronger and rich genetic composition.
It is not possible for individuals produced from asexual reproduction to survive the
vagaries of nature.
The advantages of asexual reproduction along with some of the disadvantages presented
in the article, should help us understand which method is beneficial in the cultivation of
plants. The asexual form of reproduction leads to prolific multiplication of organisms.
However, the vigor or genetic diversity required for a species to survive in the long term
is found lacking in such organisms. Finally, one could say that asexual and sexual
reproduction both have their pros and cons.
By Shashank Nakate
Read more at Buzzle: http://www.buzzle.com/articles/advantages-of-asexualreproduction.html
Asexual Vs. Sexual Reproduction
There are many organisms that reproduce either asexually or sexually. The following
article will discuss some points that will help you understand the differences between
these two reproductive strategies. Read on to know more about the potential benefits of
these strategies for different organisms.
The biological process that helps give rise or birth to new organisms from their parents is
called reproduction. Each and every organism on the world survives because they have
the ability to reproduce and propagate their own kind. There are two broad categories of
reproduction. These include asexual reproduction and sexual reproduction. These two
methods are used by plants and animals to ensure their species continues to survive. Let
us have a look at asexual vs. sexual reproduction and learn the differences between the
two.
What is Sexual Reproduction?
Sexual reproduction is a process that usually occurs in most of the complex organisms.
This method involves combination of genetic information from the parents and creating a
new organism who is the combination of both parents. Sexual reproduction is a very
complex process. Here, the organism needs to produce sex cells called gametes. These
gametes contain half the number of chromosomes containing information related to all
other cells that make up an organism. The cells undergo meiosis and produce haploid
cells (n). These haploid cells contain one copy of genome passed on from the diploid
cells (2n). The two copies of the single chromosome cross over each other and form a
completely new chromosome. This new chromosome contains a combination genes from
both the chromosomes as an effect of crossing over. This ensures, genetic information
from both parents is equally passed on to the progeny. Crossing over also ensures the
healthy genes remain active in a given population.
Sexual reproduction takes place only after the male and female mate. The males produce
sperms and females eggs. The gonads, that is, eggs and sperms are haploid cells. When
the sperm fertilizes the egg, it gives rise to a diploid cell containing genetic information
from both the parents. The diploid copy of genes favors evolution of a dominant species
with less number of mutations and genetic defects. Thus, recombination of genes is very
important in masking the recessive traits in organism. It is possible to observe sexual
reproduction in plants. Angiosperms are flowering plants that contain reproductive
organs. Some flowers can be hermaphrodites, that is, contain both male and female
organs. Few flowers may contain only one of the sexes. These flowering plants undergo
pollination where the pollen from the stamen is transferred to the stigma. This pollen like
a sperm in animals, travels down the style and fertilizes the eggs in the ovaries. This
forms a zygote that will undergo mitosis and form an embryo. This embryo develops into
a seed that on germination will develop into a new plant.
In some animals, external fertilization takes place. Here the gametes meet outside the
body of both parents. In some animals, like reptiles and birds, a hard covering is formed
around the zygote. This is called an egg or shell and the zygote develops outside the
female body. Some animals like a kangaroo have a special pouch in the body, for the
embryo to grow and develop. In most of the animals, the embryo develops in the female
womb.
What is Asexual Reproduction?
Asexual reproduction is the process where only one parent is involved giving birth to a
progeny. This progeny produced is totally identical to the parent in its genetic makeup.
There are organisms that reproduce asexually like bacteria, multicellular organisms like
fungi and even plants. Bacteria divides asexually by binary fission. Hydras and yeast
reproduce through binary fission. However, these organisms can undergo sexual
reproduction. For example, bacteria can undergo conjugation, hydra and yeast can
reproduce sexually. In some invertebrates, parthenogenesis is the process of reproduction.
This means, growth of the embryo or seed can take place without fertilization by a male.
This is seen in some parasitic wasps, water fleas, aphids, fish and sharks. Asexual
reproduction in plants includes many processes like:




Budding
Internal budding (gemmules)
Fragmentation
Regeneration
Pros and Cons of Asexual Vs. Sexual Reproduction
Both these processes involve some advantages and disadvantages. The following
paragraphs will discuss both the pros and cons of these processes.
Advantages of Sexual Reproduction




Sexual reproduction involves two organisms of the same species, thus leading to
formation of gamete cells.
The gametes fuse together causing mixing and combination of the genetic
material.
The recombination of genes helps in increasing the chances of the organism
evolving into a healthier and more adaptable species.
Combination of genes ensures, there is a lesser chance of diseases and defects
seen in parents being passed on to their children.
Disadvantages of Sexual Reproduction




Reproduction cannot occur until and unless gametes from both parents fuse
together.
Only half of the population that is, female species are capable of gestation.
There is no guarantee that the nucleus of the male gamete will fuse with the
female gamete after mating.
The time taken to produce an offspring by sexual reproduction is very long.
Advantages of Asexual Reproduction





One parent is needed to complete the process of asexual reproduction.
In majority of the cases, one is sure an offspring will be produced.
The gestation period or growth period is very short.
The offspring has the same phenotype and genetic make up as the parent.
As there is no cross over or recombination of genes involved, chance of mutation
is very less.
Disadvantages of Asexual Reproduction


As the genetic material from parent is directly passed on to the off spring, it
causes all the diseases and defects to be passed on to the offspring as well.
As no recombination of genes occurs, chances of evolution are very low.
As you can see both these methods have plenty of differences, but both lead to
reproduction of progeny. According to the various species, these methods prove to be a
successful way to sustain life.
By Batul Nafisa Baxamusa
Why is Meiosis Important
Meiosis is a phase in sexually reproductive organisms, in which cell-division takes place.
It is of great importance, because it creates genetic diversity in the population. Read on to
know more about the importance of meiosis.
Enlarge Image
Meiosis is a process of gamete formation in which diploid germ-line cells, that is, the
cells that are set aside early in animal development for sexual reproduction, yield four
genetically different haploid cells. It occurs only in sex cells, which are eggs and sperms.
Meiosis takes place in two stages, Meiosis I where DNA replication takes place and
crossing-over occurs; and Meiosis II, which lacks DNA replication but is similar to
Mitotic cell division.
Meiosis I and Meiosis II have four phases each. They are:
Meiosis I: prophase 1, metaphase 1, anaphase 1, and telophase 1
Meiosis II: prophase 2, metaphase 2, anaphase 2, and telophase 2
In meiosis, during the formation of gametes in animals and spores in plants, the
chromosome number is reduced to half. These chromosomes contain the basic DNA
chain. During the first meiotic reduction division, the chromosomal pairs are divided so
that each gamete or spore contains one of each chromosomal pair, it becomes a haploid.
When haploid gametes unite during fertilization, they result in a zygote. Zygotes, having
received one chromosome of each pair from each parent become diploid. Meiosis
involves two successive nuclear divisions, which produce four haploid cells. The meiosis
I is the reduction division, meiosis II separates the chromatids, which are the daughter
strands of a duplicated chromosome joined together by a centromere. In mitotic cell
division, new cells genetically identical to the parent cell are produced. Meiosis is
responsible for increasing genetic variation in the population.
Each diploid cell, which undergoes meiosis can produce 2n different chromosomal
combinations, where 'n' is the haploid number. In humans, the number is 223 because
there are 23 pairs of chromosomes. This number is greater than eight million different
combinations. The variation increases, because, during meiosis I, each pair of
homologous chromosomes comes together. In a process known as synapsis, each pair of
homologous chromosomes may exchange parts. The relative distance between two genes
on a given chromosome can be estimated by calculating the percentage of crossing-over
that takes place between them.
The main tasks of meiosis are
Production of haploid gametes to maintain the diploid number of species, generation after
generation.
Crossing-over, which brings together new gene combination of chromosomes.
A mechanism for comparing the two copies of each chromosome are provided with the
purpose of error correction or repairing.
In meiosis, variation occurs, because each gamete (either sperm or egg) contains a
mixture of genes from two different parent chromosomes in sexual reproduction. In other
words, the genetic coupling of non-identical DNA takes place in meiosis. It results in an
offspring, which has the genetic material of two different individuals. A new combination
of genetic information is produced in the gametes. Therefore, in meiosis, the
characteristics of parent chromosomes are combined with the characteristics of offspring
chromosomes, which ultimately results in a new and unique set of chromosomes. These
chromosomes contain the basic DNA chain, which determines the physical and genetic
characteristics of the child. It enables individuals to produce physically and genetically
unique offspring. Because of this, a high genetic diversity of a population is maintained.
With only mitosis, there would have been no sharing of genetic information, only
division would have been possible. In such a situation, there would have been only clonal
populations, which would eventually suffer from diseases or natural disasters.
What is the explanation for the diversity in populations? How can they survive variations
in the environment? The reason is meiosis. Genetic variation plays the role of a raw
material for natural selection. Some individuals who are favored by natural selection have
greater fitness than others because of their alleles (pair of alternative forms of gene). In
case of animals, males that are unable to compete for mates, for example, succumb to
predation or disease or fail to reproduce; small and weak organisms don't survive for long
time. These are the best examples of natural selection. You can also take an example of a
disease to which some individuals will be at least partially resistant while others are
susceptible to it. Populations can adapt to changes in the environment as a result of the
genetic variation resulting from meiosis. However, in clonal asexual populations,
organisms are not able to adapt to changes without mutations. Organisms which adapt to
changes in the environment, survive, while others get eliminated by natural selection. In
this way, a population contains fit individuals and the process continues for generations
together. The diversity afforded by meiosis is beneficial for the population as a whole.
Thus, meiosis helps to create a population that is not only physically and genetically
different but also one, which is perfectly fit to survive.
By Saurabha Palekar
Read more at Buzzle: http://www.buzzle.com/articles/why-is-meiosis-important.html
Asexual and Sexual Reproduction
Published by Nivu Whatani
September 7, 2008, Category: Human Biology
Pros and cons of asexual and sexual reproduction.
Plants and animals use two methods of reproduction to ensure that their species
continues. One of the two methods is sexual reproduction. It involves the fusing of two
gametes from each parent to produce the offspring.
Pros of Sexual Reproduction
In sexual reproduction, unlike in asexual reproduction two parents are involved. Two
gamete cells are involved. Fertilization takes place mixing the genetic material together.
One of the main advantages of sexual reproduction is that there is a large chance of an
evolution taking place. Thus the species would be able to adapt to new environmental
conditions. Also new species could be born. Another advantage is that a new character is
formed with the mixed gametes of the parents so there is a chance that the parent’s
diseases would not be received by the offspring.
Cons of Sexual Reproduction
One of the disadvantages is that only half the population (females) is capable of
gestation. Another one of the disadvantages is that two parents have to be involved in the
process. Also having to find a mate and producing gamete cells is also another
disadvantage. Fertilization has to take place and there is no guarantee that the nucleus of
the male sex cell would fuse with the female sex cell. Also this takes a long time and
there is no grantee that an offspring would be born.
Asexual Reproduction
Asexual reproduction is a form of reproduction which only involves one parent. Some
methods of asexual reproduction are the vegetative method. It is common in plants is
where parts of the plant fall of and form new ones. E.g. potato tubers
Fragmentation is another type of asexual reproduction common in plants.
Pros of Asexual Reproduction
There are many advantages of asexual reproduction. One of them is that only one parent
is needed. There is a large guarantee that the offspring would be born. It takes a very
short period and genetic material need not be produce. Fertilization is not there. They
have identical characteristics as their parents. They are always of the same species.
Asexual reproduction is more reliable because there are less steps to follow so less can go
wrong.
Cons of Asexual Reproduction
One of the cons of asexual reproduction is that there is almost no chance of an evolution
taking place. The offspring would almost always be of the same species and would be
identical to the parents, so diseases and bad qualities are also passed down directly from
the parents to the offspring. One of the common disadvantages of plants using the
vegetative method is that there will be a struggle for light, space, nutrients and soil so
most plants will remain less healthy.