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C HAPTER
Chapter 1. Asexual Reproduction - Advanced
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Asexual Reproduction Advanced
• Describe asexual reproduction; explain the genetic relationship between parent and offspring.
One parent or two?
That is the main difference between sexual and asexual reproduction. Sexual reproduction is combining genetic
material from two parents, usually from two gametes. How the two cells combine may take many forms, but
does not necessarily involve physical contact. Sexual reproduction produces offspring that are genetically distinct
from other offspring and from their parents. Asexual reproduction produces offspring from just one parent. These
offspring are genetically identical to that one parent.
Reproduction
Some organisms look and act exactly like their parent. Others share many similar traits, but they are definitely unique
individuals. Some species have two parents, whereas others have just one. How an organism reproduces determines
the amount of similarity the organism will have to its parent. Reproduction is the process by which organisms give
rise to offspring. It is one of the defining characteristics of living things. There are two basic types of reproduction:
asexual reproduction and sexual reproduction. Each of these processes ensures that the parental generation gives
genetic material, DNA, to its offspring.
The process of cell division is how multicellular organisms grow and repair themselves. It is also how many
organisms produce offspring. For many single-celled organisms, asexual reproduction is a similar process. The
parent cell simply divides to form two daughter cells that are identical to the parent. Asexual reproduction produces
offspring that are genetically identical to the parent, whereas sexual reproduction produces a similar, but genetically
unique offspring. In sexual reproduction, meiosis produces haploid gametes that fuse during fertilization to produce
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a diploid zygote ( Figure 1.1). In other words, a child inherits half of the genetic material from each parent. Look
at the family in Figure 1.2. The children resemble their parents, but they are not identical to them. Instead, each
has a unique combination of characteristics inherited from both parents. The children, of course, result from sexual
reproduction.
FIGURE 1.1
Fertilization of an egg cell by a sperm cell. In sexual reproduction, haploid
gametes fuse to produce a diploid zygote.
FIGURE 1.2
Family Portrait: Mother, Daughter, Father,
and Son. Children resemble their parents,
but they are never identical to them. Do
you know why this is the case?
Asexual Reproduction
No, not all animals have two parents. Sometimes animals can be made from just one parent. This Komodo dragon,
for example, only has a mother and reproduces by parthenogenesis. Asexual reproduction is the process of creating
offspring from just one parent.
Are there male and female bacteria? How could you tell? Remember, bacteria have just one chromosome; they do not
have an X or Y chromosome. So they probably have a very simplified form of reproduction. Asexual reproduction,
the simplest and most primitive method of reproduction, involves a single parent and produces a clone, an organism
that is genetically identical to the parent. Haploid gametes are not involved in asexual reproduction. A parent passes
all of its genetic material to the next generation. All prokaryotic and some eukaryotic organisms reproduce asexually.
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Chapter 1. Asexual Reproduction - Advanced
FIGURE 1.3
Do animals always have two parents?
Asexual reproduction can be very rapid. This is an advantage for many organisms. It allows these organisms to
crowd out other organisms that reproduce more slowly. Bacteria, for example, may divide several times per hour.
Under ideal conditions, 100 bacteria can divide to produce millions of bacterial cells in just a few hours. However,
most bacteria do not live under ideal conditions. If they did, the entire surface of the planet would soon be covered
with them. Instead, their reproduction is kept in check by limited resources, predators, and their own wastes. This is
true of most other organisms as well.
There are a number of types of asexual reproduction including fission, fragmentation, budding, vegetative reproduction, spore formation and agamogenesis. Spore formation occurs in plants, and some algae and fungi, and will be
discussed in additional concepts.
Fission
In fission (or binary fission), a parent separates into two or more individuals of about equal size. This type of
reproduction is common among single-celled organisms including bacteria, archaea, and unicellular eukaryotes,
such as protists and some fungi. The single cell divides into two daughter cells.
Fragmentation
In fragmentation, a body breaks into several fragments, which later develop into complete organisms. For example,
a new starfish ( Figure 1.5) can develop from a single ray, or arm. In addition to starfish, this type of reproduction
is common among some worms, fungi and plants. Many of these organisms are also capable of sexual reproduction.
Most lichens, which form from a symbiotic relationship between a fungus and photosynthetic algae or bacteria,
reproduce through fragmentation. This ensures that new individuals contain both symbionts.
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FIGURE 1.4
Binary Fission in various single-celled organisms (left). Cell division is a relatively
simple process in many single-celled organisms. Eventually the parent cell will
pinch apart to form two identical daughter
cells. In multiple fission (right), a multinucleated cell can divide to form more than
one daughter cell. Multiple fission is more
often observed among protists.
Budding
In budding, organisms reproduce by having new individuals split off from existing ones, which results in genetically
identical parent and daughter organisms. The bud may stay attached or break free from the parent. Eukaryotic
organisms, such as the single cell yeast and multicellular hydra, undergo budding ( Figure 1.5).
Vegetative Reproduction
Vegetative reproduction is a type of asexual reproduction found in plants. This type of reproduction occurs when
new individuals are formed without the production of seeds or spores. The formation of new plants out of rhizomes
or stolons is an example of vegetative reproductive, such as in the strawberry plant. Other plants use this type of
reproduction to reproduce through bulbs or tubers, or shoots and suckers that form along lateral roots. See the Plant
Biology concepts for additional information.
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Chapter 1. Asexual Reproduction - Advanced
FIGURE 1.5
Starfish reproduce by fragmentation and
yeasts reproduce by budding. Both are
types of asexual reproduction.
FIGURE 1.6
These strawberry plants reproduce by
vegetative reproduction. New strawberries grow out of stolons, a specialized type
of horizontal above-ground shoot.
Agamogenesis
Agamogenesis is any form of reproduction that does not involve a male gamete. These include are parthenogenesis
and apomixis. Parthenogenesis is a form of asexual reproduction where growth and development of embryos occur
without fertilization. Parthenogenesis occurs naturally in aphids, rotifers, nematodes and some other invertebrates,
as well as in many plants and certain lizards, such as the Komodo dragon. Apomixis is asexual reproduction, without
fertilization, in plants.
Asexual Reproduction Summary
TABLE 1.1: Asexual Reproduction Summary
Type
Fission
Fragmentation
Description
Occurs when a parent cell splits into
two identical daughter cells of the
same size.
Occurs when a parent organism
breaks into fragments, or pieces,
and each fragment develops into a
new organism.
Example Organisms
Bacteria, Protists,
Fungi
Unicellular
Starfish, some worms, fungi, plants,
lichens
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TABLE 1.1: (continued)
Type
Budding
Vegetative Reproduction
Agamogenesis
Description
Occurs when a parent cell forms
a bubble-like bud. The bud stays
attached to the parent cell while it
grows and develops. When the bud
is fully developed, it breaks away
from the parent cell and forms a
new organism.
Occurs when new individuals are
formed without the production of
seeds or spores.
Any form of reproduction that does
not involve a male gamete.
Example Organisms
Yeast, Hydra
Plants
Many plants and some animals.
Vocabulary
• agamogenesis: Any form of reproduction that does not involve a male gamete.
• asexual reproduction: Reproduction involving only one parent; occurs without a fusion of gametes; produces
offspring that are all genetically identical to the parent.
• budding: A form of asexual reproduction in which a new organism develops from an outgrowth or bud on
another one; the bud may stay attached or break free from the parent.
• clone: A genetically identical copy; may be a gene, a cell or an organism; an organism that is genetically
identical to its parent.
• diploid: The state of a cell containing two sets of chromosomes; in human somatic cells, two sets is 46 (23
pairs) chromosomes, 2n.
• fission: Asexual reproduction in which a parent separates into two or more individuals of about equal size.
• fragmentation: Asexual reproduction in which the body breaks into several fragments, which later develop
into complete organisms.
• gamete: A sexually reproducing organism’s reproductive cells, such as sperm and egg cells.
• haploid: The state of a cell containing one set of chromosomes; in human gametes, one set is 23 chromosomes,
n.
• meiosis: A type of cell division that halves the number of chromosomes and forms gametes.
• parthenogenesis: A form of asexual reproduction where growth and development of embryos occur without
fertilization.
• reproduction: Process by which living organisms give rise to offspring; making the next generation.
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Chapter 1. Asexual Reproduction - Advanced
• sexual reproduction: Reproduction involving the joining of haploid gametes, producing genetically diverse
individuals.
• spore: A haploid reproductive cell, found in plants, algae and some protists; can fully develop without fusing
with another cell.
• vegetative reproduction: A type of asexual reproduction found in plants where new individuals are formed
without the production of seeds or spores.
• zygote: A fertilized egg; the first cell of a new organism.
Summary
• Asexual reproduction involves one parent and produces offspring that are genetically identical to each other
and to the parent.
• Numerous types of asexual reproduction exist, including fission, fragmentation and budding.
• Bacteria and single-celled eukaryotic organisms must reproduce asexually, however, numerous multi celled
organisms also reproduce asexually.
Review
1.
2.
3.
4.
What is reproduction?
Describe three types of asexual reproduction?
Define gamete and zygote. What number of chromosomes does each have?
Compare and contrast asexual and sexual reproduction.
References
1. Visual by www.PDImages.com. http://commons.wikimedia.org/wiki/File:Sperm-egg.jpg . Public Domain
2. Image copyright Juan Carlos Tinjaca, 2014. http://www.shutterstock.com . Used under license from Shutterstock.com
3. Dale Harvey. http://www.flickr.com/photos/daleharvey/2718250378/ . CC BY 2.0
4. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.0
5. Starfish: Flickr:ShyViolet09; Yeast: Zappy’s. Starfish: http://www.flickr.com/photos/allthestarsthatshine/
7356697446/; Yeast: CK-12 Foundation . Starfish: CC BY 2.0; Yeast: CC BY-NC 3.0
6. James Lee. https://www.flickr.com/photos/jronaldlee/4698800604 . CC BY 2.0
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