Cell Division and Reproduction
1. Asexual Reproduction
Learning outcomes
- I can define asexual reproduction.
- I can describe how offspring are produced through asexual reproduction.
- I can compare asexual reproduction in plants, animals, and microorganisms.
- I can explain the advantages of asexual reproduction.
- I can explain the disadvantages of asexual reproduction.
What Is Asexual Reproduction?
Asexual reproduction is a form of reproduction involving:
one parent.
There is no fusion of male and female:
gametes.
Instead, a new organism develops from cells belonging to a:
single parent organism.
Because the offspring receive their genetic information from only one parent, they are usually genetically identical to that parent and to one another.
Genetically identical organisms are called:
clones.
The Main Idea
Asexual reproduction can be summarized as:
One parent → cell division → genetically identical offspring
Unlike sexual reproduction, asexual reproduction does not normally involve:
- sperm cells
- egg cells
- fertilization
- fusion of gametes
- two parents
This allows reproduction to occur:
quickly and efficiently.
Asexual Reproduction and DNA
Before a cell divides, its DNA must be:
copied.
The genetic information is then passed into the new cells.
In eukaryotic organisms, asexual reproduction usually involves:
mitosis.
Mitosis produces cells containing the same genetic information as the:
parent cell.
This is why asexual offspring are normally genetically very similar to their:
parent.
Are Asexual Offspring Completely Identical?
Asexual offspring are often described as:
genetically identical clones.
However, DNA copying is not always perfect.
Occasionally, a change in DNA called a:
mutation
can occur.
Mutations can introduce small amounts of genetic variation even in populations produced:
asexually.
The environment can also cause genetically identical organisms to develop somewhat different:
characteristics.
Different Types of Asexual Reproduction
Asexual reproduction occurs in many groups of organisms.
Different organisms use different methods, including:
- binary fission
- budding
- fragmentation
- vegetative propagation
- spore formation
- parthenogenesis
Although the methods differ, they share the same basic feature:
offspring are produced from a single parent without fertilization.
Binary Fission
Binary fission is a common method of asexual reproduction in:
bacteria and some other single-celled organisms.
During binary fission:
- The genetic material is copied.
- The cell grows.
- The copies of the genetic material separate.
- The cell divides.
- Two new cells are produced.
The resulting cells are usually genetically:
very similar or identical.
Binary Fission Can Be Very Fast
Under favourable conditions, some bacteria can reproduce:
very rapidly.
Imagine one bacterium repeatedly dividing:
1 → 2 → 4 → 8 → 16 → 32 → 64
The population can increase rapidly because each new cell can:
divide again.
This is one reason bacterial populations can grow quickly when they have sufficient:
- nutrients
- water
- suitable temperature
- appropriate environmental conditions
Budding
Budding occurs when a new organism grows from the body of its:
parent.
The new organism begins as a small:
bud.
The bud grows through cell division and may eventually separate from the parent.
One familiar example is:
yeast.
Budding in Yeast
During yeast budding:
- A small bulge develops on the parent cell.
- DNA is copied.
- The nucleus divides.
- One copy enters the growing bud.
- The bud increases in size.
- The bud may separate from the parent.
The new yeast cell contains essentially the same genetic information as the:
parent cell.
Budding in Animals
Some animals can also reproduce by:
budding.
A well-known example is the:
hydra.
A group of cells on the parent's body divides repeatedly.
A small hydra develops, eventually forming structures such as:
tentacles and a mouth.
When sufficiently developed, the young hydra can separate and live:
independently.
Fragmentation
Some organisms can reproduce when part of the parent organism:
breaks away.
The separated piece can grow or regenerate into a:
new individual.
This process is called:
fragmentation.
Fragmentation occurs in some:
- worms
- sea stars
- algae
- fungi
- plants
However, simply being able to repair an injury is not necessarily:
reproduction.
A new organism must actually be produced.
Asexual Reproduction in Plants
Plants have several highly effective methods of:
asexual reproduction.
A new plant may develop from:
- stems
- roots
- leaves
- underground structures
This is often called:
vegetative propagation.
Because the new plant develops from cells of the parent, it is usually genetically:
identical to the parent plant.
Runners
Some plants produce horizontal stems called:
runners or stolons.
These grow along the surface of the ground.
At certain points, the runner can produce:
roots and shoots.
A new plant then develops.
A common example is:
strawberry plants.
One parent strawberry plant can produce several genetically similar plants around:
itself.
Tubers
A tuber is a swollen underground plant structure that stores:
food.
A familiar example is a:
potato.
The small structures commonly called the "eyes" of a potato contain:
buds.
Under suitable conditions, these buds can develop into:
new shoots and plants.
Therefore, one potato tuber can be used to produce new plants:
asexually.
Bulbs
Some plants reproduce using:
bulbs.
Examples include:
- onions
- garlic
- tulips
- daffodils
A bulb contains a short stem surrounded by modified leaves that store:
food.
New bulbs can develop and eventually produce:
new plants.
Rhizomes
A rhizome is a horizontal underground stem.
Rhizomes can grow through the soil and produce new:
roots and shoots.
Examples include:
- ginger
- some grasses
- irises
This can allow a plant to spread across an area without producing:
seeds.
Plant Cuttings
Humans can also use a plant's natural ability to reproduce asexually.
A piece of stem or leaf can sometimes be removed and encouraged to develop:
roots.
This is called a:
cutting.
Once roots and shoots develop, the cutting can grow into an independent:
plant.
Why Use Cuttings?
Suppose a grower has a plant that produces:
excellent fruit.
Reproduction by seeds may produce offspring with different combinations of:
genes.
Using cuttings allows the grower to produce plants that retain the parent's desirable:
genetic characteristics.
This is useful in:
agriculture and horticulture.
Tissue Culture
Plants can also be cloned using a technique called:
tissue culture.
A small amount of plant tissue is collected and grown under carefully controlled:
conditions.
The cells divide and can develop into many new:
plants.
This allows large numbers of genetically similar plants to be produced from a relatively small amount of:
starting material.
Asexual Reproduction in Microorganisms
Microorganisms use several forms of asexual reproduction.
For example:
Bacteria → binary fission
Yeast → budding
Some fungi → asexual spores
Asexual reproduction can be especially effective for microorganisms because they can often reproduce rapidly when environmental conditions are:
favourable.
Asexual Spores
Some fungi produce reproductive cells called:
spores.
Asexual spores can be dispersed through the environment.
When a spore reaches suitable conditions, it may:
germinate and grow.
This can allow fungi to spread over:
large areas.
Asexual Reproduction in Animals
Asexual reproduction is less common in animals than in many microorganisms and plants, but it does:
occur.
Examples include:
Hydra → budding
Some flatworms → fragmentation and regeneration
Some insects and reptiles → parthenogenesis
Different animal groups have evolved different methods of producing offspring without:
fertilization.
Parthenogenesis
Parthenogenesis is a form of reproduction in which an embryo develops from an:
unfertilized egg.
It occurs naturally in some:
- insects
- crustaceans
- fish
- reptiles
The genetics of parthenogenesis can be more complicated than simple cloning, so the offspring are not always exact genetic copies of the:
parent.
This makes parthenogenesis somewhat different from simpler examples such as bacterial binary fission.
Comparing Asexual Reproduction
| Organism | Method | What Happens |
|---|---|---|
| Bacteria | Binary fission | One cell divides into two |
| Yeast | Budding | A new cell grows from the parent |
| Hydra | Budding | A new individual grows from the parent's body |
| Strawberry | Runners | Horizontal stems produce new plants |
| Potato | Tubers | Buds grow into new plants |
| Fungi | Asexual spores | Spores disperse and grow |
| Some animals | Fragmentation | A separated part develops into a new organism |
The mechanism varies, but the overall principle remains:
reproduction without fusion of gametes.
Advantages of Asexual Reproduction
Asexual reproduction has several important:
advantages.
Only one parent is required.
An organism does not need to:
find a mate.
This can be especially useful when organisms are:
isolated.
Asexual Reproduction Can Be Fast
Many forms of asexual reproduction occur:
quickly.
This allows organisms to produce many offspring over a relatively short:
time.
Rapid reproduction can be particularly useful when environmental conditions are:
favourable.
For example, bacteria with abundant food may increase their population very:
rapidly.
Less Energy May Be Required
Finding and competing for mates can require:
time and energy.
Asexual reproduction avoids many of these costs.
An organism can invest resources directly into:
producing offspring.
This can make asexual reproduction very:
efficient.
Successful Characteristics Are Preserved
If a parent organism is well adapted to its environment, producing genetically similar offspring can preserve those:
successful characteristics.
For example, a plant that grows particularly well in a stable environment can produce clones with similar:
genetic characteristics.
This can be advantageous while conditions remain:
similar.
Rapid Colonization
Asexual reproduction can allow organisms to colonize a new area:
quickly.
A single individual may sometimes establish an entire:
population.
It does not necessarily need another member of the same species nearby in order to:
reproduce.
Advantages in Agriculture
Humans often use asexual reproduction because it can preserve:
desirable characteristics.
For example, growers may want plants that consistently produce:
- large fruits
- attractive flowers
- high yields
- desirable flavours
- predictable growth patterns
Cloning successful plants allows these genetic characteristics to be:
maintained.
Disadvantages of Asexual Reproduction
The major disadvantage of many forms of asexual reproduction is:
low genetic variation.
Because offspring inherit nearly the same genetic information, a population may contain many individuals with:
similar characteristics.
This can create problems when the environment:
changes.
Environmental Change
Imagine a population of genetically similar plants adapted to:
warm, wet conditions.
If the climate suddenly becomes much:
drier,
many of the plants may struggle because they have similar genetic:
characteristics.
A population with greater genetic variation may contain some individuals better able to survive the:
new conditions.
Disease and Asexual Populations
Low genetic variation can also make a population vulnerable to:
disease.
If individuals are genetically very similar and one is susceptible to a pathogen, many others may also be:
susceptible.
A disease could therefore spread through the population and affect a large proportion of:
individuals.
Mutation and Asexual Reproduction
Asexual reproduction does not mean that genetic variation is completely:
absent.
Mutations can create:
new alleles and genetic differences.
Over many generations, mutations can introduce variation into an asexual:
population.
However, asexual reproduction does not generate the same genetic reshuffling associated with sexual reproduction.
Stable Environments
Asexual reproduction can be especially successful when the environment is:
stable.
If the parent is already well adapted to the conditions, producing similar offspring may be:
advantageous.
There may be little immediate benefit to creating many different combinations of:
genes.
Changing Environments
In rapidly changing environments, low genetic variation can become a:
disadvantage.
Different individuals may need different characteristics to survive changes such as:
- new diseases
- changing temperatures
- drought
- new predators
- changing food availability
Greater genetic variation increases the possibility that at least some individuals possess characteristics suited to:
new conditions.
Asexual vs Sexual Reproduction
| Feature | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Number of parents | Usually one | Usually involves genetic material from two gametes |
| Gamete fusion | No | Yes |
| Fertilization | No | Yes |
| Genetic variation | Usually low | Usually higher |
| Speed | Often faster | Often slower |
| Mate required | No | Often yes |
| Offspring | Often genetically very similar | Genetically varied |
Neither method is automatically:
better.
Each has advantages under different:
conditions.
A Simple Example
Imagine a plant growing in an environment where:
- water is plentiful
- temperature is stable
- nutrients are abundant
- there are few diseases
The plant is already well adapted.
Producing many genetically similar offspring quickly could be:
very successful.
Now imagine a new disease arrives.
If all the plants are genetically similar and susceptible, the entire population could be:
at risk.
This demonstrates both a major advantage and disadvantage of:
asexual reproduction.
Asexual Reproduction and Population Growth
Suppose one microorganism divides into two every generation.
Generation 0:
1
Generation 1:
2
Generation 2:
4
Generation 3:
8
Generation 4:
16
Generation 5:
32
The population is repeatedly:
doubling.
This demonstrates why some asexually reproducing organisms can increase their populations extremely:
rapidly.
Real-World Application: Agriculture
A farmer discovers a potato plant with desirable:
characteristics.
Rather than relying only on sexual reproduction through seeds, potatoes can be propagated using:
tubers.
New plants produced from the tuber retain much of the same genetic information.
This allows desirable characteristics to be:
preserved.
However, growing large numbers of genetically similar plants may also increase vulnerability to certain:
diseases.
Real-World Application: Horticulture
Gardeners frequently propagate plants using:
cuttings.
A cutting taken from a desirable plant can be encouraged to develop:
roots.
The resulting plant is genetically very similar to the:
parent plant.
This allows gardeners to reproduce particular flower colours, growth forms, or other:
desirable traits.
Choosing Between Reproductive Strategies
Asexual reproduction is particularly effective when:
- conditions are favourable
- conditions are stable
- rapid population growth is useful
- mates are difficult to find
- successful genetic characteristics should be preserved
Its major limitation appears when:
genetic diversity becomes important for survival.
Common Mistake: Asexual Means No Cell Division
Asexual reproduction depends heavily on:
cell division.
Cells must divide to produce:
new cells and organisms.
In eukaryotes, this commonly involves:
mitosis.
In bacteria, reproduction occurs through:
binary fission, which is not mitosis.
Common Mistake: All Asexual Offspring Are Perfectly Identical
Asexual offspring are often called:
clones.
However, mutations can occur.
Therefore, it is more accurate to say that offspring are usually:
genetically identical or very similar to the parent, depending on the reproductive mechanism.
Environmental differences can also cause genetically similar organisms to develop different:
observable characteristics.
Common Mistake: Plants Only Reproduce Sexually
Flowers and seeds are associated with:
sexual reproduction.
However, many plants can also reproduce:
asexually.
Examples include:
runners, tubers, bulbs, rhizomes, and cuttings.
Some plants can reproduce using:
both sexual and asexual methods.
Common Mistake: Regeneration Always Means Reproduction
Regeneration means replacing or regrowing:
lost tissue.
If an animal repairs a damaged body part, this is not automatically:
reproduction.
For asexual reproduction to occur, the process must result in:
a new individual.
Check Your Understanding
1. Define asexual reproduction.
2. How many parents are usually required for asexual reproduction?
3. Are gametes fused during asexual reproduction?
4. What is a clone?
5. Why are many asexual offspring genetically similar to their parent?
6. What role does DNA copying play in asexual reproduction?
7. What is binary fission?
8. Name an organism that reproduces by binary fission.
9. Describe the main stages of binary fission.
10. What is budding?
11. Name a microorganism that reproduces by budding.
12. Name an animal that can reproduce by budding.
13. What is vegetative propagation?
14. Explain how strawberry plants reproduce using runners.
15. Explain how potatoes can reproduce using tubers.
16. What is a plant cutting?
17. Explain why growers may use cuttings instead of seeds.
18. What is tissue culture?
19. Describe one example of asexual reproduction in animals.
20. Compare binary fission and budding.
21. Give three advantages of asexual reproduction.
22. Why is not needing a mate an advantage?
23. Why can asexual reproduction lead to rapid population growth?
24. What is the main genetic disadvantage of asexual reproduction?
25. Explain why genetically similar organisms may be vulnerable to disease.
26. Explain why environmental change can be a problem for asexual populations.
27. How can mutations create variation in an asexual population?
28. Why might asexual reproduction be successful in a stable environment?
29. Compare genetic variation in sexual and asexual reproduction.
30. Explain why asexual reproduction can be useful in agriculture.
Key Terms
- Asexual reproduction: Production of offspring from a single parent without fusion of gametes.
- Clone: Organism genetically identical or extremely similar to another organism from which it was produced.
- Gamete: Reproductive cell such as an egg or sperm.
- Fertilization: Fusion of male and female gametes.
- Mitosis: Cell division producing genetically identical daughter cells.
- Binary fission: Asexual reproduction in which a cell divides into two new cells.
- Budding: Asexual reproduction in which a new organism grows from the parent.
- Fragmentation: Reproductive process in which part of an organism separates and develops into a new individual.
- Vegetative propagation: Asexual reproduction in plants using vegetative structures such as stems, roots, or leaves.
- Runner: Horizontal stem that can produce new plants.
- Tuber: Swollen plant structure that stores food and can produce new plants.
- Bulb: Underground storage structure capable of producing new growth.
- Rhizome: Horizontal underground stem that can produce new roots and shoots.
- Cutting: Piece of plant used to grow a new plant.
- Tissue culture: Technique for producing plants from small amounts of plant tissue under controlled conditions.
- Spore: Reproductive cell capable of developing into a new organism under suitable conditions.
- Parthenogenesis: Development of an organism from an unfertilized egg.
- Mutation: Change in DNA.
- Genetic variation: Differences in genetic information among individuals in a population.
- Genetic diversity: Variety of genetic characteristics within a population.
Key Takeaways
- Asexual reproduction usually requires only one parent.
- Asexual reproduction does not involve the fusion of male and female gametes.
- Many asexual offspring are genetically identical or very similar to their parent.
- Genetically identical organisms are called clones.
- DNA must be copied before genetic information can be passed to new cells.
- Eukaryotic asexual reproduction commonly involves mitosis.
- Bacteria reproduce asexually through binary fission, not mitosis.
- Yeast commonly reproduces through budding.
- Hydra is an animal that can reproduce through budding.
- Some organisms reproduce through fragmentation.
- Plants can reproduce asexually through vegetative propagation.
- Runners, tubers, bulbs, rhizomes, and cuttings can produce new plants.
- Tissue culture allows many plants to be produced from small amounts of plant tissue.
- Some fungi reproduce using asexual spores.
- Some animals can reproduce through parthenogenesis.
- Asexual reproduction can be fast and efficient.
- A mate is not required.
- A single organism may be able to establish a new population.
- Asexual reproduction can preserve successful genetic characteristics.
- Humans use asexual reproduction extensively in agriculture and horticulture.
- The major disadvantage is usually low genetic variation.
- Low genetic diversity can make populations vulnerable to disease or environmental change.
- Mutations can introduce genetic variation into asexual populations.
- Asexual reproduction can be particularly effective in stable, favourable environments.
- Sexual reproduction generally produces more genetic variation than asexual reproduction.
- Neither reproductive strategy is universally better; their advantages depend on the organism and environmental conditions.