2.1.7 Asexual Plant Reproduction — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions

Section 1 of 9What is asexual reproduction?

In asexual reproduction a single parent produces offspring without gametes and without fertilisation. The offspring are produced by mitosis, so they are genetically identical to the parent and to each other. Such genetically identical organisms are called clones. Differences between clones arise only from mutations or from the environment.

Many organisms reproduce this way

bacteria divide by binary fission;
- strawberry plants send out runners;
- potato plants make tubers;
- daffodils form bulbs;
- some fungi release spores;
- and Hydra and yeast produce buds.

Section 2 of 9Mitosis and the cell cycle

Mitosis is the type of division in which one cell makes two genetically identical cells.

1

Before it begins, the cell grows and copies its DNA, so that each chromosome consists of two identical copies.

2

During mitosis the copies are separated and move to opposite ends of the cell; then two nuclei form and the cell splits in two.

3

The cell cycle is therefore: growth and DNA replication, mitosis, and cytokinesis, after which the process can begin again.

4

Mitosis also allows growth of multicellular organisms, repair of damaged tissue and replacement of cells.

Section 3 of 9Comparing sexual and asexual reproduction

Feature	Asexual	Sexual
Number of parents	One	Two
Gametes	None	Haploid gametes fuse
Type of division	Mitosis	Meiosis then fertilisation
Offspring	Genetically identical clones	Genetically different
Speed and numbers	Fast, many offspring	Slower, fewer offspring
Response to change	Little variation, whole population may be wiped out	Variation gives some survivors

Asexual reproduction is advantageous in a stable environment: the parent is already well adapted, no mate is needed and energy is saved. Sexual reproduction is better when the environment changes or disease strikes. Some organisms can use both, such as the malaria parasite and many fungi and plants.

Section 4 of 9Cloning plants and animals

Gardeners use cuttings: a piece of stem is cut, dipped in rooting powder (containing plant hormone) and planted, and grows into a new plant identical to the parent. This is cheap and quick.

Tissue culture uses tiny groups of cells from a plant, grown on a sterile nutrient agar with hormones. Large numbers of identical plants can be made quickly, even of rare species. Aseptic technique is needed to prevent microbial growth.

Embryo transplants in cattle: an embryo is split into several cells while it is still a ball of cells, and each is implanted into a different surrogate mother, producing several identical calves.

Adult cell cloning

In adult cell cloning (as with Dolly the sheep) the nucleus of an adult body cell is placed into an egg cell with its nucleus removed.

An electric shock stimulates it to divide, and the embryo is implanted in a surrogate.

Section 5 of 9Issues with cloning

Benefits include producing many identical plants with desired features, saving endangered species and getting more offspring from high-quality animals.

Issues with cloning

Risks include reduced genetic variation,

which makes a population vulnerable to a single disease, and the fact that cloned animals may have health problems and shortened lives.

Cloning humans, and ethical issues such as the use of embryos, are widely debated.

Cloned crops can also be at greater risk since all are susceptible to the same pest.

Section 6 of 9Required practical thinking: growing clones

A typical method for tissue culture or cuttings in school uses a clean working area.

1

Instruments are sterilised with ethanol or a flame, hands are washed, and the agar plate or container is covered.

2

The reasoning behind each step matters: sterile technique stops bacteria and fungi,

which grow quickly on nutrient-rich agar,
from competing with the plant tissue.

3

A rooting hormone can be used because auxin stimulates root growth in cuttings.

4

To compare how well the cuttings root, several cuttings are used for each treatment, the same size and cut at the same type of position on the stem,

and the roots are counted and measured after a fixed time.

5

A control cutting is placed without rooting powder.

6

Variables to keep constant include the light, temperature, water supply and the number of leaves left on each cutting,

because leaves lose water by transpiration and a cutting has no roots to replace it.

7

Results are compared as the mean number or length of roots per cutting, so the effect of the hormone can be judged fairly.

Section 7 of 9Exam-style questions

Describe how a gardener could produce 100 identical plants from one parent plant with a prized flower colour. [3]3 marks
Suggest one advantage and one disadvantage to a farmer of growing a field of cloned plants. [2]2 marks
Explain why Dolly the sheep was genetically identical to the sheep that donated the nucleus. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Asexual reproduction	Reproduction by one parent producing genetically identical offspring.
Clone	A genetically identical organism or cell.
Mitosis	Cell division producing two genetically identical cells.
Tissue culture	Growing new plants from small pieces of tissue in sterile nutrient medium.
Cutting	A piece of plant stem used to grow a new plant.
Embryo transplant	Moving embryos into surrogate mothers to produce offspring.
Adult cell cloning	Transferring a body cell nucleus into an egg without its nucleus.
Binary fission	Division of a bacterium into two.

Section 9 of 9Exam technique

1

Saying asexual offspring are 'exactly' identical in every way. The environment may still cause differences.

2

Using meiosis in a description of asexual reproduction. Only mitosis is involved.

3

Forgetting the surrogate mother in animal cloning.

4

Claiming cloned organisms have more variation. They have very little.

5

Assuming asexual reproduction is only used by simple organisms; many plants and fungi use it too.