Cambridge IGCSE0970

Asexual reproduction

Biology 0970 Chapter Notes

What this chapter covers

Asexual reproductionSexual reproductionSexual reproduction in plantsSexual reproduction in humansSex hormones in humansSexually transmitted infections
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In this chapter

  1. 1The Basics of Asexual Reproduction
  2. 2Natural Asexual Reproduction in Plants
  3. 3Asexual Reproduction in Microbes
  4. 4Artificial Cloning in Plants
  5. 5Advantages and Disadvantages

1. The Basics of Asexual Reproduction

Asexual reproduction is a method of producing new organisms that involves only one parent. Unlike sexual reproduction, it does not involve the fusion of sex cells (gametes). The entire genetic code of the single parent is passed on to its offspring. This process relies on a type of cell division called mitosis, where a cell divides to produce two genetically identical daughter cells. The result is that the offspring are exact genetic copies of the parent, known as clones.

Key term

Asexual Reproduction: A type of reproduction involving a single parent that results in offspring that are genetically identical to the parent.

Examiner insight

Examiners reward students who use precise terminology. Stating that offspring are 'genetically identical clones' is stronger than saying they are just 'the same'.

Common pitfall

Confusing asexual reproduction with self-pollination in plants. Self-pollination is a form of sexual reproduction as it still involves the fusion of male and female gametes, even though they come from the same plant.

Worked example 13 marks

List three key features of asexual reproduction.

  1. 1
    1. Involves only one parent.
  2. 2
    1. There is no fusion of gametes and therefore no mixing of genetic information.
  3. 3
    1. The offspring are genetically identical to the parent (they are clones).

Recap

  • Asexual reproduction requires only one parent.
  • It does not involve sex cells (gametes) or fertilisation.
  • The cell division process involved is mitosis.
  • Offspring are genetically identical to the parent and are called clones.
  • This method is common in plants, fungi, and bacteria.

Quick check

  1. How many parents are involved in asexual reproduction?1 mark
  2. What type of cell division is used in asexual reproduction?1 mark

2. Natural Asexual Reproduction in Plants

Many plants have evolved natural methods of asexual reproduction, also known as vegetative propagation. This allows them to spread and colonise an area quickly, especially if the conditions are favourable. For example, strawberry plants send out horizontal stems called 'runners' that grow along the surface of the soil. New plants sprout from nodes on the runner, which then grow their own roots and become independent. Other plants, like potatoes, reproduce from tubers, while daffodils and onions reproduce from bulbs. In all cases, the new plant is a clone of the parent.

Key term

Vegetative Propagation: A type of asexual reproduction in plants where new individuals arise from vegetative parts of the parent, such as stems, roots, or leaves.

Examiner insight

When asked for causes of variation in cloned organisms, always focus on environmental factors (like light, water, temperature, nutrients), as their genes are identical.

Fun fact

The largest known organism on Earth is a clone! A grove of quaking aspen trees in Utah, named 'Pando', is a single genetic individual connected by a massive root system, covering over 100 acres.

Worked example 12 marks

A strawberry plant produces new plants via runners. A scientist measures the length of the first runner (X) and second runner (Y) from 5 parent plants. The results are: Plant 1 (X=40, Y=20), Plant 2 (X=35, Y=28), Plant 3 (X=42, Y=26), Plant 4 (X=61, Y=26), Plant 5 (X=52, Y=30). All lengths are in cm. Calculate the mean length for runner X.

  1. 1

    Step 1: Sum the lengths for runner X. 40 + 35 + 42 + 61 + 52 = 230 cm.

  2. 2

    Step 2: Divide the sum by the number of plants. Mean = 230 cm / 5 plants.

  3. 3

    Step 3: Calculate the final answer. Mean length of runner X = 46 cm.

Worked example 22 marks

Suggest two environmental factors that could cause the variation in runner length seen in the data.

  1. 1
    1. Variation in light intensity: Plants in sunnier spots may have more energy for growth, producing longer runners.
  2. 2
    1. Variation in soil nutrients or water availability: A better supply of minerals and water would support more vigorous growth and longer runners.

Recap

  • Natural asexual reproduction in plants is called vegetative propagation.
  • Strawberry plants use runners to reproduce asexually.
  • Potato plants reproduce asexually from tubers.
  • Daffodils and onions reproduce asexually from bulbs.
  • All new plants formed this way are clones of the parent plant.

Quick check

  1. Name the structure used by strawberry plants for asexual reproduction.1 mark

3. Asexual Reproduction in Microbes

Microorganisms are masters of asexual reproduction, allowing them to multiply at incredible speeds when conditions are right. Bacteria reproduce by a simple cell division process called 'binary fission'. The circular DNA inside the bacterium replicates, and the cell then splits into two identical daughter cells. Yeast, a single-celled fungus, reproduces by 'budding'. A small outgrowth, or bud, forms on the parent cell, receives a copy of the nucleus, and eventually detaches to become a new, smaller cell. Other fungi, like the mould Mucor, can produce vast numbers of lightweight spores asexually, which are dispersed by the wind to colonise new surfaces.

Key term

Binary Fission: The method of asexual reproduction used by bacteria, where one cell divides into two genetically identical cells.

Common pitfall

Thinking that budding and binary fission are the same. In budding (yeast), the division is unequal, with a small bud growing on a large parent cell. In binary fission (bacteria), the cell splits into two equal-sized cells.

Worked example 13 marks

A single bacterium is placed in a nutrient broth. It divides by binary fission every 20 minutes. How many bacteria will there be after 2 hours, assuming no bacteria die?

  1. 1

    Step 1: Calculate the number of divisions. Time = 2 hours = 120 minutes. Number of divisions = 120 minutes / 20 minutes per division = 6 divisions.

  2. 2

    Step 2: Calculate the number of bacteria. The number doubles with each division. Number = 1 x 2^n, where n is the number of divisions.

  3. 3

    Step 3: Substitute the value of n. Number = 1 x 2^6.

  4. 4

    Step 4: Calculate the final answer. 2^6 = 2x2x2x2x2x2 = 64. There will be 64 bacteria.

Recap

  • Bacteria reproduce asexually by binary fission.
  • Yeast, a single-celled fungus, reproduces asexually by budding.
  • Some fungi, like bread mould, reproduce asexually by producing spores.
  • These methods allow for very rapid population growth in favourable conditions (e.g., warmth, moisture, food).

Quick check

  1. What is the name for asexual reproduction in yeast?1 mark
  2. How do bacteria reproduce?1 mark

4. Artificial Cloning in Plants

Humans have used asexual reproduction for centuries to produce plants with desirable features. This is called artificial propagation or, more commonly, cloning. The simplest method is taking 'cuttings'. A piece of stem or leaf is cut from the parent plant, often dipped in a rooting hormone to encourage root growth, and then planted in compost. Since the cutting is a piece of the original plant, it will grow into a new plant that is genetically identical to the parent. This is extremely useful in commerce for producing large numbers of uniform plants, such as crop plants with high yield or decorative plants with attractive flowers, quickly and reliably.

Key term

Clone: A group of genetically identical organisms or cells produced asexually from a single parent.

Examiner insight

When describing how to take a cutting, marks are often awarded for practical details like using a clean knife, removing lower leaves, and using rooting hormone. Be specific.

Worked example 14 marks

A gardener wants to create more of a specific rose bush with beautiful flowers. Describe the steps they should take to do this using a cutting.

  1. 1

    Step 1: Use a clean, sharp knife or secateurs to cut a 10-15 cm piece of a healthy, young stem from the parent rose bush.

  2. 2

    Step 2: Remove the leaves from the lower half of the cutting to prevent water loss.

  3. 3

    Step 3: Dip the cut end of the stem into rooting powder. This contains hormones that stimulate root development.

  4. 4

    Step 4: Plant the cutting into a pot containing moist compost. Place a plastic bag over the pot to maintain a humid atmosphere, and keep it in a warm, light place but out of direct sunlight.

  5. 5

    Step 5: After a few weeks, roots will have formed, and the cutting will begin to grow into a new, genetically identical rose bush.

Recap

  • Humans can clone plants using artificial vegetative propagation.
  • Taking cuttings is a common method of artificial cloning in plants.
  • Rooting hormone is used to encourage cuttings to develop roots.
  • This method is used to produce large numbers of genetically identical plants with desirable characteristics.
  • Examples of desirable characteristics include high crop yield, disease resistance, or specific flower colour.

Quick check

  1. What is the purpose of dipping a cutting in rooting powder?1 mark

5. Advantages and Disadvantages

Asexual reproduction has significant advantages and disadvantages. Its main advantage is speed and efficiency. Since only one parent is needed, an organism can rapidly colonise a habitat without needing to find a mate. If the parent is well-adapted to a stable environment, all its cloned offspring will also be well-adapted. However, the major disadvantage is the complete lack of genetic variation. All individuals in the population are genetically identical. If the environment changes, or a new disease appears, the entire population could be wiped out as they all share the same vulnerabilities. The Irish Potato Famine in the 1840s is a stark example, where a fungal blight destroyed the potato crop because the potatoes grown were all a single, susceptible variety (a clone).

Key term

Genetic Variation: The differences in the genetic material of individuals within a population.

Examiner insight

For 'evaluate' or 'compare' questions, examiners look for a balanced answer. You must present both sides of the argument (advantages and disadvantages) to gain full marks.

Common pitfall

Forgetting the context. A lack of variation is a disadvantage in a changing environment, but it can be an advantage in a stable environment where the parent is already perfectly adapted.

Worked example 14 marks

Evaluate the use of asexual reproduction by a farmer to grow a commercial crop like bananas.

  1. 1

    Advantage 1: All the banana plants will be genetically identical, leading to a uniform crop. This means all fruits will ripen at the same time and be of a similar size and quality, which is efficient for harvesting and selling.

  2. 2

    Advantage 2: It is a rapid method of production. Large numbers of plants can be produced quickly from a single parent with desirable features (e.g., high yield, good taste).

  3. 3

    Disadvantage 1: The lack of genetic variation is a major risk. If a new disease, like Panama disease which affects bananas, infects the farm, the entire crop is susceptible and could be destroyed.

  4. 4

    Conclusion: While commercially beneficial for uniformity and speed, relying solely on asexual reproduction is a high-risk strategy due to the vulnerability to disease. It is advantageous in the short term but potentially disastrous in the long term.

Recap

  • Advantage: Reproduction is rapid and can exploit favourable conditions.
  • Advantage: Only one parent is required, no need to find a mate.
  • Advantage: Desirable traits from a parent are always passed on to offspring.
  • Disadvantage: There is no genetic variation among offspring.
  • Disadvantage: The entire population is vulnerable to the same diseases or environmental changes.

Quick check

  1. State one advantage of asexual reproduction.1 mark
  2. State one disadvantage of asexual reproduction.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'clone'.2 marks
  2. Describe the process of binary fission in bacteria.3 marks
  3. Explain why offspring produced by asexual reproduction show no genetic variation.2 marks
  4. A gardener wants to produce many new fuchsia plants that are identical to her prize-winning one. Describe, in detail, the method she should use.4 marks
  5. Compare the advantages and disadvantages of asexual reproduction with sexual reproduction for a species.5 marks
  6. The vast majority of bananas grown for export are clones of a single variety, the Cavendish. Explain one commercial advantage and one major risk of this practice.4 marks
  7. A single yeast cell is placed in a sugar solution. It reproduces by budding every 90 minutes. How many yeast cells will there be after 6 hours, assuming none die?3 marks
  8. Explain why a plant that is well-adapted to its stable environment might benefit more from asexual reproduction than sexual reproduction.3 marks
  9. Some organisms, like aphids, reproduce asexually during the summer when conditions are good, but switch to sexual reproduction in the autumn. Suggest the advantages of this life cycle strategy.4 marks
  10. A potato is a tuber, which is a swollen underground stem used for asexual reproduction. If you plant a potato, a new plant grows. If this new plant produces flowers and seeds, will the plants grown from these seeds be genetically identical to the parent plant? Explain your answer.3 marks

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