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Plant cloning summary
CLONING means producing many genetically identical individual organisms, cells or copies of a
gene. In section 6.2.1 the emphasis is on whole organisms, that is, many individual plants or
animals that share the same genotype. In order to be genetically identical, cells and the individuals
that grow from them must have been produced by cell division involving MITOSIS only. Natural
clones include populations of bacteria and of some plants, such as:

strawberry plants from runners

clumps of daffodils from one original bulb

a clonal patch of elm trees developed from root suckers of one parent elm tree.
Many plant species clone themselves readily, and gardeners and farmers have long taken
advantage of this to make more plants by vegetative propagation. For example, twigs cut from many
bushes will grow roots if stuck in the ground. The success of these ‘cuttings’ can be improved by
dipping the cut ends in commercial rooting powder, which contains the plant growth regulator auxin.
Other traditional techniques of cloning plants include grafting, layering and dividing clumps of bulbs
or large crowns of perennial plants.
Tissue culture is a modern development of this, where much smaller pieces of plant
tissue are caused to grow by mitotic cell division and to differentiate, forming new
roots and leaves. This is also known as micropropagation. The key stages in growing
many new plants by tissue culture are listed as a flow diagram on the next page.
Notice the use of PGRs (plant growth regulators) – a synoptic link with 5.1.5 (b) - (f).
Advantages of cloning plants
1. Vegetative propagation and tissue culture are quick methods of producing new plants
compared to growing plants from seed.
2. If tissue culture is done using the apical bud as an explant, the new plants will be diseasefree (no viruses).
3. The plants chosen for cloning will be high-yielding and have other desirable features such as
maybe being resistant to a common pest or disease of that crop. The original plant may have
its good characteristics due to selective breeding or genetic modification.
4. The new plants all have the same phenotype, that is, they are uniform. The uniformity makes
growing and harvesting easier.
5. Infertile plants such as triploids can be reproduced, e.g. commercially grown bananas.
6. Plants that are hard to grow from seed can be reproduced, e.g. orchids for the horticulture
industry.
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Disadvantages of cloning plants

Tissue culture is an expensive process because it is labour intensive.

The tissue culture process can fail due to microbial contamination.

All the cloned offspring are susceptible to the same pest or disease, so that if one dies, they
probably all will. Crops are grown in monocultures and pests and disease spread easily
between the closely-grown crop plants.

There is no or very little genetic variation. The only source of variation in a clone is mutation.
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Growing plants by tissue culture
1 The plant material is cut into many small pieces called explants. These could
be tiny pieces of leaf, stem, root or bud. The meristem (region of mitotically
dividing cells) at the tip of the plant is often used, because these cells are free
of viruses.
2 The explants are sterilised with bleach or alcohol to kill bacteria and fungi.
Steps 3-5 that follow are carried out under aseptic conditions.
3 The sterilised explants are placed on a growth medium of agar jelly
containing nutrients such as glucose, amino acids and phosphates, and also
high concentrations of the PGRs auxin and cytokinin. This causes the cells of
each explant to divide by mitosis to form a callus. A callus is a mass of
undifferentiated, totipotent cells.
4 Each callus can then be subdivided, to increase the final number of plants
that will be made.
5 To make the totipotent cells of the callus differentiate into organs to form a
plant, the callus must be moved twice onto new growth media. These contain
different ratios of auxin and cytokinin.
One ratio (100 auxin : 1 cytokinin) causes roots to form,
the other (4 auxin : 1 cytokinin) causes shoots to form.
6 The tiny plantlets are transferred to a greenhouse to be grown on in sterile
soil and be acclimatised to normal growing conditions.
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