Cloning in Plants and Animals

Question 1

(a) Outline the differences between reproductive and non-reproductive cloning;

Answer 1

Reproductive cloning: the production of a new individual with the genotype of an existing one; a complete organism that’s genetically identical to another organism.

Non-reproductive cloning (therapeutic cloning): the use of stem cells to generate replacement cells, tissues or organs (instead of entire organisms) which may be used to treat particular diseases or conditions of humans.

Question 2

(b) Describe the production of natural clones in plants using the example of vegetative propagation in elm trees;

Answer 2

Vegetative propagation: the natural production of plant clones from non-reproductive tissues (e.g. roots, leaves and stems). ‘Structures’ are produced which grow into new individual organisms, clones of the original.

• The English Elm is adapted to reproduce asexually following damage to the parent plant.
• New growth in the form of basal sprout suckers (sucker = shoot that grows from the shallow roots of an elm tree) appear within two months of the destruction of the main trunk.
• These suckers grow from meristem tissue in the trunk close to the ground where the least damage is likely to have occurred.
• Suckers grow from sucker buds (undeveloped shoots) that are scattered around the tree’s root system. They are normally dormant, but stress (drought, damage, disease) or when the parent plant is dying, the buds are activated and the suckers begin to form.
• Suckers can spout meters away to avoid the stress that triggered their growth.
• Separate trees (clones) eventually form, with a clonal patch being a circle of English elm basal sprouts that occur around 2 months after the original elm tree has died or been coppiced.

Question 3

(c) Describe the production of artificial clones of plants from tissue culture (micropropagation);

Answer 3

Tissue culture: the separation of cells of cells of any tissue type and their growth on or in a nutrient medium.

• Explants (small pieces of tissue is taken from the plant to be cloned, usually from the shoot tip) are taken from young, developing stems that have been surface sterilised.
• In aseptic conditions, the explants are placed on a nutrient growth medium (contains sucrose for energy and other organic nutrients such as amino acids and vitamins, inorganic ions and plant hormones to stimulate mitosis).
• Undifferentiated (meristematic) cells in the explant divide by mitosis (no differentiation) to produce a callus (a mass of undifferentiated cells).
• Callus can be subdivided many times to increase number of plants that be produced.
• After a few weeks, single callus cells can be removed from the mass and placed on a new growth medium containing cytokinins (plant hormone) to encourage shoot growth
• After a further few weeks, the growing shoots are transferred on a different medium with auxins (plant hormone) to encourage root growth, creating new plantlets.
• The growing plantlets are then transferred to a greenhouse to be acclimatised and grown further before they are planted outside in soil. Will be genetically identical to parent plant.

Question 4

(d) Discuss the advantages and disadvantages of plant cloning in agriculture;

Answer 4

Advantages
• Very many genetically identical plants can be produced from one original plant
• Plants can be produced at any time of the year (any season) and air-freighted around the world
• Callus can be genetically engineered; desirable genetic characteristics can be chosen
• Plants that take a long time to produce seeds can be reproduced quickly

Disadvantages
• Plants are all susceptible to a newly mutated pathogen/pest or to changing environmental conditions as the plants are all genetically identical; no genetic variability.
• Process is labour-intensive; more difficult to plant plantlets than to sow seed.
• Production costs are very high due to high energy use and training of skilled workers.

Question 5

(e) Describe how artificial clones of animals can be produced;

Answer 5

Nuclear transfer
A nucleus from an adult differentiated cell is placed in an enucleated (egg cell with nucleus removed) egg cell, via electro-fusion to produce a zygote. The zygote is cultured (stimulated to divide, embryo formed) in a tied oviduct, recovered, and in reproductive cloning, implanted into a surrogate uterus to produce a genetically identical organism to the nucleus donor. In non-reproductive cloning, stem cells are harvested from the embryo.

Splitting embryos
In vivo/in vitro grown embryos are subdivided and split up into separate segments, implanted into surrogate mothers, each producing genetically identical offspring. Desirable female not put at risk in pregnancy, available for further superovulation (treating a female mammal with hormones so that many egg cells mature in the ovaries at the same time). Increases stock of selectively bred animals.

Question 6

(f) Discuss the advantages and disadvantages of cloning animals

Answer 6

Advantages
• Reproductive rate of a genetically superior animal is increased
• Number of animals with a wanted trait is increased; high value animals (e.g. high milk yield cows) cloned in high numbers; allele always passed on too.
• The cloned embryo can be sexed/tested for certain genetic diseases before implantation into a surrogate
• A fertile female of an endangered species is not needed for somatic nuclear transfer.
• Rare animals can be cloned to preserve the species
• Genetically modified animals- e.g. sheep that produce pharmaceutical chemicals in their milk- can be quickly reproduced.
• Infertile animals can be reproduced.
• Animals can be cloned at any time; allow breeding season.

Disadvantages
• Unethical; high value animals are not necessarily produced with animal welfare in mind. (e.g. some strains of meat producing chickens have been developed that are unable to walk)
• As with plants, excessive genetic uniformity in a species makes it unlikely to be able to cope with/adapt to changes in the environment, disease etc.
• Unclear whether animals cloned using the nuclear material of adult cells will remain healthy in the long term; evidence suggests clones may not live as long as natural offspring.
• Reproductive cloning is difficult, time-consuming and expensive (e.g. Dolly the sheep took 277 nuclear transfer attempts).