(d) Cloning
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
517B Micropropagation processDescribe the process of micropropagation (tissue culture) in which explants are grown in vitro.518B Commercial micropropagationUnderstand how micropropagation can be used to produce commercial quantities of genetically identical plants with desirable characteristics.519B Cloned mammalsDescribe the stages in the production of cloned mammals involving the introduction of a diploid nucleus from a mature cell into an enucleated egg cell, illustrated by Dolly the sheep.520B Uses of cloned transgenic animalsUnderstand how cloned transgenic animals can be used to produce human proteins.
Grow plant clones by micropropagation
Micropropagation is tissue culture in which small plant explants are grown in vitro under sterile, controlled conditions and develop into whole plants.
| Stage | Procedure and purpose |
|---|---|
| 1 choose tissue | take a small explant, often from a shoot tip, from a healthy plant with the desired genotype |
| 2 work aseptically | sterilise instruments, vessel and explant surface without killing plant cells; this prevents bacteria and fungi overgrowing the culture |
| 3 place on medium | transfer the explant to sterile agar containing water, sugar, mineral ions and suitable plant growth regulators |
| 4 multiply shoots | cells divide by mitosis and form a mass of tissue or many shoots; pieces can be subcultured to increase number |
| 5 form roots | move shoots to a medium with conditions that promote root development |
| 6 acclimatise | transfer plantlets to compost and humid protected conditions, then gradually to a glasshouse or outside |
In vitro means outside the organism in laboratory culture, such as a sterile vessel. Every new cell forms by mitosis, preserving the explant's genotype unless mutation occurs.
Sterilisation must remove contaminating microbes without boiling or otherwise killing the living explant. Nutrient medium supports growth but does not replace light once plantlets photosynthesise.
Scale a desirable plant genotype commercially
Commercial micropropagation repeatedly subdivides tissue from one selected plant, producing large numbers of genetically identical plants that preserve its desirable characteristics.
| Commercial advantage | Why micropropagation provides it |
|---|---|
| genetic uniformity | all clones retain the selected genotype, so a GM gene, flower form, fruit quality or other inherited trait is copied |
| rapid multiplication | many explants and repeated subculture produce far more plants than one parent could by conventional cuttings |
| year-round production | in vitro culture is controlled and not restricted to the outdoor growing season |
| propagation of difficult plants | plants with few seeds, poor germination or slow conventional reproduction can be multiplied |
| clean starting stock | carefully selected and tested tissue can provide uniform pathogen-free material |
Uniformity is valuable only when the selected genotype suits the environment. A genetically uniform crop has little variation, so one disease or environmental change may affect most plants similarly.
Micropropagation copies a desirable genotype; it does not create the characteristic. The useful parent must first be selected or genetically modified, and sterile culture still requires acclimatisation before field planting.
Clone a mammal by nuclear transfer
A cloned mammal can be produced by transferring a diploid nucleus from a mature body cell into an egg cell whose own nucleus has been removed.
| Stage | Nuclear-transfer process |
|---|---|
| 1 donor nucleus | take a mature diploid body cell from the animal to be cloned and isolate its nucleus |
| 2 enucleated egg | obtain an unfertilised egg cell from a donor female and remove its haploid nucleus |
| 3 nuclear transfer | insert the diploid body-cell nucleus into the enucleated egg, or fuse the body cell with it |
| 4 activation | apply an electric shock to stimulate the reconstructed cell to divide |
| 5 embryo | mitosis produces an early embryo with nuclear DNA from the adult donor |
| 6 implantation | place the embryo into the uterus of a surrogate mother for development and birth |
Dolly's nuclear DNA matched the mature-cell nucleus donor, not the egg donor or surrogate. Many reconstructed eggs are needed because activation, division, implantation and development often fail.
The transferred nucleus must be diploid and comes from a mature body cell; meiosis and fertilisation are not part of this cloning route. A clone can still differ in phenotype because environment and non-nuclear factors differ.
Clone transgenic animals to manufacture human proteins
A transgenic animal can carry and express a human gene; cloning that animal creates many individuals with the same inserted gene, allowing repeated production of the human protein.
| Stage | Production logic |
|---|---|
| 1 create a transgenic founder | insert a human gene with suitable control DNA into an animal cell or embryo |
| 2 confirm expression | identify an animal that produces the required human protein, often in an accessible secretion such as milk |
| 3 clone the selected genotype | use nuclear transfer from the successful transgenic animal to create genetically identical embryos |
| 4 produce a herd | implant embryos into surrogates and raise cloned transgenic offspring |
| 5 recover protein | collect the protein-containing material and extract and purify the human protein for its intended use |
Cloning preserves the exact gene insertion and productive genotype instead of relying on sexual reproduction, which would reshuffle alleles and may not pass the transgene to every offspring.
The animal produces a human protein because it expresses transferred human genetic material; the whole animal does not become human. Cloning scales a verified transgenic genotype but does not replace purification or safety testing.