What is a clone?
A clone is a living thing with the same genes as another one. Genes are the instructions in DNA.
Nature makes clones without any lab. Identical twins come from one fertilised egg that splits in two. A bacterium splits into two copies of itself. A cutting of a money plant grows into a new plant with the same genes.
Clones are copies of genes, not copies of memories or habits. A cloned cat can have a different pattern or nature, because life and surroundings also matter.
Animal cell culture and cell fusion
Cell culture means growing animal cells in a clean flask with food and warmth (37 °C). More on this is in the lesson on animal cell culture.
Cell fusion means joining two cells into one. Chemicals (like PEG), a harmless virus or a small electric pulse can do it. The new cell has both nuclei, and may then merge them into one.
- Fusion of two plant cells without their walls (protoplasts) gives a somatic hybrid.
- Fusion of an antibody-making B cell with a long-living myeloma cell gives a hybridoma.
Monoclonal antibodies
A B cell makes one kind of antibody, but it dies soon. A myeloma (cancer-type) cell divides for ever. A hybridoma does both: it lives long and makes one kind of antibody.
All its cells are clones of one cell, so the antibody is called monoclonal (“one clone”). Steps: inject the antigen into a mouse, take B cells from the spleen, fuse them with myeloma cells, pick the hybridoma that makes the right antibody, and grow it in large tanks.
Uses: pregnancy test strips, finding germs in blood, treating some cancers and immune diseases. See the full lesson on monoclonal antibodies.
Cloning by nuclear transfer
Every body cell has all the genes of the animal, but most are switched off. Nuclear transfer wakes them up again.
- Donor cell: take a body cell (for example from the udder or skin) from the animal to copy.
- Empty egg: take an egg from another animal and remove its nucleus.
- Transfer: put the donor nucleus in the empty egg. A small electric pulse fuses them and starts division.
- Embryo: the egg divides in a dish for a few days.
- Surrogate: place the embryo in the womb of a female of the same species.
- Birth: a baby with the donor’s genes is born.
Dolly was born in 1996 in Scotland. Many tries were needed for one healthy lamb. She lived about 6 years.
Uses, limits and ethics
- Farming: copies of high-milk animals.
- Conservation: help for very rare species.
- Medicine: animals that carry a human disease gene, to test treatments; animals that make useful proteins in milk.
- Stem cells (therapeutic cloning): embryo cells that could grow tissues for repair. This is still research.
Limits: the success rate is very low (often about 1 in 100 or less), many clones are born with health problems, and the cost is high. A clone is also not 100% identical, because a little DNA stays in the egg’s mitochondria and the environment shapes the animal.
Ethics: making a cloned human baby (reproductive cloning) is banned or not allowed in most countries, for safety and dignity. Read the lesson on bioethics.
Try it
In the 3D, tap the three donor buttons and see how the lamb’s colour follows the nucleus. At home, put a money-plant or mint cutting in water: its new roots grow into a plant that is a clone of the parent. Draw a table: who gave the nucleus, who gave the egg, who carried the baby.
Key formulas and definitions
- Clone = same genes as the donor
- Nuclear transfer = donor nucleus + empty egg + surrogate mother
- Hybridoma = B cell + myeloma cell (makes one kind of antibody for ever)
- Cells after n divisions = 2ⁿ
- Success rate (%) = healthy clones ÷ tries × 100
Worked examples
1. A black sheep gives the nucleus, a white sheep gives the egg, a brown sheep is the surrogate. What colour is the lamb?
Black. Colour genes are in the nucleus. The egg donor and the surrogate do not pass on their genes.
2. Scientists make 200 reconstructed eggs and get 4 live, healthy lambs. What is the success rate?
4 ÷ 200 × 100 = 2%.
3. Why can we not use a red blood cell of a human for nuclear transfer?
A mature human red blood cell has no nucleus, so it has no genes to transfer. We must choose a body cell that has a nucleus, such as a skin cell.
Common mistakes
- Thinking the surrogate mother passes her looks to the baby. She gives only a womb and food; the genes come from the nucleus donor.
- Thinking cloning joins sperm and egg. In nuclear transfer there is no sperm; a body-cell nucleus goes into an egg.
- Believing a clone is a perfect copy in every way. Mitochondrial DNA, the womb, food and life experience make small differences.
- Mixing up a clone (same genes) with a hybrid (genes from two different species).