By the end of this lesson
- Describe the stages in cloning a mammal.
- Explain why the clone is genetically identical to the nucleus donor.
- Apply the stages to the example of Dolly the sheep.
BIG QUESTION
How was Dolly the sheep made without a father?
An egg cell with its own nucleus removed received the nucleus from an adult udder cell.
Preparing the cells
A diploidDescribes a cell with two sets of chromosomes, one from each parent (46 in humans). body cell is taken from the mature animal to be cloned. Its nucleus contains a full set of chromosomes.
An egg cell is taken from a second female. Its own haploid nucleus is removed, giving an enucleated egg cellAn egg cell whose nucleus has been removed, used in cloning by nuclear transfer..
Fusing and developing
The diploid nucleus is inserted into the enucleated egg, often by fusing the cells with a small electric shock, which also stimulates the egg to divide.
The cell divides by mitosisCell division that produces two genetically identical diploid cells, for growth, repair and asexual reproduction. to form an embryoAn early stage in the development of an organism, formed by mitosis from the zygote., which is implanted into the uterus of a surrogate motherA female who carries and gives birth to an embryo that is not genetically her own., where it develops until birth.
Dolly the sheep
For DollyThe sheep, born in 1996, that was the first mammal cloned from an adult body cell, by transferring its nucleus into an enucleated egg cell., born in 1996, the nucleus came from an udder cell of a Finn Dorset sheep and the egg came from a Scottish Blackface sheep. A Scottish Blackface surrogate gave birth.
Dolly was white-faced like the Finn Dorset, showing that she was genetically identical to the nucleus donor, not the egg donor or surrogate. Many attempts failed before one succeeded.
Three sheep, one clone
Track what each animal provides.
| Animal | What it provided | Is Dolly genetically identical to it? |
|---|---|---|
| Finn Dorset (nucleus donor) | Diploid nucleus from an udder cell. | Yes. |
| Scottish Blackface (egg donor) | Enucleated egg cell. | No (nuclear DNA removed). |
| Scottish Blackface (surrogate) | Uterus for development. | No. |
Language that earns precision
- enucleated
- Having had the nucleus removed.
- diploid
- Having two sets of chromosomes.
- surrogate mother
- A female who carries an embryo that is not genetically hers.
- embryo
- An early stage of development formed by mitosis.
Common misconceptions
Claim: Dolly is identical to the surrogate who gave birth to her.
Correction: She is genetically identical to the nucleus donor.
Claim: A sperm cell fertilises the egg in cloning.
Correction: No fertilisation occurs; the diploid nucleus is inserted.
Claim: The egg’s nucleus is used.
Correction: The egg nucleus is removed; the body cell nucleus is used.
Match the answer to the command word
Diploid nucleus from mature cell → enucleated egg → fuse (electric shock) → embryo → surrogate.
Use enucleated, diploid and surrogate; say which animal the clone is identical to.
Recall, apply, then justify
Describe the stages in producing a cloned mammal.
Hint
Two cells, one embryo, one surrogate.
Suggested marking points
- Nucleus removed from an egg cell (enucleated).
- Diploid nucleus taken from a mature body cell of the animal to be cloned.
- Nucleus inserted / fused into the enucleated egg (electric shock).
- Embryo develops and is implanted into a surrogate mother.
Explain why Dolly had the same characteristics as the Finn Dorset sheep rather than the Scottish Blackface sheep.
Hint
Where were Dolly’s genes from?
Suggested marking points
- Her nucleus / DNA came from the Finn Dorset udder cell.
- The Scottish Blackface egg had its nucleus removed / surrogate only carried the embryo.
Explain why cloned mammals are genetically identical to the nucleus donor and suggest why cloning has a low success rate.
Planning hint
Nucleus, mitosis, development.
What a strong design includes
- The nucleus carries the genetic information.
- The egg’s own nucleus was removed.
- The embryo cells are produced by mitosis.
- So all cells carry the donor’s DNA.
- The mature nucleus may not reprogramme correctly to direct development.
- Many embryos fail to develop or implant, so success rates may be low.