Transferring DNA

Sheep, calves, and monkeys were cloned using the same basic techniques. A nucleus is removed from one cell and placed into an enucleated unfertilized egg cell. Basically the nucleus is introduced into the egg just like sperm are injected into an egg.

Research has indicated that the nucleus can be removed from an eight-cell embryo (about 3 days old), a nine-day embryo, and an adult, with similar results. It is even possible to transfer DNA into an enucleated egg to allow an older woman a chance to reproduce.

It was assumed that the nucleus is basically set back to time zero so that the developmental process can start over.

• Why do they use unfertilized egg cells to receive the nuclear transplant instead of cells from developing embryos, juveniles or adults?
• What do you think is in the ooplasm (cytoplasm of the egg) that allows the nuclear material to get set back to zero developmental time?

In part this is true, development does seemingly begin from time zero. However, research now indicates that the offspring produced by nuclear transfer is the cumulation of the age of the nuclear donor and the age of the offspring. The 3 year old Dolly was created from the nucleus of a 6 year old ewe. Dolly's cells are acting like they are 9 years old.

• What age-related changes occur in the nucleus and its component DNA that cannot be reversed?

The eggs that are used for cloning must be healthy.

• What criteria would you use to determine if an egg is healthy or not?

When the nucleus was transplanted into the egg, the researchers chose to use cells where the DNA was not being duplicated.

• In which stage of the cell cycle is DNA not being duplicated?
• How can you "force" a cell into this stage of the cell cycle?

Enough about nuclear transplants per se. Getting the raw material, DNA, and eggs, is relatively easy. Now you have to jump start the developmental process.