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Genetics Index

Chromosomes
Meiosis
Meiosis: Crossing over
Mitosis and Meiosis
Introduction to Mendelian Genetics
Test Cross
Codominance
Multiple Alleles
Pedigree Charts
Twin Studies
The dihybrid cross
Dihybrid Cross : Test Cross
Autosomal Linkage
The Genetic Diagram for Linked Genes
Calculating the cross over value using a test cross
Sex determination and sex linkage
Sex linkage
Genetic diagram for sex linked genes
Blood Clotting and Haemophilia
The Retina and Daltonism
Genetic Modification
Cloning Plants

Topic Chapters Index

 

Cloning embryos

Much of the success in cloning animals has been achieved using early embryo cells. Embryo cells up to the 8-cell stage retain their totipotence. They can be split apart and each one will grow into a complete individual.

Sex cells are selected from the male and female. In vitro fertilisation (IVF) is carried out. Zygotes are allowed to develop to the 8-cell stage. The shell or zona pellucida that surrounds the embryo is broken open and the ball of cells is split up. Each embryo cell is reinserted into a fresh zona pellucida and the cloned cells are implanted into surrogate mothers. This produces eight clones of the original zygote. In theory the technique can be repeated indefinitely, letting the cloned embryos develop to the 8-cell stage and breaking them apart again.

 

Zygotes are hybrids

The disadvantage of this method is that to clone you need a zygote that produces embryo cells. The zygote is a hybrid of two animals. Because of the variation generated by meiosis and fertilisation you are not sure of what you will obtain until the embryos grow up. Thus, if you want to conserve the characteristics of an animal you should clone from that adult. This means cloning from fully differentiated adult cells.

 

GENETICS

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Cloning Animals

Animals are not as easy to clone as plants. Whereas simpler animal groups such as Cnidaria (jellyfish and sea anemones) do naturally use asexual reproduction, it is rare to find it in higher groups. The power to regenerate tissues from amputated body parts is also restricted. Animal cells seem to lose their totipotence once they have differentiated.

 

Cloning from adult cells

In 1885, August Weismann put forward the theory that development of cells was unidirectional. As a cell differentiates it loses some of its genetic information, which stops it from returning to the undifferentiated state. Wilhelm Roux experimented with frog embryos, which are large and develop in pond water. He let them grow to the 2-cell stage and destroyed one of the two cells with a hot needle. The remaining cells developed into half frog embryos.

This seemed to confirm Weismann's theory.

Adolf Dreisch tried the same experiment with sea urchin embryos. These are smaller than frogs so he could not destroy one of the cells without damaging the other. Instead he shook the 2-cell embryos apart. To his surprise they developed into complete but smaller sea urchins. He found similar results when he used the 4-cell stage embryo. Unfortunately he could not shake frog cells apart.

In 1902 Hans Spemann developed a technique for separating salamander embryo cells. He tied a noose in a hair (taken from the head of his baby son) and tightened it around 2-cell salamander embryos until they split. Each cell developed into a normal salamander.

Spemann used his technique to go further. He looped the hair round the single cell of a zygote and tightened until there were two halves joined by a thin bridge of cytoplasm. The half with the nucleus divided. The half without did not. He let the dividing part develop to the 16-cell stage. He then loosened the noose to allow a nucleus from the dividing cells into the part that had been left without and then cut it off. The transferred nucleus developed with the cytoplasm into a complete salamander.

Weismann theory seemed to be incorrect.

No further progress was made on nuclear transplants until 1952. Robert Briggs and Thomas King invented the technique of sucking nuclei out of cells using glass needles. Their success rate using nuclei from early embryo cells was high (about 44%) but this success rate rapidly diminished with older embryos. Form young tadpoles it was no more than 2%. Adult cells seemed out of reach.

So maybe Weismann was not so wrong after all.

During the 1970s Karl Illmensee claimed he had clone mice using nuclei transplanted from early mice embryos but no one else could repeat his results. It was declared that it was impossible to clone mammals. However, in 1986 that scientists succeed in cloning 8-cell embryos from sheep (Steen Willadsen) and cattle (Randall Prather), Willadsen went on to clone from 64-cell and 128-cell cow embryos. The inventions that helped them succeed was the use of a small electric shock to fuse cells together and the use of unfertilised egg cells which had had their nuclei removed. The embryo cell and the enucleated egg cell were fused using the electric shock treatment.

It seems that nuclei in differentiated cells do not lose their genetic material but it is covered (masked) by proteins during development. These proteins prevent some genes from expressing themselves.

Ultimately fully differentiated adult cells were cloned in 1996 by Ian Willmut and Keith Campbell working on sheep. Their success was the result of the idea that the adult cell needed to be in the same phase of the cell cycle as the egg cell. To do this they starved the adult cells which put them into a stage called G0. The hypothesis is that in G0 the proteins covering the DNA on the chromosomes of a cell are reset. They removed the nucleus from the egg cell and fused the starved adult cell with the egg cell using the electric shock. The embryos were allowed to develop for 1 week, then they were implanted into the surrogate mothers.

Cloning from adult cells means that animals with known characteristics can be cloned. An animal with a valuable genetic characteristic can be copied again and again. The clones will show the same characteristics.

 

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