Title: Clones in animals

Homework: learning package 11 due in today

Ewe were always on

my mind….

Learning Outcomes

(e) describe how artificial clones of animals can be produced;

(f) discuss the advantages and disadvantages of cloning animals (HSW4, 6a, 6b, 7c).

Stem Cells• All cells start of with the same

DNA, so why do they end up looking so different?

• Some genes “switch on” while others “switch off”, leading to a process called differentiation.

• Cells that have differentiated cells lose their ability to divide and produce more cells of different kinds.

Stem Cells• What happens to cells that

remain undifferentiated?

• These undifferentiated cells are called stem cells – they have the ability to become any other cell in the body (or more stem cells).

• Why is this useful?

Obtaining stem cells - embryos• Most useful at a few days

old – ball of undifferentiated cells.

• Cells at this stage are totipotent – divide to form a whole new organism!

• 50-100 cell stage is described as pluripotent – ability to become any cell type, but not a new organism because some cell differentiation has already started.

Embryo formation

• Embryo is formed by transferring DNA from embryonic stem cells to an egg whose nucleus has been removed (enucleated).

• Reconstructed egg containing DNA from a donor cell is treated with an electric current to stimulate cell division and blastocyst formation.

Cloning by nuclear transfer

Cloning by splitting embryos

What are the differences between cloning by splitting of embryos and cloning by nuclear transfer?

Splitting of embryos• The nucleus comes from an egg

fertilised in vitro and allowed to divide to form an embryo

• The embryos produced are clones of the original zygote

• The zygote is a product of a fertilised egg. Since this is formed from fusion of sperm and egg cells which are themselves a product of meiosis, it is impossible to know exactly what characteristics the cloned organism will possess

• All cellular components are derived from the original zygote. The mitochondrial DNA component will be identical in each clone

Nuclear transfer– The nucleus comes from an adult,

differentiated cell taken from the animal to be cloned

– The embryos produced are clones of the donor adult organism

– The adult cell is a product of an adult with known characteristics. It is therefore known what characteristics the clones will possess

– The cellular components are derived from the egg cell used therefore the mitochondrial DNA component will be different from that of the original adult cell

Reproductive vs non-reproductive

• Reproductive cloning– Cloning to produce a

whole organism– Examples

• Embryo transplantation• Dolly the sheep

• Non-reproductive cloning– Using cloning to produce

cells– Examples

• Stem cell research• Production of cells,

tissues or organs

Dolly the Sheep

• Dolly the sheep is believed to have suffered from a serious health problem which developed at a relatively early age

Non-reproductive cloning in animals

• Non-reproductive cloning involves the production of genetically identical cells

– 1951 – HeLa cells• Cancerous cells which divide repeatedly in culture solution• Used in medical research

– Stem cells• Totipotent or pluripotent• Undifferentiated cells that are able to differentiate into specialised

cells• Embryonic stem cells – ethical arguments

Possibilities of non-reproductive cloning

• Potential future uses include– Regeneration of heart tissue

following a heart attack– Repair of nervous tissue– Repairing the spinal cord

• Stem cells taken from the patient to produce the tissues mean that tissue rejection by the immune system is less likely

Advantages and disadvantages of cloning animals

Advantages Disadvantages

High-value animals, for example cows giving high milk yield, can be cloned in

large numbers

High-value animals are not necessarily produced with animal welfare in mind.

Some strains of meat-producing chickens have been developed that are unable to

walk

Rare animals can be cloned to preserve the species

As with plants – excessive genetic uniformity in a species makes in unlikely

to be able to cope with, or adapt to, changes in the environment

Genetically modified animals – for example sheep that produce

pharmaceutical chemicals in their milk – can be quickly reproduced

It is still unclear whether animals cloned using the nuclear material of adult cells

will remain healthy in the long term. Dolly was put down at 6 years old due to lung cancer caused by a virus, although post-mortem showed nothing unusual

Artificial Cloning in animals

• Advantages– High value animals– Rare animals can be

cloned– Quickly reproduced

• Disadvantages– Animal welfare– Genetic uniformity, loss

of genetic variation– Uncertainties of health

of cloned animals

Moral and ethical Arguments Is it right to clone an aging pet? Is it right to clone an animal which leads to a

cure for human diseases?

Give reasons why repairing a damaged heart using cloned cells could potentially be less dangerous than

receiving a heart transplant

• Heart transplant surgery carries with it risk of infection associated with any major surgery.

• There are also risks associated with use of general anaesthetic.

• The transplanted heart is foreign tissue – as such it may be rejected by the host’s immune system.

• Cloned cells are derived from the host and it may be possible to get them to the right location using relatively minor surgical methods