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Stem cells

Cells which can divide repeatedly without becoming differentiated and have the capacity to develop into a diverse range of specialised cell types.


A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism.


The name for a group of cells that are developing into a fetus. In humans this is from implantation to the 8th week of development

embryonic stem cells

Embryonic stem cells (ESCs) are undifferentiated, pluripotent cells derived from the inner mass of an early embryo at the blastocyst stage

Tissue culture

The propagation of new plants using cells taken from a parent plant. The cells are grown in a suitable medium with hormones and the new plants are clones of the parent.

Nuclear membrane

The thin, flexible structure enclosing the contents of the nucleus in a cell.

Asexual reproduction

Reproduction not involving the fusion of gametes.


A fertilised cell produced as the result of the combination of an ovum and a sperm.


The male sex cell or gamete. The full name is spermatazoan, abbreviated to sperm cell or sperm.


Division of a cell nucleus which results in each daughter cell having the same number of chromosomes as the parent cell.


An asexual method of plant propagation where the parent plant is bent over so that part of it touches the soil. This part eventually produces roots and shoots and, when big enough, can be detached from the parent plant.


An asexual process of joining the tissues of two plants together to produce one plant with desirable characteristics. It is a method often used in the commercial production of many shrubs and trees.


An organism that is genetically identical to its parent.


Copies of the original chromosomes involved in mitosis and meiosis.


A form of asexual reproduction in which a new individual is produced as an outgrowth (bud) of the parent and is later released as a clone of the parent.

DNA & Cloning

How a cell divides - mitosis

Mitosis is the process by which a cell divides to produce two identical daughter cells

diagram of mitosis
  • Firstly, every chromosome makes a copy of itself and each new pair is then called a pair of sister chromatids
  • Then the membrane of the nucleus dissolves so that the chromatids spill out into the rest of the cell.
  • They then line up across the middle of the cell and the sister chromatids are pulled apart to opposite ends of the cell.
  • Now they are called chromosomes again and each set becomes enclosed in a new nuclear membrane.
  • Eventually the mother cell splits into two giving rise to two daughter cells each with the same set of chromosomes as the original cell.

When an egg has been fertilised, and thus becomes a zygote, it undergoes mitosis, dividing to become two cells, then four, then eight etc. In a human embryo, up to the eight-cell stage all the cells are identical and have the potential to become any kind of cell required by the body. These eight cells are known as embryonic stem cells.

Cloning in nature

spider plant
A spider plant

Mitosis occurs in the natural process of growth, but many plants and some animals also use mitosis as a method of reproduction known as asexual reproduction.

An amoeba

The strawberry plant sends a runner out from the main stem and, where this touches the ground, new plants develop from buds on the runner. Spider plants regularly send out drooping branches, which, if they touch the ground, will root and produce new plants. The single-celled amoeba is one of a small number of animals that reproduce asexually. The new plants or animals produced are exact copies of the parents and are known as clones.

photo of identical twins
photo of identical twins

Since identical twins have both developed from the same fertilised egg, they can be regarded as naturally occurring clones.

Artificial cloning

Cloning is a natural process, but scientists have also developed methods of producing clones in an artificial way. Many plants are produced by layering, cuttings and by grafting and budding. Plantlets can also be grown from one original plant by tissue culture. Just a few cells of the plant are treated with hormones in a special environment and many thousands of plants can be produced quickly, disease free and all year round.

Cloned plants Cloned plants
Cloned plants
images courtesy of

Prize bull
Prize bull

Cloning of animals has also been done. It is quite common, for example, to use the sperm from a prize bull to artificially inseminate selected cows, which have been given hormones to make them produce lots of eggs. The fertilised eggs develop into embryos each of which can be divided to create several identical embryos. These cloned embryos are then implanted into other cows, or they can be frozen and used later. Cloning was also used in the case of Dolly the sheep, which was the first mammalian clone produced from one adult cell.

Click here for more information on Dolly the Sheep

Embryo Cloning
Embryo cloning (created for ABPI by Webucators)

Question 2

Quiz Print
Put the following phrases into the order they occur in the mitosis process:

To reorder an item; select it, then use the up or down arrow to the right of the list to move it.

The first answer is already set for you at the top of the list.

What kind of cell has the potential to become any other kind of cell?
Name two plants which naturally produce clones: