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Asexual reproduction

Reproduction not involving the fusion of gametes.

embryonic stem cells

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

Nuclear membrane

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

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.

Chromatids

Copies of the original chromosomes involved in mitosis and meiosis.

Grafting

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.

Layering

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.

Membrane

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

Mitosis

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

Budding

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.

Zygote

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

Embryo

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

Clone

An organism that is genetically identical to its parent.

Sperm

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

DNA and Cloning

How a cell divides – mitosis

diagram of mitosis

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

  • 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 become 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 (Chlorophytum comosum)

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.

amoeba
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, taking 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.

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

Exam style questions

1. What is mitosis?
The process by which a cell divides to produce two identical daughter cells.
The process by which a cell divides to produce two non-identical daughter cells
The process by which two cells merge to produce one cell
The process by which a cell divides to produce 4 non-identical daughter cells
The process by which a cell divides to produce two identical daughter cells.
The process by which a cell divides to produce two non-identical daughter cells
(The correct answer is:
'The process by which a cell divides to produce two identical daughter cells.')
The process by which two cells merge to produce one cell
(The correct answer is:
'The process by which a cell divides to produce two identical daughter cells.')
The process by which a cell divides to produce 4 non-identical daughter cells
(The correct answer is:
'The process by which a cell divides to produce two identical daughter cells.')
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(The correct answer is:
'The process by which a cell divides to produce two identical daughter cells.')

2. Name some examples of plants that naturally produce clones.

Examples include strawberry plants and spider plants.