B1b - Reproduction and cloning

The genetic information passed from parent to offspring is contained in genes carried by chromosomes in the nucleus. Sexual reproduction produces offspring that resemble their parents, but are not identical to them. Asexual reproduction produces offspring - clones - which are genetically identical to their parents.

Plants can be cloned artificially using cuttings or tissue culture. Animals can be cloned using embryo transplants or fusion cell cloning. Genetic information from one species can be transferred to another species using genetic engineering.

Offspring resemble their parents because they contain genetic information passed on to them by their parents.

A gene is a section of DNA that carries the code for a particular protein. Different genes control the development of different characteristics of an organism. Many genes are needed to carry all the genetic information for a whole organism. Chromosomes, found in the cell nucleus, contain many genes.

The number of genes and chromosomes varies from species to species. For example, cells in human beings have 46 chromosomes that carry about 30,000 genes in each cell; and cells in fruit flies have eight chromosomes that carry about 13,600 genes.

Organisms have sex cells called gametes. In human beings, the male sex cells are called sperm and the female sex cells are called eggs or ova.

Sexual reproduction happens when a male gamete and a female gamete join. This fusion of gametes is called fertilisation. Sexual reproduction allows some of the genetic information from each parent to mix, producing offspring that resemble their parents, but are not identical to them. In this way, sexual reproduction leads to variety in the offspring. Animals and plants can reproduce using sexual reproduction.

In human beings, each gamete contains 23 chromosomes, half the number found in the other cells of the body. When the male and female gamete fuse, the new embryo contains the full 46 chromosomes – half from the father and half from the mother.

Asexual reproduction only needs one parent, there is no fusion of gametes and no mixing of genetic information. As a result, the offspring are genetically identical to the parent and to each other. They are clones.

Asexual reproduction in plants can take a number of forms. Many plants develop underground food storage organs that later develop into the following year’s plants. Potato plants and daffodil plants are examples that do this.

Some plants produce side branches with plantlets on them. Busy Lizzy does this. Other plants, such as strawberries, produce runners with plantlets on them.

Asexual reproduction in animals is less common than sexual reproduction. It happens in sea anemones and starfish, for example.

Cloning of plants has many important commercial implications. It allows a successful variety of a plant to be produced commercially and cheaply on a massive scale in a short space of time.

The simplest way to clone a plant involves taking a cutting. A branch from the parent plant is cut off, its lower leaves removed and the stem planted in damp compost. Plant hormones are often used to encourage new roots to develop. The cutting is usually covered in a clear plastic bag at this stage to keep it moist and warm. After a few weeks, new roots develop and a new plant is produced. The method is easy enough for most gardeners to do successfully.

Another way of cloning plants is by tissue culture, which works not with cuttings but with tiny pieces from the parent plant. Sterile agar jelly with plant hormones and lots of nutrients is needed. This makes tissue culture more expensive and difficult to do than taking cuttings.

1. Small amounts of parent tissue or a number of cells are taken
2. The plant material is transferred to plates containing sterile nutrient agar jelly
3. Plant hormones are added to stimulate the cells to divide
4. Cells grow rapidly into small masses of tissue
5. More growth hormones are added to stimulate the growth of roots and stems
6. The tiny plantlets are transferred into potting trays where they develop into plants

The cloning of animals has many important commercial implications. It allows an individual animal with desirable features. But the process takes much longer than it does with plants. A developing embryo is removed from a pregnant animal at an early stage, before the embryo’s cells have had time to become specialised. The cells are separated, grown for a while in a laboratory and then transplanted into host mothers.  When the offspring are born, they are identical to each other and genetically related to the original pregnant animal. They are not related to their host mothers because they contain different genetic information.

Fusion cell cloning involves replacing the nucleus of an unfertilised egg with the nucleus from a different cell. The replacement nucleus can come from an embryo, but if it comes from an adult cell, it is called adult cell cloning. 'Dolly the sheep' was the first mammal to be cloned using adult cell cloning. She was born in the UK in 1996 and died in 2003. She was produced with the following steps:

1. Egg cell removed from ovary of an adult female sheep, nucleus removed.
2. Nucleus from udder cell of donor sheep inserted into the empty egg cell.
3. Fused cell began to develop, using genetic information from the donated DNA.
4. Before cells became specialised, embryo was implanted in uterus of foster mother sheep. The result was Dolly, genetically identical to the donor sheep.

Genetic engineering is also called genetic modification or GM. It is not the same as cloning. Although cloning techniques are used in genetic engineering, the two things should not be confused.

Certain enzymes can cut pieces of DNA from one organism, and join them into a gap in the DNA of another organism. This means that the new organism with the inserted genes has the genetic information for one or more new characteristics. For example, the organism might produce a useful substance, or be able to carry out a new function. We say that the organism has been genetically modified.

There are strong arguments for and against cloning and genetic engineering. It is possible to produce genetically modified animals and plants. Sheep that produce human proteins for treating the symptoms of cystic fibrosis have been produced, and even tobacco plants that glow in the dark when they need watering.  Some people are excited by the almost limitless possibilities of cloning and genetic engineering, while some people believe the process is unethical and should be banned. Others are concerned about what might happen in the future.

1. Which statement about sexual reproduction is correct?

• It needs just one parent
• The offspring are not identical to the parents

2. Which statement about asexual reproduction is correct?

• The offspring are not identical to the parents
• It needs just one parent

 3. What is one advantage of taking cuttings from plants?

• New, genetically identical plants can be grown quickly and cheaply.
• New plants can be grown quickly and cheaplu
• All the offspring will be susceptible to the same diseases

4. In general, where can the donor nucleus come from in fusion cell cloning?

• Only from an adult cell
• From the red blood cells of the donor animal
• An embryo cell or an adult cell