Module One 11.6 Why are individuals of the same species different from each other? What new methods

Genes, Chromosomes and DNA

Every nucleus of every cell contains strand like structures called chromosomes. Each chromosome is made up of DNA. Each chromosome is divided into sections called genes. You have a gene that determines every characteristic, e.g. eye colour, height, hair colour. Every chromosome is divided into a large number of genes.

Every gene has a number of different forms, these are called alleles. For example you have a gene that determines eye colour, but you could have an allele for brown eyes while someone else has an allele for blue eyes.

In body cells the chromosomes are found in pairs. We know this because each pair is identical in size and shape. They are called homologous pairs.

DNA - an introduction

Body Cell

 

 

This cell has four chromosomes, there are two pairs of chromosomes present. One pair is large and the other is small. It also has two sets of chromosomes, each set consists of one large and one small chromosome.

Gamete

This cell has two chromosomes, there is only one of each Chromosome. There are no homologous pairs. The cell has only one set of chromosomes.

Body cells divide to produce identical cells that are needed during growth or to replace cells that have been damaged. Body cells can also give rise to gametes that are needed for sexual reproduction.

Reproduction

There are two types of reproduction.

Sexual reproduction – it involves the fusion of two gametes (one male and one female) from two parents. Humans reproduce sexually using sperm and eggs (ova). Plants also reproduce sexually using pollen and ovules.

Asexual reproduction – it only involves one parent, there is no fusion of cells or gametes needed. Very few animals can reproduce asexually, greenfly is an example of one. They lay eggs without them having been fertilised.

Many plants reproduce asexually.
 

Cell Division

Biology in Motion - Mitosis and Meiosis

 

 

Comparing Asexual and Sexual Reproduction

Asexual Reproduction	Sexualreproduction
1 parent involved	2 parents involved
Quicker	Slower
Produces clones	Produces varied offspring

Producing clones

If a scientist produces a plant or animal with desired characteristics, it would be an advantage to produce large numbers of identical copies (clones) of this plant or animal.

Clones are usually made from plants that have been genetically engineered or selectively bred. There are two main ways in which organisms are cloned.

a. Cuttings

This has already been covered. It is a useful procedure but doesn't really produce a large number of clones quickly. Tissue culture is better because it can produce a very large number of identical plants.

b. Tissue culture

A section of a plant is removed. It could be a section of stem, root or leaf. The piece of plant is sterelised by placing it in bleach, this removes all bacteria. It is then wounded (its surface it scraped with a knife) to stimulate cell division. The piece of plant is then placed in a jar of jelly and left. The cells of the piece of plant carry out mitosis and a ball of thousands of cells is the result. The cells are taken and placed in a solution of hormones and food. Placed in the correct conditions each cell will grow into a new plant. One plant will produce an unlimited number of identical offspring.

A few things are important during this process.

· The plant must be kept in conditions to encourage its growth. The jelly contains nutrients, minerals and antibiotics and the jar must be provided with plenty of light.

¨ All plants produced by tissue culture are identical. Plants grown from seeds are varied, and so not all of them are desirable.

¨ Many seeds are attacked by diseases and don't germinate. Tissue culture is carried out in a sterile, disease free environment, so this is not a problem in plants produced by tissue culture.

¨ Tissue culture can produce an unlimited number of offspring from only one plant. Seeds are always limited in number.

¨ Some plants are selectively bred or genetically engineered so that they are sterile. If they are sterile they cannot produce seeds, so tissue culture is the only option.

Embryo Transplants in Animals

Instead of waiting for normal breeding cycles a farmer can obtain more offspring by using their best animals to produce offspring that are clones, who will also have all the desirable characteristics.

Example

Farmers choose a prize ram and a prize ewe and obtain gametes from them (sperm and egg).

After the egg cell has been fertilised it is called a zygote. The zygote is allowed to divide into a ball of cells, it is then called an embryo. The cells of the embryo are taken and split before they become specialised. The smaller groups of cells are then transplanted into surrogate mothers. Each surrogate mother will give birth to offspring that are clones of each other. These clones are not identical to either of the parents or to the surrogate mothers.

Two important points:

1. The surrogates are merely incubators. They are not in any way related to the offspring. They are individuals with no particular desirable characteristics.

2. The embryo cells must be taken before they become specialised. If a cell is specialised it has changed. E.g. The cell becomes a blood or bone cell, once this happens scientists cannot use the cells for embryo transplants. 
 
Fusion cell cloning

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. Here's how she was produced:

1. An egg cell was removed from the ovary of an adult female sheep, and its nucleus removed.
2. The nucleus from an udder cell of a donor sheep was inserted into the empty egg cell and given an electric shock.
3. The electric shock then causes the fused cell to start developing normally, using genetic information from the donated DNA.
4. Before the dividing cells became specialised, the embryo was implanted into the uterus of a foster mother sheep. The result was Dolly, genetically identical to the donor sheep.

Benefits of cloning.

· A large number of identical plants or animals can be produced.
- If one has superior characteristics, they all will.
- Very quick
- Economically viable

Disadvantages of cloning.

· If all plants or animals are the same then they are susceptible to the same diseases. If a disease comes along and kills one then they could all die.

· It greatly reduces the number of alleles in a population. This will reduce the possibilities of selective breeding and the long term survival of a species. This was explained earlier in the selective breeding section (the example of the cows and mastitis).

Transferring genes into plant and animal cells

Scientists are now taking genes from one plant or animal chromosome and inserting them into another plant or animal's chromosome. The new gene will then pass the characteristic on to the other organism. This produces what we now call genetically modified foods. The procedure is the same as the one used in bacteria, it follows three main steps:

1. A desirable gene is found in a plant or animal chromosome and "cut out."

2. The gene is then inserted into the chromosome of another plant or animal cell.

3. The cell is used to produce an organism with the added gene and therefore the added characteristic.  

Examples of genetic engineering possibilities

1. Genes for resistance to pest and diseases.

Farmers have to add a lot of pesticides and herbicides to their land to prevent pests and diseases from attacking crops. This is expensive and bad for the environment. If a gene was inserted into plants so that they were resistant to the pests and diseases then no chemicals will need to be added. This saves money and the environment.

2. Genes for Pesticide resistance.

Companies develop a plant that is resistant to their pesticides. They will sell the pesticide with the plant seed, and the farmers can spray the crop with the pesticide, harming other plants and animals (pests) without harming the crop that they are growing.

3. Anti freeze genes from fish have been inserted into tomatoes so that they are not as easily damaged by frost.

4. Genes have increased shelf life of tomatoes so that they can be kept for longer in supermarkets and homes.

5. Genes have been inserted into some plants to increase their nutritional value, e.g. oil seed rape has an increased oil content.

Other possibilities for plants are:

· Finding the gene for drought resistance. This would mean that you could insert it into cereals and they could grow with very little water. This would mean less famine in countries like Africa.

· Inserting a "salt resistant" gene into plants so that they could grow in soil with high concentration of salt. This would mean that we could use sea water to water crops. Again this would be useful in countries that do not have a lot of rain

· Increasing growth rates in pigs. Humans growth rates are controlled by a human growth gene. This gene was found in humans and has been cut out and transferred into pigs. The theory was that the pigs would now grow faster and have leaner meat. This will make more money. The result was faster growing pigs, but at a cost. The pigs grew so fast that their legs could not cope with the weight and they developed arthritis. For some reason they also developed ulcers.

· Changing pig cells to aid organ transplants. Human genes have been transferred into pigs so that pig cells are similar to human cells. The hearts of these pigs can then be transplanted into humans and there will be less chance that the heart will be rejected. If this is developed animal organs will be available on demand to be transplanted in humans. This will reduce the need for using humans as organ donors.

Benefits of genetic engineering In bacteria

· Huge quantities of safe human proteins or hormones can be made very cheaply.

· The insulin made by genetic engineering is safer as it is human insulin and not animal insulin that could sometimes cause reactions in patients.

· Without genetic engineering in bacteria there would be a shortage of many life saving proteins. Before insulin was made in this way many people died of diabetes.

In plants and animals

· In the case of crops this means that less pesticides, herbicides and fertilisers will be used, benefiting the environment.

· Crops will grow faster, in conditions that are less than ideal, such as in frost and drought.

· Crops could also have higher nutritional value.

· Animals can grow faster with less fat and more lean meat.

· Cows will make more milk, sheep more wool.

There are no limitations of what can be done to both plants and animals. Genetic engineering could solve the world's food problems and increase the quality of living for many people.

Disadvantages.

Genetic engineering poses ethical and social concerns including the following:

· It cannot tell us what its role and behaviour are in the organism it came from or what it might do if we place it into a completely different species. Salmon genetically engineered with a growth hormone gene not only grew too big too fast but also turned green. These are unpredictable side effects.

· Genetically engineered organisms that escape or are released from the laboratory could wreak environmental havoc.

· Genetically engineered organisms could be dangerous because they are alive, genetically engineered products that are unpredictable. Once released, it will be virtually impossible to recall genetically engineered organisms back to the laboratory.

· Mutations may occur. Researchers conducting experiments recently found that genetically altering plants to resist viruses can cause the viruses to mutate into new, more dangerous forms, or forms that can attack other plant species.

· Foreign genes from genetically engineered plants could get into other crops, as well as wild and weedy relatives. Disaster would follow if genetically engineered DNA for things such as insect and virus resistance, found their way into weeds, for instance.

· Resistant pests may eventually evolve, then stronger chemicals will be needed to get rid of the pests. And what will happen when the pesticide gene spreads to weeds and other unwanted plants?

 

· It could decrease biological diversity. Biological diversity is the numbers of different varieties of different species that exist. Genetically engineered plants and animals may have "superior" genes and overpower wild species. What will happen to wild species, for example, when scientists release into the environment carp, salmon, and trout that are twice as large, and eat twice as much food, as their wild counterparts?

 

· The food may taste differently or cause reactions and illnesses in humans.

Summary of procedures.

Controlling Inheritance

Learn About the GM Process

How Stuff Works - Cloning

Active Science - Selective Breeding and Genetic Engineering

BBSRC Life - The Science Behind Genetic Modification