DNA

Variation can be due to inheritance, and also to environmental factors such as climate and diet. Different forms of a certain gene are called alleles. Alleles can be recessive (showing up only when there are two of them) or dominant (showing up always). Cystic fibrosis is an inherited disorder caused when an individual inherits two faulty alleles of a certain gene.

A transgenic organism is one that is created by genetic modification (GM), also called genetic engineering. GM organisms can produce useful substances for us.

In vitro fertilisation (IVF) is a process that helps women who have difficulty in getting pregnant normally. It involves fertilising a woman's egg outside her body, then implanting it back. Some people are worried that IVF will lead to “designer babies”.

Environmental variation
Recessive and dominant alleles
Transgenic animals
Designer babies
Environmental variation
Our genetic make-up, the sum total of our genes, is called a genotype, but the characteristics which show up in our appearance are called a phenotype.



Genetic variation gives rise to differences between individuals that are inherited. For example, our eye colour is inherited from our parents. But our phenotype is also affected by environmental variation such as:

climate
diet
physical accidents
culture
lifestyle
Many kinds of variation are influenced by both environmental and genetic factors, because although our genes decide what characteristics we inherit, our environment affects how these inherited characteristics develop. For example:

a person might inherit a tendency to be tall, but a poor diet during childhood will cause poor growth
plants may have the potential for strong growth, but if they do not receive sufficient mineral resources from the soil, they may hardly grow at all
Identical twins are a good example of the interaction between inheritance and environment, as such twins are genetically the same. Any differences you may see between them – for example in personality, tastes and particular aptitudes – are due to differences in their experience or environment.
Recessive and dominant alleles
You will recall that genes have different forms called alleles. An allele can be recessive or dominant.

A recessive allele only shows if the individual has two copies of the recessive allele. For example, the allele for blue eyes is recessive. You need two copies of the allele to have blue eyes.

A dominant allele always shows, even if the individual only has one copy of the allele. For example, the allele for brown eyes is dominant. You only need one copy of the allele to have brown eyes (and two copies will still give you brown eyes).


Cystic fibrosis
Cystic fibrosis (CF) is caused by a recessive allele. You need to inherit two copies of the faulty allele to have CF. If you just have one copy of the faulty allele, you are a carrier but have no symptoms. If two carriers have a child between them, there is a one in four chance of that child having the disorder. The inheritance diagram shows why this happens.

Transgenic animals
Genetic engineering, also called genetic modification (or GM), involves putting a gene from one organism into the cells of another organism. An organism treated by GM is called a transgenic organism. Cloning techniques are used in genetic engineering, but cloning and genetic engineering should not be confused. The table shows some of the differences.

Cloning/ Genetic engineering
Produces exact copies /Produces a unique set of genes.
Genes copied within the same species /Genes can be swapped across species

How it works
Certain enzymes can cut pieces of DNA containing a particular gene 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.

The animation shows how the method can be used to produce bacteria that produce insulin. This is a human hormone and valuable to people with diabetes. Bacteria reproduce quickly, so a lot of insulin can be made quickly.


Problems?
There are strong arguments for and against genetic engineering. Some people are excited by the almost limitless possibilities, while some people believe the process is unethical and should be banned. Others are concerned about what might happen in the future.
Designer babies

IVF
If a couple are having difficulty conceiving a child, because the quantity or quality of the man's sperm is poor, in vitro fertilisation - or IVF - can be used. This is where the egg is fertilised outside the woman's body and then implanted back into her uterus.

Designer babies
Some people worry about the ethical implications of IVF. They are concerned that couples may want “designer babies” with “desirable” qualities, so may only want certain fertilised eggs. For example, they may want a girl if they have lots of boys in the family, or they may wish to avoid producing a baby with an inherited defect.

DNA replication
In order to grow and reproduce, living cells need to duplicate their chromosomes, as happens during cell divisions such as mitosis. This is possible because DNA can replicate itself.

It is complementary base pairing that enables DNA to make exact copies of itself - because the bases always pair off in the same way:

Adenine always pairs with Thymine
Guanine always pairs with Cytosine

Here's how DNA replication works:

Enzymes move up the DNA spiral, breaking the hydrogen bonds between the base pairs and 'unzipping' the spiral


Each strand or backbone now has a sequence of single bases which act as a template. Remember: each base can only pair up with one other base.


'Free' nucleotides now move in and begin to line themselves up on each of the backbones, forming a new partner for each of the two original backbones. At the end of this process...


there will be two identical copies of the original DNA spiral.

Genes and proteins
Inside the nucleus of a cell long strings of DNA are coiled up and spooled onto chromosomes, and on each chromosome there are several thousand genes - shorter sections of DNA which act as a code for a particular characteristic. Since we inherit particular chromosomes through the egg and sperm, we also inherit the particular characteristics coded for by the genes on those chromosomes.


Each gene acts as a code or set of instructions for making a particular protein. These proteins control the cell's internal chemistry - telling the cell what to do, giving the organism particular characteristics and determining the way its body functions.

To enable genes to code for proteins, the bases A, T, G and C get together not in pairs but in groups of three or triplets. It works like this:

Each protein is made up of large numbers of amino acid molecules; and each triplet of bases codes for one particular amino acid.


So amino acids are made in the number and order dictated by the number and order of base triplets. Finally...


The amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein results.
The world is an astonishingly diverse place, inhabited by millions of different plant and animal species, with probably millions more that have not yet been discovered.

The term biodiversity refers not only to the sheer number of different species, but to all the genetic variations within and between species - and all the differences between the many, many habitats and ecosystems that make up the earth's biosphere.

Why is it important to maintain the huge variety of life? Some answers are that :

we have a moral responsibility to look after the planet and its resources, rather than simply use them up


biodiversity provides us with many direct benefits - eg clean air and water, food, medicines, fertile soil, and pollination for our crops


biodiversity represents the earth's total gene pool - a source of future variation which is vital for all species


reduction in biodiversity in ecosystems may reduce climatic stability, for example by upsetting the balance between intake and output of carbon dioxide



Impact of humans

By far the biggest threat to the variety of life is posed by human exploitation of the environment. Our impact on the global environment is greater than that of any other species because of:

Our technologies: we use tools and techniques which can change the shape of the earth in a short space of time (eg clearing forests or changing the course of rivers)

Our population is increasing at a phenomenal rate. There are 6 billion of us now and by 2050 we are expected to number around 9 billion. Our sheer numbers will mean that even small activities multiplied 9 billion times will have a huge impact on the environment

Consumption and waste: we consume vast amounts of natural resources (eg water, fossil fuels) and produce vast amounts of waste (eg greenhouse gases). Both pose a threat to other forms of life - as well as to ourselves!

Human population growth
Like all living things, humans exploit their surroundings for resources. Before the beginning of agriculture about 10,000 years ago, small groups of humans wandered across large areas hunting and gathering just enough to keep alive. Population numbers were kept low by the scarcity of food.

The invention of agriculture began a population explosion which has accelerated enormously in the last 500 years. Unlike other species, humans can adapt to and survive in almost all habitats and climates, from the cold Arctic regions to dry, hot deserts. Currently around 6 billion and rising fast, human population growth now poses a threat to the global environment.


Human population growth over the last 10,000 years


In the United States and many European countries, changing work patterns, universal education and health care, and family planning have all helped to halt population growth. But in developing countries, like those of India and Africa, population numbers continue to rise fast.




Growth-limiting factors

In every ecosystem and every species, population sizes have limits beyond which they cannot safely expand. They are constrained by growth-limiting factors which not even humans can easily control. These are:

the availability of food and water
invasion of parasites, pathogens or disease
over-crowding (increasing competition for food, water and space)
severe or sudden climatic changes
pollution of air, soil and water

Waste pollution: a mountain of household and industrial rubbish.

If we do not take steps to control population growth ourselves, it is likely that one or more of these growth-limiting factors will eventually kick in to forcibly reduce our numbers.
Urbanisation and industrialisation



Urbanisation means the growth of cities. Around 3 billion of us - half the world's population - now live in cities, and virtually all future population growth will take place in cities. Moreover our cities are getting bigger and bigger: by 2015 it is predicted that the world's 6 largest cities will each have more than 20 million inhabitants.

Some of the effects of urbanisation are:

greatly increased pollution as a result of urban traffic, energy consumption and waste production


land taken out of food production, as more and more farmland is built on


loss of natural habitats, as cities and roads proliferate


fragmentation of rural communities and cultures, as more and more people leave to live in towns



Industrialisation

Closely linked with population growth and the rise of cities has been a worldwide rapid development of industries such as manufacturing, mining, power generation and transport. These have a major impact on the environment, because:

they use up huge amounts of water and energy (usually non-renewable fossil-fuel energy), and


they are a major agent of climate change, because they release greenhouse gases into the atmosphere and speed up global warming

Impact of farming: 1
Farming is what makes possible the production of food surpluses and settled living. It also brings about big changes in the relationships between living things and in their habitats. Farming - especially modern, intensive farming - can damage the environment in many different ways.




Effect of fertilisers

Fertilisers containing plant nutrients are sprayed onto fields to make plants grow faster and boost crop yields. When it rains the nutrients may get washed down from the fields and into rivers and lakes (this is called run-off). The result is eutrophication - which can kill almost everything living in the aquatic environment. It works like this:

Impact of farming: 2
Effect of pesticides

Pesticides are chemicals used to kill insects, weeds and micro-organisms that might damage crops. However, pesticides damage other organisms apart from those they are intended to kill - for example, depriving insect-eating birds of food.

Pesticides can also enter local food chains. Organisms that ingest them cannot break them down, so they persist in their bodies. (Substances that cannot be broken down are called persistent substances: the pesticide DDT is an example.) The pesticides may then build up at ever-higher levels until they become toxic to much larger organisms. Here's how it works:







Other impacts of farming

Agriculture can impact on the environment in many other ways. For example:

farming takes up land, reducing habitats and wildlife


monocultures (large amounts of one type of food) provide lots of food for pests as well as humans


irrigation (watering of crops) may take too much water from rivers, depriving downstream habitats of water


clearing land for farming may result in soil erosion, damaging ecosystems and leaving land barren


Intensive livestock farming produces huge amount of faeces, which may pollute waterways