DNA is the complex chemical that carries genetic information. DNA is contained in chromosomes, which are found in the nucleus of most cells. The gene is the unit of inheritance and different forms of the same gene are called alleles.

Cystic fibrosis is an inherited disorder caused by a faulty allele.

The Human Genome Project has worked out the human DNA sequence, and its data are useful for forensic science and medical research.

What is DNA?

You will remember from your Key Stage 3 studies that the cell's nucleus controls the activities of the cell. Look there if you need to remind yourself of animal and plant cells.


Chromosomes are X-shaped objects found in the nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] of most cells. They consist of long strands of a substance called deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene.

The gene is the unit of inheritance, and each chromosome may have several thousand genes. We inherit particular chromosomes through the egg of our mother and sperm of our father. The genes on those chromosomes carry the code that determines our physical characteristics, which are a combination of those of our two parents.

chromosome, showing gene as section of DNA

Nucleus, chromosome and gene

The bases in the DNA molecule carry the different codes needed for different amino acids. The code for a particular amino acid is made from three bases in a particular order. The animation shows the structure of DNA in more detail, but note that you do not need to know this for your examination.

Alleles and genetic disorders


Different forms of the same genegene: The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. are called alleles - pronounced 'al-eels'. You inherit one allele for each gene from your father and one allele for each gene from your mother. For example, the gene for eye colour has an alleles for blue eye colour and an alleles for brown eye colour. Your eye colour will depend on the combination of alleles you have inherited from your parents.

Genetic disorders

Diseases can be caused by a number of things, including:

  • infections eg influenza
  • poor diet eg scurvy
  • environmental factors eg asbestosis
  • spontaneous degeneration of tissues eg multiple sclerosis

Some diseases are inherited from our parents through our genes: they are called genetic disorders. They occur because of faulty alleles. Cystic fibrosis is an example of a genetic disorder.

Cystic fibrosis

People with cystic fibrosis have inherited two faulty alleles, one from their father and one from their mother. They produce unusually thick and sticky mucusmucus: Slimy white protein, which lines the respiratory tract and alimentary canal. in their lungs and airways. Their lungs become congested with mucus, and they are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics [antibiotics: Substances that kill bacteria. ] are used to fight infection. The disorder also affects the gut and pancreas, so that food is not digested efficiently.

The Human Genome Project

DNA shown as a double twisted string with interlinking parts

Base pairs on a DNA molecule

The genetic information in an organism is called its genome. The Human Genome Project, or HGP for short, was started at the end of the last century. It was very ambitious and had several aims, including:

  • to work out the order or sequence of all the three billion base pairsbase pairs: The pairs of nitrogenous bases that connect the complementary strands of DNA. in the human genome
  • to identify all the genesgenes: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production.
  • to develop faster methods for sequencing DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ]

The sequencing project was finished in 2001, and work continues to identify all the genes in the human genome. The HGP used the DNA of several people to get a sort of average sequence, but each person has a unique sequence (unless they have an identical twin).

Forensic science


You may wish to view this BBC News item from 2006 about how scientists are unlocking the DNA code of extinct mammoths.

Information about a person's DNA can be useful for forensic science. Genetic fingerprinting was invented in 1985 by Sir Alec Jeffreys at the University of Leicester. It uses some of the small differences between the DNA from different people to make a picture rather like a barcode. If enough parts of the DNA are tested, it is very unlikely that two identical DNA fingerprints would belong to two different people. This makes the method very useful for matching samples found at the scene of a crime to people suspected of committing the crime.

Genetic treatment of disease

It is hoped that information from the Human Genome Project will allow scientists to develop new ways of treating or diagnosing illnesses, especially genetic disorders and cancer.

Genetic disorders

A person with cystic fibrosis has inherited two faulty alleles for a certain gene on one of their chromosomes, chromosome 7. It is hoped that it may one day be possible to repair the faulty alleles using gene therapy, perhaps by putting the normal allele into the cells of the lungs. This would greatly improve the lives of people with cystic fibrosis, who often need lung transplants as their illness progresses.

Breast cancer

Members of some families are particularly at risk of developing certain types of breast cancer, because they carry faulty alleles. These alleles have been identified, and it is now possible to test people to see if they have an increased risk of developing breast cancer. This allows them to make decisions, if they wish, about whether to have surgery to remove breast tissue before any cancer develops in their breasts.

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