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Cystic fibrosis

A genetic disease caused by a defective, recessive gene. It is characterised by the production of thick, sticky mucous in the lungs and pancreas which cause respiratory and digestive problems.

DNA-directed RNA polymerase

Enzyme involved in the transcription of DNA and the production of mRNA. The enzyme is involved in the breakdown of the hydrogen bonds between the two strands of DNA in the formation of mRNA, and the build up of the mRNA strand from 5-3 end.

gene promoter

The genetic material directly upstream of a gene (towards the 5’ region of the sense strand). This region of DNA initiates transcription.

Gene therapy

A new, experimental method of fighting disease by replacing a defective gene with a healthy gene

Active site

The specially shaped site on an enzyme where the substrates of the reaction bind. It is formed by the folding of the amino acid chains which make up the protein.


Any substance that causes cancer.


A long chain of hundreds or even thousands of amino acids joined by peptide bonds.


A change in the arrangement or amount of genetic material in a cell.


A mass of abnormal cells which keep multiplying in an uncontrolled way.

What can go wrong?

Mutations can occur randomly during DNA replication. The chance of a mutation occurring is increased by exposure to certain chemicals and radiations- these are known as mutagens. If a mutation results in cancer, the mutagens responsible are known as carcinogens.

Examples of mutations include addition, deletion and substitution of nucleotide bases. The majority of mutations are not harmful or fatal. Most mutations are neutral, a small number are harmful and a small number give a new characteristic that gives the organism a selective advantage.

If the mutation occurs in a gene, it can alter the amino acid sequence coded. Modifying the amino acid sequence of a polypeptide chain can have various consequences including modifying protein structure. Hydrogen, ionic and disulphide bonds hold the tertiary and quaternary structures of proteins together. Amino acid changes can disrupt these bonds- or even create new bonds. This can transform the structure of a protein. Structural modifications of a protein can change the shape of its active site, change its function, or stop it functioning.

If the mutation occurs within a non-coding gene promoter region, it can result in increased or decreased levels of protein expression. These mutations can influence the frequency of transcription by altering the affinity of RNA polymerase for the sequence.

Individuals can also be missing a chromosome or have an extra one. Down’s syndrome is caused in humans by the presence of an extra copy of chromosome 21.

Down syndrome human karyotype
Down syndrome human karyotype 47, XY, +21

Inherited diseases

Cystic Fibrosis, one of the most common genetic diseases, develops as a result of a faulty recessive gene (c) which was identified in 1989. To develop the disease a child must inherit the gene from both parents. If, as in the diagram, both parents are carriers, there is a one-in-four chance that a child will have the disease, a two-in-four chance of a child being a carrier and a one-in-four chance of the child being unaffected.

Unlike cystic fibrosis, Huntington’s Disease is dominant, that is, it can be inherited if just one parent has this gene.

Huntington's family tree and cross diagram (affected people in blue)
Huntington's family tree (affected people in blue)

The diagram shows what can happen if one parent has the HD gene (and therefore has Huntington’s disease) and the other parent does not. In this diagram the presence of the disease is shown by a capital H. In Huntington’s the symptoms do not show until the sufferer is 35-40 years of age, by which time the faulty gene may have been passed on to children and even grandchildren.

Continuing genetic research has meant development of tests for some genetic diseases both in utero and after birth and treatments such as gene therapy.

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