Dna Definition Biology Gcse Coursework

Structure of DNA

Two scientists, James Watson and Francis Crick, worked out the structure of DNA. By using data from other scientists they were able to build a model of DNA.

The data they used showed that bases occurred in pairs. Further x-ray data showed that there were two chains wound into a double helix.

A double helix

Base pairs on a DNA molecule

Each strand of DNA is made of chemicals called bases [nitrogenous bases: the chemical 'letters' that make up the language of DNA: Adenine (A), Cytosine (C), Guanine (G) and Thymine (T) ]. Do not confuse these with the bases you meet when you study acids and alkalis [alkali: A base which is soluble in water. ] in chemistry. There are four different types of bases, shown as A, T, C and G in the diagram.

In DNA, two strands coil together to form a double helix [double helix: The shape of the DNA molecule with two strands twisted together in a spiral. ]. There are chemical cross-links between the two strands, formed by pairs of bases.

Genes and proteins

Each gene in a molecule of DNA contains:

  • A different sequence of bases
  • Codes for a particular protein

Proteins are made in the cytoplasm of a cell, not in the nucleus. Genes cannot leave the nucleus, so a copy of the gene is needed. This copy is able to leave the nucleus to go into the cytoplasm so that proteins can be made by the cell.

Now try a Test Bite.

Read on if you are taking the Higher tier paper.

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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.

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.

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