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The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.


Bases, sometimes called nitrogenous bases, are the parts of the DNA molecule that join the two helix strands. They are like rungs on a ladder. There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). Each base can only join with one other base; i.e. they join together in pairs: A with T and G with C.

Nitrogenous bases

Four nitrogen-containing molecules, sometimes just called bases, adenine, A, Thymine, T, Guanine, G and Cytosine, C, which form the 'rungs' of the DNA molecule 'ladder'.

Human genome

The complete sequence of all 20,000-25,000 human genes. That is, which chromosomes they are in and whereabouts the gene appears on that chromosome's piece of DNA.

Genetic code

Sequence of bases in a DNA molecule which codes for the sequence of amino acids in a protein. Each group of three bases, codes for an amino acid.

Cell division

The process by which a parent cell divides into two daughter cells

Amino acids

The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body.


The description of an allele which is overridden by an equivalent dominant allele.


A large molecule which is made up of repeated individual units called monomers.


The part of a cell that controls the cell function and contains the chromosomes.


Monomer unit of the nucleic acids DNA and RNA. Each nucleotide is made up of three parts: a pentose sugar, a phosphate group and a nitrogenous base.


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




The description of an individual who has two identical alleles for one particular gene.


The description of an individual who has two different alleles for one particular gene.


The description of an allele which overrides an equivalent recessive allele.


Alternative forms of the same gene.

Genes and DNA

Where are the genes?

Most cells in the human body have a nucleus containing 23 pairs of chromosomes. A small section of a chromosome that controls one particular character, e.g. eye colour, is called a gene and each chromosome carries hundreds of genes. Scientists have identified all the human genes and the complete set, of 20,000-25,000, is known as the human genome. Only some of the full set of genes is used in any one cell - some genes are "switched off".

Click here for more information on the Human Genome Project


Chromosomes are made from very long molecules of DNA. Each molecule is in a ladder-like shape, with the "rungs" being made from a pair of nitrogenous bases, known as A, T, G and C, and the sides made from repeated pentose sugars and phosphate groups A group of one base, one pentose molecule and one phosphate together is known as a nucleotide and DNA can be regarded as a polymer of these nucleotides.

drawing of a nucleotide
drawing of base pairs
Model of a small section of a DNA helix
Model of a small section of a DNA helix

The bases are always paired A with T and G with C and it is the exact sequence of these base pairs in the DNA molecule that maps out the genetic code of the individual.

The DNA molecule is twisted into a double helix and coiled so tightly, that the total length of the human genome, about 1.8m, fits inside a nucleus of diameter about 6 micrometres.

In this animation chromosomes are shown condensed as they appear during cell division.

Genes and alleles

The chromosomes in a pair carry the same genes at the same positions. One of each pair has come from the mother and the other from the father, so the genes are also in pairs and both code for the same characteristic. These alternative forms of one gene are called alleles

chromosomes showing matching genes with alleles shown in different colours
Chromosomes showing matching genes with alleles shown in different colours

An individual may have two alleles the same or two different alleles for any one gene. For example, there are two alleles of one of the genes controlling eye colour. They may both code for blue eyes or one may code for blue eyes and the other for brown eyes. If both alleles are the same (e.g. both for brown eyes) they are known as homozygous alleles and if they are different they are known as heterozygous When alleles are heterozygous it may be that one of the alleles is dominant and the other recessive. A dominant allele always shows its related characteristic in the individual.

A gene is a piece of DNA which has the code for a particular protein. Bases in the gene determines the order in which amino acids are put together to make the protein. Proteins have special functions in the body so it is very important that the correct proteins are made. Occasionally there is a change in the order of the bases so the correct protein cannot be made and the cell cannot function properly. Such a change is known as a mutation.

Read more about mutations in What Can Go Wrong.

Question 1

Quiz Print
How many chromosomes are there in the majority of human cells?
What is the approximate number of genes in the human genome?
How do the bases (A, G, C and T) pair up in the DNA molecule?
What three molecules make up a nucleotide?
What is meant by an allele being described as dominant?
What is the term used when both alleles are the same?
What are the smaller units of which proteins are made?