Site author Richard Steane
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Genes, DNA and Chromosomes

A gene is a unit of inheritance, and different organisms therefore have many genes in every cell of their body. Some genes are only active in certain cells.

Genes are probably responsible for most of an organism's characteristics - visible and non-visible. It must be said that the environment will also have an influence on this. Genes effectively control everything that a cell does, and this is because they contain the specification for proteins that cells produce - for example enzymes, hormones and the very proteins that cells are built from.

Genes are passed from cell to cell when cells divide in growth, and also passed from generation to generation when organisms reproduce - whether this is by sexual or asexual reproduction.

Genes are made of a chemical called DNA which is found in the nucleus of almost every cell. In higher organisms it is found in roughly cylindrical structures called chromosomes.

In bacterial cells the DNA is present as a diffuse blob.

Another way of describing a gene would be to call it a part of a chromosome, composed of a section of DNA which carries the genetic information to code for a particular protein.

More about chromosomes

Humans have 46 chromosomes in every normal cell of the body. Sex cells (sperms and eggs) have exactly half this number (23).

At fertilisation, the fertilised egg (zygote) contains equal amounts of genetic information from both parents. There are 2 versions of each chromosome - one from the father (paternal) and one from the mother (maternal). These are passed to every cell of the developing embryo and the new individual person that is grows into.

As a result, there are therefore usually 2 copies of most genes in each cell. Different forms of genes (alleles) may work in different ways. So-called faulty or defective genes may produce different proteins or not work at all. In reality, it is the combination of (pairs of) genes that determine what characteristics are seen. Some versions of genes are said to be dominant and have their effect whether one or two of them are present on chromosomes in cells, whilst others are called recessive and will only be noticed if both of them are present on chromosomes in cells.

More about DNA

DNA is said to be a long chain molecule. It is in fact shaped as a double helix, with two parallel strands wound into a twisted shape like strands within a piece of rope. In between the two strands - in fact holding them together - are sections called bases which form an order or sequence which can be read by the cell to make proteins, in the same way that letters can be read from a line in a book to make words and sentences. There are in fact 4 bases which are often known by the initial letters of their name: A which pairs with T, and C which pairs with G on the other strand.

It has been found that unlike words in a book which may only make sense if you know the language it is written in, the code in DNA is universal, so it works in the same way in all living organisms. This means that genes may be taken from one organism and hopefully function when transferred to another completely unrelated organism. This is the basis for genetic engineering.


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