Genes make proteins

The function of genes is to provide the information needed to make molecules called proteins in cells. Cells are the smallest independent parts of organisms: the human body contains about 100 trillion cells, while very small organisms like bacteria are just a single cell. A cell is like a miniature and very complex factory that can make all the parts needed to produce a copy of itself, which happens when cells divide. There is a simple division of labor in cells – genes give instructions and proteins carry out these instructions, tasks like building a new copy of a cell, or repairing damage. Each type of protein is a specialist that only does one job, so if a cell needs to do something new, it must make a new protein to do this job. Similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible. Genes tell cells what to do by telling them which proteins to make and in what amounts.

Genes are expressed by being transcribed into RNA, and this RNA then translated into protein.

Proteins are made of a chain of 20 different types of amino acids. This chain folds up into a compact shape, rather like an untidy ball of rope. The shape of the protein is determined by the sequence of amino acids along its chain and it is this shape that, in turn, determines what the protein will do. For example, some proteins have depressions in their surface that perfectly match another molecule, allowing the protein to bind to this molecule very tightly. Other proteins are enzymes, which are like tiny machines that can alter other molecules.


The information in DNA is held in the sequence of the repeating units along the DNA chain.  These units are four types of nucleotides (A,T,G and C) and the sequence of nucleotides stores information in an alphabet called the genetic code. When a gene is read by a cell the DNA sequence is copied into a very similar molecule called RNA (this process is called transcription). Transcription is controlled by other DNA sequences (such as promoters), which show a cell where genes are, and control how often they are copied. The RNA copy made from a gene is then fed through a structure called a ribosome, which translates the sequence of nucleotides in the RNA into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain. The new protein then folds up into its active form. The process of moving information from the language of DNA into the language of amino acids is called translation.

DNA replication. DNA is unwound and nucleotides are matched to make two new strands.

If the sequence of the nucleotides in a gene changes, the sequence of the amino acids in the protein it produces may also change – if part of a gene is deleted, the protein produced will be shorter and may not work any more. This is the reason why different alleles of a gene can have different effects in an organism. As an example, hair color depends on how much of a dark substance called melanin is put into the hair as it grows. If a person has a normal set of the genes involved in making melanin, they make all the proteins needed and they grow dark hair. However, if the alleles for a particular protein have different sequences and produce proteins that do not do the job correctly, no melanin will be produced and the hair will be white. This condition is called albinism and the person with this condition is called an albino.

Genes are inherited as units, with parents dividing out their genes to their offspring. You can think of this process like mixing two hands of cards, shuffling them, and then dealing them out again. Humans have two copies of each of their genes (i.e, two alleles ) and when people reproduce they make copies of their genes in eggs or sperm, but only put in one copy of each type of gene. An egg then joins with a sperm to give a child with a new set of genes. This child will have the same number of genes as its parents but for any gene one of their two copies will come from the father, and one from the mother.

The effects of this mixing depends on the types (the alleles) of the gene you are interested in. If the father has two alleles specifying green eyes, and the mother has two alleles specifying brown eyes, all their children will get two alleles giving different instructions, one for green eyes and one for brown. The eye color of these children depends on how these alleles work together. If one allele overrides the instructions from another, it is called the dominant allele, and the allele that is overridden is called the recessive allele. In the case of a daughter with both green and brown alleles, brown is dominant and she ends up with brown eyes.

Green eyes are a recessive trait.

However, the green eye color allele is still there in this brown-eyed girl, it just doesn’t show. This is a difference between what you see on the surface (the set of observable traits of an organism, also called its phenotype) and which genes are in this organism (its genotype). In this example you can call the brown allele “B” and the green allele “g”. (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown-eyed daughter has the “brown eye phenotype” but her genotype is Bg, with one copy of the B allele, and one of the g allele.