OVERVIEW OF GENETIC INFORMATION AND ITS EXPRESSION |
| Use these links to navigate to selected parts of this section of the Introductory Biochemistry module. |
Introduction |
| Deoxyribonucleic acid (DNA) is the molecule used to store genetic information in the chromosomes of the cell. |
This stored information
will be used in two ways :
|
DNA replication |
| When a cell divides it need to make two exact copies of its DNA, one for each of the cells produced by the division. This involves replication of the parent cell DNA molecule. |
| The steps in this process are shown in the diagram below. |
| The two strands separate (to allow access to the enzymes which catalyse the process) and both strands are copied simultaneously. |
| The products are two exact copies of
the parental DNA molecule.
|
Protein synthesis |
| DNA contains a store of genetic information in the chromosomes of the cell. |
| This information is used solely to direct the synthesis of proteins. |
The proteins then are used for a
variety of other purposes :
|
| There are two parts to the process of protein synthesis. |
Transcription |
| Chromosomes are very large structures containing extremely long DNA molecules. The information contained in the DNA is read as short regions called genes. |
| In the simplest case each gene contains the information for the synthesis of one protein. (This relationship is largely true in bacteria but becomes more complex in higher organisms.) |
| A copy is made of one strand of the DNA which comprises the gene to be expressed. The selected strand is termed the sense strand. There are controlling mechanisms associated with the gene which determine which of the two DNA strands is the sense strand. |
| The copy that is made is the other type of nucleic acid, ribonucleic acid (RNA) - called messenger RNA (mRNA). RNA is a single strand of nucleotides. |
| Another type of RNA is copied from specific regions of the DNA within the cell. This is called transfer RNA (tRNA) and has a different role from mRNA in protein synthesis. |
Translation |
| Proteins are composed of chains of amino acids so the information originally stored in the DNA and now encoded in the mRNA has to be converted to a sequence of amino acids. |
| The mRNA (and the stored DNA) is a chain of nucleotides. |
| Each group of three successive nucleotides specifies the insertion of one particular amino acid into the protein chain being synthesised. Each group of three successive nucleotides is referred to as a codon. |
| This relationship between the sequences of three nucleotide groups in mRNA and particular amino acids is called the genetic code. |
| Consider as an example the
hypothetical segment of mRNA shown below. (remember, RNA contains uracil (U) in place of thymine (T) A U G U U U G G U This is read as blocks of three nucleotides. A U G | U U U | G G U This would be translated into a sequence of amino acids. methionine - phenylalanine - glycine |
| The amino acids are brought to the site of protein assembly by molecules of tRNA; each is specific for a particular amino acid. There is at least one unique tRNA for each of the twenty amino acids which are used in protein synthesis. |
| The actual protein synthesis occurs in the cytoplasm of the cell on units called ribosomes which are large complexes of many proteins (including enzymes) and several RNA species. |
Mutation |
| The process is rigidly controlled by a number of mechanisms to ensure the information originally contained in the stored DNA is converted to the correct sequence of amino acids in the protein. |
| It is the sequence of amino acids in the protein (primary structure) which determines the protein's 3-D shape and hence its biological activity. |
| As described in the section on protein structure, there is an absolute requirement for the correct protein shape for that protein to be biologically active. |
| A change in the sequence of nucleotides in the DNA (or a deletion of some of the gene sequence) will result in a change in the sequence of amino acids in the protein which may result in a loss of biological activity. |
| This type of change in nucleotide sequence is termed a mutation. |
This completes the section containing an overview of genetic information and its expression.
