Cells are able to build up proteins directly from amino acids, but only the L amino acids react. The instructions specifying the primary structure of a protein are carried by molecules of ribonucleic acid (RNA). There are many different RNA molecules. Messenger RNA (mRNA) molecules provide a code which tells the cell which amino acids to put together, in which order, to make a protein. There are many different proteins hence there are many different mRNA molecules. Transfer RNA (tRNA) molecules select and separate the amino acids needed for protein synthesis.
RNA molecules consist of strands formed from alternate ribose sugar molecules and phosphate groups. Attached to each ribose group is one of four bases. The bases are uracil (U), cytosine (C), adenine (A) and guanine (G). The sequence is different in different RNA molecules. It is the bases which form the code for protein synthesis.
The cells catalyst for protein production is a particle called ribosome. This contains the ribonucleic acid ribosomal RNA (rRNA).
In order for four bases to code for 20 amino acids a triplet code is needed, where a combination of three bases tells the cell which amino acids to use. The different combinations of three bases are called codons. tRNA molecules recognise and bind to the codons on mRNA through anticodons. Base G in the anticodon recognises C in the codon and vice versa. Bases U and A recognise one another similarly. Hence anti-codon GUA will bind to codon CAU.
How does this work in practice?
The bases in RNA all have flat shapes and the correct pairs U & A and C & G can fit neatly together and place groups in just the right positions for hydrogen bonds to form.
mRNA is destroyed after its corresponding protein production is no longer required. A cell keeps a permanent record in the nucleus using deoxyribonucleic acid (DNA). DNA like RNA consists of sugar-phosphate strands with attached bases, but ribose is replaced by deoxyribose and the base uracil (U) is replaced by thymine (T).
When a cell starts protein production, the record in the DNA has to be turned into an RNA message carried by mRNA, which is then 'read' to form the protein.
Also each DNA molecule contains the information for the production of many different mRNA molecules, but each mRNA molecule is a set of instructions for just one protein. A DNA segment responsible for a particular protein is called a gene. The full set of all genes of an organism is called its genome.
Taking genes from the cells of one type of organism and putting them into the cells of another type of organism is called genetic engineering. An important example is the manufacture of insulin. People who suffer from diabetes are unable to make their own insulin, and as a result their blood sugar level is not controlled properly. The disease can be treated reasonably effectively if diabetics regularly inject themselves with insulin.
Originally, insulin for diabetics was isolated from animals. Now it is made by introducing the genetic information contained in parts of DNA into the bacterium Escherichia coli (E. coli). The E. coli makes insulin in sufficient quantities to produce the twp tonnes of insulin needed by the population of diabetics each year.
If the amino acid residue at position B9 in insulin is changed from serine to aspartic acid, it results in the insulin acting faster after injection. This can be carried out by inserting insulin genes into yeast or bacteria. The human insulin gene is altered so that a codon for serine (e.g. AGG) is changed to a codon for aspartic acid (e.g. CTG). The yeast or bacterial cells will make the insulin analogue for us.
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