Mechanism for Mutation to Occur
There are two mechanisms for making mutations. They are physical mechanism and chemical mechanism.
(1) Physical induced mutation,
The nitrogen bases of nucleotides can absorb short wavelength electromagnetic waves, as ultra-violet light, X-ray and gamma ray etc. Therefore, DNA can absorb these electromagnetic waves. The effect of change is dependent on the dosage. After the absorption of high-energy electromagnetic wave, the nitrogen bases or DNA would be ionized and changed.
(2) Chemical induced mutation,
Some chemicals can react with the nitrogen bases in DNA and cause changes in DNA. If the nitrogen bases in DNA are changes, the codons are also changed. This makes the use of inappropriate amino acids and changes the primary conformation of the polypeptide (or protein), then, the property of the polypeptide (or protein) is also changed.
As nitrous acid, HNO2 can change adenine into hypoxanthine. But hypoxanthine has the complementary property of guanine. So, in the next replication, the A-T pair in the DNA would be changed into the G-C pair. In transcription, the original uracil would be replaced with cytosine. The codon is then, changed. After this change, the amino acids sequence of the protein is also changed. The new protein would have new properties and different from the original ones. Therefore, the expression is changed, and we say, mutation is formed.
In avian flu, it is found that the H5 (haemoagglutinine protein) has one amino acid different from the H1 antigen. H1N1 was the pathogen of the endemic Spanish flu in 1919. The Spanish flu could be endemic in 1919, because the conformation of the H1 antigen has a big hole, and this big hole can combine to the human receptors. So, the virus can infect human and spread rapidly. Now, the H5 antigen does not have the hole and cannot combine with the human receptors. So, it is difficult for H5 to infect human. The hole is blocked by that different amino acid. If the H5 antigen of the avian flu makes a mutation and get back the amino acid as that in the H5 of the Spanish flu, the molecule of the avian flu H5 will get the hole and become possible to combine with the human receptor easily. Then, it would become endemic. The mortality of the avian flu is about 40% for the high pathogenic strain. If it becomes endemic, the mortality of human would be terrible.
(23.05.2007)
