Question

Describe the difference between spontaneous and induced mutations AND provide THREE sources from which these mutations might arise.

Answer 1

Answer:

Spontaneous and induced mutations

Spontaneous mutations are those that occur without a known cause. Induced mutations are those that occur due to the exposure of the organism to mutagenic agents such as ionizing radiation, ultra violet light or various chemicals that react with DNA.

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The factors which cause mutations are called “mutagens”. There are two main classes of mutagens.

1. Physical mutagens and
2. Chemical mutagens

a. Physical mutagens:

All forms of energy radiations that are capable of disrupting the chemical structure of chromosomes have been found to be mutagenic in almost all the organisms. Physical mutagens are mainly radiations. Radiations are of two types- Ionizing and nonionizing.

i).

Ionizing radiations cause ionization and will force ejection of an electron from the atom. Among ionizing radiations, X- rays, gamma rays, beta rays and neutrons are more commonly used for inducing mutations. These radiations produce breaks in the chromosomes and abnormal mitosis in cells. Chromosome breaks lead to loss of chromosomes and result in abnormal functioning of the cells.

ii).

The non ionizing radiations like UV rays cause excitation through energy transfer. They have longer wave lengths and carry much lower energy. Therefore, their penetration power is much less than X- rays. The UV rays are absorbed by nucleic acid and cause alterations in the bond characteristic of purines and pyrimidines. Two adjacent pyrimidines of the same DNA are found to form covalent bonds forming dimers. Dimer interferes with normal base pairing.

b. Chemical mutagens:

Chemicals can also be used for inducing mutations. The chemicals used for inducing mutations are mustard gas, ethyl urethane, phenol, formaldehyde etc. The first chemical mutagen discovered was mustard gas (sulphur mustard). Chemical mutagens can be classified into two classes.

The first class is mutagenic to both replicating and non replicating DNA such as the alkylating agents and nitrous acid. The second class is mutagenic only to replicating DNA which includes the acridine dyes which bind to DNA and increase the probability of mistakes during replication and base analogs (purines and pyrimidines with structures similar to the normal bases of DNA). The base analogs get incorporated into DNA chain in the place of normal bases.