How do certain types of radiation and chemicals cause mutation?

Mutations happen when the structure of DNA is changed. These changes can occur naturally, but in many cases, they are caused by external agents called mutagens. Two important types of mutagens are radiation and chemicals. These agents damage the DNA either directly or indirectly, and if the damage is not repaired properly, it becomes a permanent mutation in the genetic code. The way radiation and chemicals cause mutations is explained below:

1. Radiation-Induced Mutations

Radiation is a physical mutagen. It causes mutations depending on how much energy it carries. Radiation is mainly of two types: ionizing and non-ionizing.

(a) Ionizing Radiation

Ionizing radiation includes X-rays, gamma rays, and radioactive particles like alpha and beta rays. These have very high energy and when they pass through cells, they can remove electrons from atoms, creating ions.

This energy causes direct damage to DNA by breaking the sugar-phosphate backbone or bases. It can lead to single-strand or double-strand breaks in DNA. Double-strand breaks are very serious because if they are not repaired correctly, they can lead to loss of genetic material, chromosomal rearrangements, or even cell death.

Also, ionizing radiation produces free radicals like hydroxyl radicals from water molecules inside the cell. These free radicals are highly reactive and can attack the DNA bases, modifying them chemically and causing mutations during replication.

(b) Non-Ionizing Radiation

This includes ultraviolet (UV) radiation, which is found in sunlight. UV rays do not have enough energy to remove electrons, but they still cause serious damage to DNA.

One of the main effects of UV radiation is the formation of thymine dimers. This means that two thymine bases, which are next to each other in a DNA strand, become abnormally linked. This bond bends the DNA and blocks the normal function of DNA polymerase during replication. If the cell cannot repair this damage properly, it can lead to base substitutions or frameshift mutations.

So, ionizing radiation breaks the DNA, while UV radiation changes the structure of bases like thymine, both leading to mutation.

2. Chemical-Induced Mutations

Many chemicals found in the environment, industry, or even inside the body can act as chemical mutagens. They affect DNA in different ways depending on their structure.

(a) Base Analogs

Some chemicals look like natural DNA bases. When cells are replicating DNA, these chemicals get inserted in place of real bases. For example, 5-bromouracil is similar to thymine but sometimes pairs with guanine instead of adenine. This wrong pairing results in a base substitution mutation during replication.

(b) Alkylating Agents

These chemicals add alkyl groups (like methyl or ethyl) to DNA bases. When the shape of a base is changed, it may pair incorrectly with another base. For example, ethyl methanesulfonate (EMS) adds an ethyl group to guanine, which then pairs with thymine instead of cytosine, leading to mutation.

(c) Deaminating Agents

These remove amino groups from bases. Nitrous acid, for example, converts cytosine into uracil. Since uracil pairs with adenine, this results in a C-G to T-A transition mutation.

(d) Intercalating Agents

These are flat molecules like acridine orange or proflavine that insert themselves between base pairs of DNA. This disturbs the spacing of DNA and causes the DNA polymerase to either skip a base or add an extra base during replication. This results in insertions or deletions, which can shift the reading frame and damage the whole protein.

(e) Reactive Oxygen Species (ROS)

These are formed inside cells naturally during metabolism or by radiation exposure. They are highly reactive and can attack DNA bases. For example, guanine may be oxidized to 8-oxoguanine, which wrongly pairs with adenine, causing mutations.






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