Molecules comprises of atoms, which have electrons revolving around their nuclei (much like the planets around the sun, a star; though the forces that govern their rotation are entirely different). To the utter vexation of the electrons, Wolfgang Pauli, a great scientist had laid down his 'exclusion principle' which states that no 2 electrons (fermions) can have the same quantum number. Thus, if 2 electrons are occupy an orbital, they must have opposite spins! They (electrons) still do not complain, as long as they stay paired (like husband and wife : opposite spins!) but when they stay single (unpaired), and uncared for (unshared: not contributing to bond formation), they become agitated and highly reactive. Free radicals are molecules that contain unshared electrons and being highly reactive, pounce on nearby molecules and harm them.
Superoxide anion, hydroxyl radicals and nitric oxide free radicals are some examples. They are grouped into a common category called ROS (Reactive Oxygen Species), since all these free radicals contain oxygen in their molecules. To fight against these free radicals our body has developed its own machinery: speroxide dismutase (SOD), catalase (CAT), glutathione peroxidase are some examples. Vitamin C, E, A and some other nutrients assist them in free radical scavenging and serve individually as antioxidants.
Free radicals particularly attack the DNA, both in the nucleus (nDNA) and in the mitochondria (mtDNA), mitochondria being particularly vulnerable due to its inefficient repair mechanism, and also that the mtDNA is not covered by protective histone coverings. As a result of this assault by free radicals, mutations of genes and even deletions occur in the DNA, specially in the mitochondria. Oxidative phosphorylation, the process by which mitochondria produces ATP (the energy currency of the cell; produced in the mitochondria and disposed off to other locations in the cell, for energy utilization, as if portable batteries), is compromised, ATP production diminishes and the cells begin to die. Thus cumulative genetic damage results in aging.
Related Article: Aging: From a General and Evolutionary Perspective
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