Dr. Holloway has written a book about biological aging at the level of DNA and cells. Her central focus is on the cumulative effects of random events within a cell and how these effects contribute to cellular damage and ?

Free radicals

Explanation:

The free radical theory of aging says that many of the changes that occur as our bodies age are caused by free radicals. Damage to DNA, protein cross-linking and other changes have been attributed to free radicals. Over time, this damage accumulates and causes us to experience aging.

Oxidative stress occurs when an oxygen molecule splits into single atoms with unpaired electrons, which are called free radicals. Electrons like to be in pairs, so these atoms, called free radicals, scavenge the body to seek out other electrons so they can become a pair. This causes damage to cells, proteins and DNA.

Free radicals can damage the skin by trying to grab an extra electron from atoms in the skin. When atoms are taken away from molecules in the skin, it causes damage to our skin's DNA that can speed along skin aging. This is called the "free radical theory of aging."

FREE RADICALS.

Explanation:

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Generally the atomic structure of an atom is made up of a central  nucleus of proton and neutron(p+n) surrounded by continuously revolving electrons. Atoms with full outermost shells of electron are stable.Some atoms do not have full outermost  shell of electron, therefore they are unstable. Such atom always reacts with other atoms with extra unpaired electron, looking for extra electron  to pair with to gain stability for  octet or full outermost  electron  shell. Thus they  are highly reactive. Therefore atom, which do not have full outermost  shell electron,(unstable)and therefore very  reactive, always looking for  lone  pair of  electrons to pair with  are called Free Radicals.

Thus  in our bodies they therefore highly unstable,   reactive, looking for  electrons in our body cells and tissues to pair with. They are byproducts of   biochemical reactions  or products of chemical reaction on exposure to certain radioactive materials or toxic materials  

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Oxygen radical are the most ubiquitous in our body. It occurs when oxygen atom is split to  unstable unpaired electron,  which is reactive, with need to pair with another atoms to gain stability, reaction with molecules in the cells and tissues leads to oxidative stress, which damages the genetic composition of  living cells and tissues leading to aging.

the answer would be d : p