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Telomeres may shorten.

Telomeres are protective caps at the ends of each strand of DNA. Some scientists have compared them to the plastic tips of shoelaces that keep them from fraying. 

Some research suggests that every time cells divide, the tips of the chromosome become shorter. When the telomeres are lost, chromosomes become unstable and all kinds of problems arise. The most notable is that chromosomes can't replicate correctly, and end up fragmented or with extra parts that aren't supposed to be there. These abnormalities usually kill cells or make them dangerous.

Scientists have figured out how to increase levels of telomerase – an enzyme that can extend the length of telomeres — in mice, and a study suggested that can extend mice's lifespan. When they lowered levels of telomerase in mice, the mice lived shorter lives.

Proteins become less stable and accurate in their roles.

Proteins, like the motor proteins above, perform integral functions in the cell.Kateryna Kon / Shutterstock

In our cells, proteins are produced constantly, and they control almost every function inside the cell. They move materials, carry signals, turn processes on and off, and provide structural support for the cell. 

But proteins have to be recycled regularly because they lose their effectiveness over time. As we age, our bodies lose the ability to eliminate old proteins.

If our bodies can't turn over unusable proteins, they can build up and become toxic. Protein accumulation is one of the major features of Alzheimer's disease — proteins called beta-amyloid aggregate in the brain and result in the loss of nerve cells. 

Cells don't die when they're supposed to.

As cells undergo stress and become damaged, they sometimes stop dividing and become resistant to death. They turn into something scientists call "zombie cells," which can infect other cells in their vicinity and spread inflammation throughout the body. These cells are also called senescent cells.

Senescent cells accumulate with time and age, and scientists have found that eliminating senescent cells in old mice seems to reverse some of the effects of aging. Similarly, when senescent cells were injected into young mice, they had debilitating and inflammatory effects, and were detrimental to overall health. 

Several drugs called senolytics are now being developed with the goal of reducing senescent cells in the elderly to treat age-related disease. 

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