PTE · Reorder Paragraphs

Mechanisms of Biological Ageing

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1

Telomeres and Cellular Senescence

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  1. ABecause standard enzymes cannot copy the extreme tips of chromosomes, these structures gradually shorten with each cell division.
  2. BOnce they reach a critically diminished length, a permanent arrest of growth known as replicative senescence is triggered.
  3. CAt the ends of eukaryotic chromosomes lie repetitive DNA sequences called telomeres, which protect genetic code during replication.
  4. DIn this dormant state, cells no longer divide, contributing to gradual tissue degradation across the organism.

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2

Mitochondria and Oxidative Stress

  1. AHowever, this metabolic machinery constantly leaks small amounts of reactive oxygen species as chemical by-products.
  2. BDeprived of vital energy, the affected tissues experience declining functional capacity and accelerated senescence.
  3. CSuch structural degradation steadily impairs organelle efficiency, reducing the overall energy yield of the host cell.
  4. DOver time, these unstable molecules inflict progressive oxidative damage on surrounding mitochondrial DNA and membrane lipids.
  5. EMitochondria generate the chemical energy required for cellular function through the process of oxidative phosphorylation.
3

Autophagy and Cellular Waste

  1. AConsequently, toxic molecular aggregates accumulate within the cytoplasm, disrupting vital cellular operations and accelerating tissue deterioration.
  2. BDuring this internal clearance process, specialised vesicles engulf biological waste and deliver it to lysosomes for enzymatic breakdown.
  3. CAs organisms grow older, however, the regulatory signalling pathways governing this digestive mechanism suffer a progressive decline.
  4. DLiving cells rely on an essential intracellular recycling system known as autophagy to degrade damaged organelles and misfolded protein structures.
4

DNA Methylation and Epigenetic Clocks

  1. AOver decades, predictable shifts in this methylation landscape occur at hundreds of distinct genomic sites.
  2. BThroughout an organism's lifetime, small chemical tags called methyl groups attach to specific locations across the genome.
  3. CThis calculated metric often reflects an individual's true physiological health far better than chronological age alone.
  4. DBy measuring these predictable chemical changes, researchers can calculate a precise biological age known as the epigenetic clock.
  5. EThese modifications alter gene expression patterns without changing the underlying DNA sequence itself.
5

Stem Cell Exhaustion in Ageing

  1. ASustaining this regenerative capacity requires these specialised precursor cells to maintain a fine balance between self-renewal and differentiation.
  2. BThis resulting state of stem cell exhaustion ultimately limits organ renewal, leaving vital physiological systems unable to repair everyday wear.
  3. CWith advancing age, however, cumulative DNA mutations and altered signalling within the surrounding niche gradually erode their proliferative potential.
  4. DAdult stem cells serve as an indispensable biological repair reservoir, continually dividing to replace damaged cells throughout mature tissues.

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