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BMEN 515 Lecture Notes - Dna Mismatch Repair, Selective Breeding, Mutation

Biomedical Engineering
Course Code
BMEN 515
William Huddleston

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Basic Issues in Life History Analysis
Fig 13.2 shows how a female opossum got her energy at different stages of her life, and the functions to which she
allocated that finite energy supply
Female possum before she became sexually mature, the female used her energy for growth, metabolic functions like
thermoregulation, and the repair of damaged tissues
o After she became sexually mature, the female stopped growing, thereafter using her energy for metabolism,
repair, and reproduction
Changes in life history are caused by changes in the allocation of energy
o For example, a different female opossum might stop allocating energy to growth at an earlier age, thereby
reaching sexual maturity more quickly
This strategy involves a trade-off: The female also matures at a smaller size, which means that she will
produce smaller litters (babies)
o Still another female might, after reaching sexual maturity, allocate less energy to reproduction and more to repair,
thereby keeping her tissues in better condition
Again there is a trade-off: Allocating less energy to reproduction means having smaller litters (babies)
Why Do Organisms Age and Die?
Aging or Senescence- is a late-life decline in an individual’s fertility and probability of survivial
Documentation of a bird, a mammal, and an insect, all show declines in both fertility and survival
If everything else remains equal, aging reduces an individual’s fitness
o Therefore aging should be opposed by natural selection
Two theories on why aging persists:
o Rate-of-Living theory
Invokes an evolutionary constraint
Posits that populations lack the genetic variation to respond any further to selection against aging
o Evolutionary theory
Invokes a trade-off between the allocation of energy to reproduction versus repair
The Rate-of-Living Theory of Aging
Holds that aging is caused by the accumulation of irreparable damage to cells and tissues
Damage to cells and tissues is caused by errors during replication, transcription, and translation, and by the accumulation
of poisonous metabolic by-products
Under this theory-all organism have been selected to resist and repair cell and tissue damage to the maximum extent
physiologically possible
o They have reached the limit of biologically possible repair
o In other words, populations lack the genetic variation that would enable them to evolve more effective repair
mechanisms than they already have
This theory makes two predictions:
o Because cell and tissue damage is caused in part by the by-products of metabolism, the aging rate should be
correlated with the metabolic rate
o Because organisms have been selected to resist and repair damage to the maximum extent possible, species
should not be able to evolve longer life spans, whether subjected to natural or artificial selection
o This theory holds that aging is a function of metabolic rate but data on variation in metabolic rate and aging
among mammals deny this theory
o Many populations are not, in fact, up against intrinsic limits to longevity
o They harbour genetic variation that would allow the evolution of longer life spans
And yet, longer life spans have not evolved
The Evolutionary Theory of Aging
Under the evolutionary theory, aging is caused not so much by cell and tissue damage itself as by the failure of organisms
to completely repair such damage
o This failure to fully repair leads to gradual decay and ultimate collapse
Given that organisms are capable of constructing themselves from scratch, they should also be capable of maintaining
their organs and tissues once formed
Organisms do have remarkable abilities to replace or repair damaged parts; yet in many organisms repair is incomplete
Under the evolutionary theory of senescence, the failure to completely repair damage is ultimately caused by either :
o Deleterious mutations
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