write a note on biology of sensecence.
1. Aging
Aging, also known as biological aging or senescence, is a complex biological process
defined as the time-related deterioration of physiological functions necessary for
survival and fertility. It is characterized by the progressive accumulation of damage
over time, leading to disturbed function at cellular, tissue, and organ levels, eventually
resulting in disease and death. Aging is considered a multifactorial process influenced by
genetic, endogenous, and environmental factors.
• Concepts of Life Span and Life Expectancy:
o Maximum life span is a characteristic of the species, representing the
maximum number of years a member has been known to survive. Examples
include an estimated 121 years for humans, over 150 years for tortoises and
lake trout, about 20 years for a domestic dog, 4.5 years for a laboratory
mouse, and 3 months for a Drosophila fruit fly.
o Life expectancy is the age at which half the population still survives. It is the
length of time an individual of a given species can expect to live and is
characteristic of populations, not species. It is a key metric for assessing
population health. Improvements in public health and medical services,
particularly the control of infectious diseases, have significantly increased
life expectancy in many populations. For example, in India, life expectancy
rose from around 32 years in 1947 to 60 years by 1990. The global average life
expectancy is 72.6 years, while India's is 69.7 years.
• Population Aging: This is a significant emerging demographic phenomenon
worldwide. The proportion and number of older persons (aged 60 and above) are
increasing globally. In 1950, they constituted 8.2% (205 million) of the world
population; by 2000, this increased to 10% (606 million). Projections indicate this
proportion will rise to 21.1% (two billion) by 2050. Asia currently holds the largest
number of elderly (53%), followed by Europe (25%). By 2050, 82% of the world's
elderly are projected to reside in developing regions.
• Negligible Senescence: Some species apparently do not age or have negligible
senescence, meaning older individuals do not appear less fit than younger ones.
Examples include Rougheye Rockfish (oldest recorded specimen 205 years), Lake
Sturgeon (152 years), Aldabra Tortoise (152 years), Koi (226 years), and Bowhead
Whale (211 years). These species challenge simple deterioration theories of aging
and suggest the possibility of dramatically longer human life spans.
,explain different theories of aging and its mechanisms
• Theories of Aging: Aging is a multifactorial process with more than three hundred
theories, generally classified into three main groups: genetic mutation theories,
wear and tear theories, and cellular waste accumulation theories. Many hypotheses
involve the deterioration of maintenance processes like homeostatic metabolic,
inflammatory, and redox processes.
o General Wear-and-Tear Theory: Introduced by Dr. August Weismann in
1882, this theory posits that the body and cells are damaged by overuse,
abuse, toxins, and small traumas that build up over time. This wear and tear
occur at both organ and cellular levels.
o Neuroendocrine Theory: Proposed by Dr. Vladimir Dilman in 1954 and also
called the aging clock theory, it states that the body's homeostatic
adjustments decline with aging, leading to the failure of adaptive
mechanisms, aging, and death. This theory highlights the loss of efficiency in
regulatory systems like the Hypothalamic-Pituitary-Adrenal (HPA) axis, which
controls body functions via hormonal signals and negative feedback. A drop
in hormone production with age impairs the body's ability to repair and
regulate itself. Hormone replacement therapy is mentioned as a way to
potentially reverse or delay aging effects by resetting the hormonal clock.
o Genetic Control Theory: This theory suggests humans are born with a
genetic code that predetermines physical and mental functioning, and
influences the rate of aging and lifespan. It proposes a "biological clock" that
signals aging and death, though timing can vary based on growth and lifestyle
factors. This theory might be restricted to species where aging onset is
specifically controlled genetically and has a functional role, like the rapid
aging and death of Pacific salmon after spawning.
o Waste (undegradable by-products of metabolism) Accumulation Theory:
This theory states that cells produce more waste than they can eliminate,
such as toxins (free radicals, histones, aldehydes, lipofuscins), which
accumulate and interfere with normal cell function, eventually killing the
cell. In multicellular organisms, many cell types lose or reduce replicative
capacity (like cardiomyocytes and brain neurons), preventing waste dilution
through cell division and leading to accumulation. Lipofuscin (age pigment)
is a common by-product, accumulating in long-lived post-mitotic cells.
Extracellular deposits like cholesterol plaques and beta-amyloid protein
polymers also accumulate. Protein homeostasis is maintained by
chaperones and the proteasomal system. Aging and stress conditions
, disrupt this balance, leading to accumulation of oxidized proteins,
aggregates, and lipofuscin, compromising cell viability.
o Errors and Repairs Theory: Dr. Leslie Orgel suggested in 1963 that errors in
protein production or DNA reproduction can occur. While natural repair
processes exist, they aren't perfect, and the accumulation of flawed
molecules can cause diseases and age changes. Faulty DNA polymerases
are seen in senescent cells, and efficient DNA repair is linked to longer
lifespans. Genetic defects in DNA repair enzymes can cause premature aging
syndromes.
o Autoimmune Theory of aging (Immunosenescence): Proposed by Dr. Roy
Walford in 1969, this theory links aging to declines in adaptive and innate
immunity, termed immunosenescence. With age, the immune system's
ability to produce disease-fighting antibodies and distinguish "self" from
"nonself" declines, potentially causing it to react against itself. This can lead
to chronic low-level inflammation, higher infection rates, and chronic
diseases in the elderly.
o Cross-Linkage Theory of Ageing: First proposed by Johan Bjorksten in 1942,
this hypothesis suggests that with age, proteins, DNA, and other molecules
develop inappropriate attachments or cross-links to one another. These links
decrease mobility and elasticity of molecules, affecting tissues like skin,
blood vessels, and the immune system. Cross-linking can lead to irreversible
protein structural changes and altered function. These cross-links can
obstruct nutrient/waste passage and inhibit enzyme activity (like proteases
that normally break down damaged proteins), causing damaged proteins to
persist. A main way crosslinking occurs is through glycation, the non-
enzymatic reaction between reducing sugars (like glucose) and proteins,
lipids, or nucleic acids. This forms Advanced Glycation End products
(AGEs), which can form permanent cross-links disabling protein function.
AGEs are a heterogeneous group of molecules, interacting with specific
receptors like RAGE (Receptor for AGEs). Binding to RAGE stimulates
signaling pathways (MAPKs, ERK, PI3K, Ras, JNK, Janus kinases), activating
transcription factor NFκB and promoting proinflammatory gene transcription,
creating a cycle of inflammation.
o The Free Radical Theory or Oxidative Stress Theory of Ageing: Popularized
by Denham Harman, this theory proposes that organisms age due to the
accumulation of oxidative damage caused by Reactive Oxygen Species
1. Aging
Aging, also known as biological aging or senescence, is a complex biological process
defined as the time-related deterioration of physiological functions necessary for
survival and fertility. It is characterized by the progressive accumulation of damage
over time, leading to disturbed function at cellular, tissue, and organ levels, eventually
resulting in disease and death. Aging is considered a multifactorial process influenced by
genetic, endogenous, and environmental factors.
• Concepts of Life Span and Life Expectancy:
o Maximum life span is a characteristic of the species, representing the
maximum number of years a member has been known to survive. Examples
include an estimated 121 years for humans, over 150 years for tortoises and
lake trout, about 20 years for a domestic dog, 4.5 years for a laboratory
mouse, and 3 months for a Drosophila fruit fly.
o Life expectancy is the age at which half the population still survives. It is the
length of time an individual of a given species can expect to live and is
characteristic of populations, not species. It is a key metric for assessing
population health. Improvements in public health and medical services,
particularly the control of infectious diseases, have significantly increased
life expectancy in many populations. For example, in India, life expectancy
rose from around 32 years in 1947 to 60 years by 1990. The global average life
expectancy is 72.6 years, while India's is 69.7 years.
• Population Aging: This is a significant emerging demographic phenomenon
worldwide. The proportion and number of older persons (aged 60 and above) are
increasing globally. In 1950, they constituted 8.2% (205 million) of the world
population; by 2000, this increased to 10% (606 million). Projections indicate this
proportion will rise to 21.1% (two billion) by 2050. Asia currently holds the largest
number of elderly (53%), followed by Europe (25%). By 2050, 82% of the world's
elderly are projected to reside in developing regions.
• Negligible Senescence: Some species apparently do not age or have negligible
senescence, meaning older individuals do not appear less fit than younger ones.
Examples include Rougheye Rockfish (oldest recorded specimen 205 years), Lake
Sturgeon (152 years), Aldabra Tortoise (152 years), Koi (226 years), and Bowhead
Whale (211 years). These species challenge simple deterioration theories of aging
and suggest the possibility of dramatically longer human life spans.
,explain different theories of aging and its mechanisms
• Theories of Aging: Aging is a multifactorial process with more than three hundred
theories, generally classified into three main groups: genetic mutation theories,
wear and tear theories, and cellular waste accumulation theories. Many hypotheses
involve the deterioration of maintenance processes like homeostatic metabolic,
inflammatory, and redox processes.
o General Wear-and-Tear Theory: Introduced by Dr. August Weismann in
1882, this theory posits that the body and cells are damaged by overuse,
abuse, toxins, and small traumas that build up over time. This wear and tear
occur at both organ and cellular levels.
o Neuroendocrine Theory: Proposed by Dr. Vladimir Dilman in 1954 and also
called the aging clock theory, it states that the body's homeostatic
adjustments decline with aging, leading to the failure of adaptive
mechanisms, aging, and death. This theory highlights the loss of efficiency in
regulatory systems like the Hypothalamic-Pituitary-Adrenal (HPA) axis, which
controls body functions via hormonal signals and negative feedback. A drop
in hormone production with age impairs the body's ability to repair and
regulate itself. Hormone replacement therapy is mentioned as a way to
potentially reverse or delay aging effects by resetting the hormonal clock.
o Genetic Control Theory: This theory suggests humans are born with a
genetic code that predetermines physical and mental functioning, and
influences the rate of aging and lifespan. It proposes a "biological clock" that
signals aging and death, though timing can vary based on growth and lifestyle
factors. This theory might be restricted to species where aging onset is
specifically controlled genetically and has a functional role, like the rapid
aging and death of Pacific salmon after spawning.
o Waste (undegradable by-products of metabolism) Accumulation Theory:
This theory states that cells produce more waste than they can eliminate,
such as toxins (free radicals, histones, aldehydes, lipofuscins), which
accumulate and interfere with normal cell function, eventually killing the
cell. In multicellular organisms, many cell types lose or reduce replicative
capacity (like cardiomyocytes and brain neurons), preventing waste dilution
through cell division and leading to accumulation. Lipofuscin (age pigment)
is a common by-product, accumulating in long-lived post-mitotic cells.
Extracellular deposits like cholesterol plaques and beta-amyloid protein
polymers also accumulate. Protein homeostasis is maintained by
chaperones and the proteasomal system. Aging and stress conditions
, disrupt this balance, leading to accumulation of oxidized proteins,
aggregates, and lipofuscin, compromising cell viability.
o Errors and Repairs Theory: Dr. Leslie Orgel suggested in 1963 that errors in
protein production or DNA reproduction can occur. While natural repair
processes exist, they aren't perfect, and the accumulation of flawed
molecules can cause diseases and age changes. Faulty DNA polymerases
are seen in senescent cells, and efficient DNA repair is linked to longer
lifespans. Genetic defects in DNA repair enzymes can cause premature aging
syndromes.
o Autoimmune Theory of aging (Immunosenescence): Proposed by Dr. Roy
Walford in 1969, this theory links aging to declines in adaptive and innate
immunity, termed immunosenescence. With age, the immune system's
ability to produce disease-fighting antibodies and distinguish "self" from
"nonself" declines, potentially causing it to react against itself. This can lead
to chronic low-level inflammation, higher infection rates, and chronic
diseases in the elderly.
o Cross-Linkage Theory of Ageing: First proposed by Johan Bjorksten in 1942,
this hypothesis suggests that with age, proteins, DNA, and other molecules
develop inappropriate attachments or cross-links to one another. These links
decrease mobility and elasticity of molecules, affecting tissues like skin,
blood vessels, and the immune system. Cross-linking can lead to irreversible
protein structural changes and altered function. These cross-links can
obstruct nutrient/waste passage and inhibit enzyme activity (like proteases
that normally break down damaged proteins), causing damaged proteins to
persist. A main way crosslinking occurs is through glycation, the non-
enzymatic reaction between reducing sugars (like glucose) and proteins,
lipids, or nucleic acids. This forms Advanced Glycation End products
(AGEs), which can form permanent cross-links disabling protein function.
AGEs are a heterogeneous group of molecules, interacting with specific
receptors like RAGE (Receptor for AGEs). Binding to RAGE stimulates
signaling pathways (MAPKs, ERK, PI3K, Ras, JNK, Janus kinases), activating
transcription factor NFκB and promoting proinflammatory gene transcription,
creating a cycle of inflammation.
o The Free Radical Theory or Oxidative Stress Theory of Ageing: Popularized
by Denham Harman, this theory proposes that organisms age due to the
accumulation of oxidative damage caused by Reactive Oxygen Species
