Programmed cell death
Encyclopedia
Programmed cell-death is death
of a cell
in any form, mediated by an intracellular program. PCD is carried out in a regulated process
which generally confers advantage during an organism's life-cycle
. PCD serves fundamental functions during both plant
and metazoa (multicellular animal
s) tissue development.
Besides these three types of PCD, other pathways have been discovered.
Called "non-apoptotic programmed cell-death" (or "caspase
-independent programmed cell-death" or "necrosis-like programmed cell-death"), these alternative routes to death are as efficient as apoptosis and can function as either backup mechanisms or the main type of PCD.
Other forms of programmed cell death include anoikis
, almost identical to apoptosis except in its induction; cornification, a form of cell death exclusive to the eyes; excitotoxicity
and Wallerian degeneration
.
Plant cells undergo particular processes of PCD which are similar to autophagic cell death. However, some common features of PCD are highly conserved in both plants and metazoa.
to die
. Only natural forces on the cell are considered to be atrophic factors. The following are generally considered to be common types of atrophic factors;
& Williams in 1964 in relation to insect
tissue development, around eight years before "apoptosis" was coined. Since then, PCD has become the more general of these two terms.
PCD has been the subject of increasing attention and research efforts. This trend has been highlighted with the award of the 2002 Nobel Prize in Physiology or Medicine
to Sydney Brenner
(United Kingdom
), H. Robert Horvitz
(US) and John E. Sulston
(UK).
, but it also has differences, most obviously the presence of a cell wall
and the lack of an immune system
which removes the pieces of the dead cell. Instead of an immune response, the dying cell synthesizes substances to break itself down and places them in a vacuole
which ruptures as the cell dies.
In "APL regulates vascular tissue identity in Arabidopsis
", Bonke and colleagues state that one of the two long-distance transport systems in vascular plants, xylem
, consists of several cell-types "the differentiation of which involves deposition of elaborate cell-wall
thickenings and programmed cell-death." The authors emphasize that the products of plant PCD play an important structural role.
Basic morphological and biochemical features of PCD have been conserved in both plant and animal kingdoms
. It should be noted, however, that specific types of plant cells carry out unique cell-death programs. These have common features with animal apoptosis—for instance, nuclear DNA
degradation—but they also have their own peculiarities, such as nuclear
degradation being triggered by the collapse of the vacuole
in tracheary
elements of the xylem.
Janneke Balk and Christopher J. Leaver, of the Department of Plant Sciences
, University of Oxford
, carried out research on mutation
s in the mitochondrial genome of sun-flower
cells. Results of this research suggest that mitochondria play the same key role in vascular plant PCD as in other eukaryotic
cells.
, plants enforce self-incompatibility
(SI) as an important means to prevent self-fertilization
. Research on the corn poppy
(Papaver rhoeas) has revealed that protein
s in the pistil on which the pollen
lands, interact with pollen and trigger PCD in incompatible (ie. self) pollen. The researchers, Steven G. Thomas and Veronica E. Franklin-Tong, also found that the response involves rapid inhibition of pollen-tube
growth, followed by PCD.
Dictyostelium discoideum
has the peculiarity of adopting either a predatory amoeba
-like behavior in its unicellular
form, or coalescing into a mobile slug
-like form when dispersing the spore
s which will give birth to the next generation
.
The stalk is composed of dead cells which have undergone a type of PCD that shares many features of an autophagic cell-death: massive vacuoles forming inside cells, a degree of chromatin
condensation, but no DNA fragmentation
. The structural role of the residues left by the dead cells is reminiscent of the products of PCD in plant tissue.
D. discoideum is a slime mold, part of a branch which may have emerged from eukaryotic
ancestors about a billion years
before the present. They apparently emerged after the ancestors of green plants
and the ancestors of fungi and animals had differentiated. But in addition to their place in the evolutionary tree
, the fact that PCD has been observed in the humble, simple, six-chromosome
D. discoideum has additional significance: it permits the study of a developmental PCD path which does not depend on the caspases which are characteristic of apoptosis.
which had been incorporated as endosymbiont
s ("living together inside") of larger eukaryotic cells. It was Lynn Margulis
who from 1967 on championed this theory
, which has since become widely accepted. The most convincing evidence
for this theory is the fact that mitochondria possess their own DNA
and are equipped with gene
s and replication
apparatus.
This evolution
ary step would have been more than risky for the primitive eukaryotic cells which began to engulf the energy-producing
bacteria and conversely, a perilous step for the ancestors of mitochondria which began to invade their proto-eukaryotic hosts
. This process is still evident today, between human
white blood cells
and bacteria. Most of the time, invading bacteria are destroyed by the white blood cells; however, it is not uncommon for the chemical warfare
waged by prokaryote
s to succeed, with the consequence known as infection
by its resulting damage.
One of these rare evolutionary events, about two billion years
before the present, made it possible for certain eukaryotes and energy-producing prokaryotes not only to coexist, but to mutually benefit from their symbiosis
.
Mitochondriate eukaryotic cells live poised between life
and death, because mitochondria still retain their repertoire of molecule
s which can trigger cell suicide. This process has now been evolved to happen only when programmed. Given certain signals
to cells (such as feedback from neighbors, stress or DNA damage
), mitochondria release caspase
activators which trigger the cell-death inducing biochemical
cascade. As such, the cell suicide mechanism
is now crucial to all of our lives.
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
of a cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
in any form, mediated by an intracellular program. PCD is carried out in a regulated process
Biological process
A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation....
which generally confers advantage during an organism's life-cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
. PCD serves fundamental functions during both plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
and metazoa (multicellular animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s) tissue development.
Types
- ApoptosisApoptosisApoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
or Type I cell-death. - Autophagic or Type II cell-death. (CytoplasmCytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
ic: characterized by the formation of large vacuoles which eat away organelles in a specific sequence prior to the nucleusCell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
being destroyed.) - Necrotic cell death or Type-III
Besides these three types of PCD, other pathways have been discovered.
Called "non-apoptotic programmed cell-death" (or "caspase
Caspase
Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases that play essential roles in apoptosis , necrosis, and inflammation....
-independent programmed cell-death" or "necrosis-like programmed cell-death"), these alternative routes to death are as efficient as apoptosis and can function as either backup mechanisms or the main type of PCD.
Other forms of programmed cell death include anoikis
Anoikis
Anoikis is a form of programmed cell death which is induced by anchorage-dependent cells detaching from the surrounding extracellular matrix . Usually cells stay close to the tissue to which they belong since the communication between proximal cells as well as between cells and ECM provide...
, almost identical to apoptosis except in its induction; cornification, a form of cell death exclusive to the eyes; excitotoxicity
Excitotoxicity
Excitotoxicity is the pathological process by which nerve cells are damaged and killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate such as the NMDA receptor and AMPA receptor are...
and Wallerian degeneration
Wallerian degeneration
Wallerian degeneration is a process that results when a nerve fiber is cut or crushed, in which the part of the axon separated from the neuron's cell body degenerates distal to the injury. This is also known as anterograde degeneration, or orthograde degeneration...
.
Plant cells undergo particular processes of PCD which are similar to autophagic cell death. However, some common features of PCD are highly conserved in both plants and metazoa.
Atrophic factors
An atrophic factor is a force that causes a cellCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
to die
Atrophy
Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations , poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, disuse or lack of exercise or disease intrinsic to the tissue itself...
. Only natural forces on the cell are considered to be atrophic factors. The following are generally considered to be common types of atrophic factors;
- Decreased workload
- Loss of innervation
- Diminished blood supply
- Inadequate nutrition
- Loss of endocrine stimulation
- Senility
- Compression
History
The concept of "programmed cell-death" was used by LockshinRichard A. Lockshin
Richard A. Lockshin is an American cellular biologist known for his work on apoptosis.He was educated at Harvard University where, in 1959, he obtained his bachelor's degree. This was followed by doctoral studies at Harvard University under the guidance of Carroll Williams...
& Williams in 1964 in relation to insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
tissue development, around eight years before "apoptosis" was coined. Since then, PCD has become the more general of these two terms.
PCD has been the subject of increasing attention and research efforts. This trend has been highlighted with the award of the 2002 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...
to Sydney Brenner
Sydney Brenner
Sydney Brenner, CH FRS is a South African biologist and a 2002 Nobel prize in Physiology or Medicine laureate, shared with H...
(United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
), H. Robert Horvitz
H. Robert Horvitz
Howard Robert Horvitz is an American biologist best known for his research on the nematode worm Caenorhabditis elegans.-Life:Horvitz did his undergraduate studies at MIT in 1968, where he joined Alpha Epsilon Pi...
(US) and John E. Sulston
John E. Sulston
Sir John Edward Sulston FRS is a British biologist. He is a joint winner of the 2002 Nobel Prize in Physiology or Medicine.He is currently Chair of the Institute for Science, Ethics and Innovation at the University of Manchester....
(UK).
Programmed cell-death in plant tissue
Programmed cell death in plants has a number of molecular similarities to animal apoptosisApoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
, but it also has differences, most obviously the presence of a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
and the lack of an immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
which removes the pieces of the dead cell. Instead of an immune response, the dying cell synthesizes substances to break itself down and places them in a vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
which ruptures as the cell dies.
In "APL regulates vascular tissue identity in Arabidopsis
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
", Bonke and colleagues state that one of the two long-distance transport systems in vascular plants, xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
, consists of several cell-types "the differentiation of which involves deposition of elaborate cell-wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
thickenings and programmed cell-death." The authors emphasize that the products of plant PCD play an important structural role.
Basic morphological and biochemical features of PCD have been conserved in both plant and animal kingdoms
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
. It should be noted, however, that specific types of plant cells carry out unique cell-death programs. These have common features with animal apoptosis—for instance, nuclear DNA
Nuclear DNA
Nuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In mammals and vertebrates, nuclear DNA encodes more of the genome than the mitochondrial DNA and is composed of information inherited from two parents, one male, and one female, rather than...
degradation—but they also have their own peculiarities, such as nuclear
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
degradation being triggered by the collapse of the vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
in tracheary
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
elements of the xylem.
Janneke Balk and Christopher J. Leaver, of the Department of Plant Sciences
Department of Plant Sciences, University of Oxford
The Department of Plant Sciences, at the University of Oxford, focusses on research and teaching in plant and fungal biology.- History :Forestry was an important part of the university under the name of the Imperial Forest Institute , later the Commonwealth Forest Institute from 1939...
, University of Oxford
University of Oxford
The University of Oxford is a university located in Oxford, United Kingdom. It is the second-oldest surviving university in the world and the oldest in the English-speaking world. Although its exact date of foundation is unclear, there is evidence of teaching as far back as 1096...
, carried out research on mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s in the mitochondrial genome of sun-flower
Sunflower
Sunflower is an annual plant native to the Americas. It possesses a large inflorescence . The sunflower got its name from its huge, fiery blooms, whose shape and image is often used to depict the sun. The sunflower has a rough, hairy stem, broad, coarsely toothed, rough leaves and circular heads...
cells. Results of this research suggest that mitochondria play the same key role in vascular plant PCD as in other eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cells.
PCD in pollen prevents inbreeding
During pollinationPollination
Pollination is the process by which pollen is transferred in plants, thereby enabling fertilisation and sexual reproduction. Pollen grains transport the male gametes to where the female gamete are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself...
, plants enforce self-incompatibility
Self-incompatibility in plants
Self-incompatibility is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage outcrossing...
(SI) as an important means to prevent self-fertilization
Biological reproduction
Reproduction is the biological process by which new "offspring" individual organisms are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction...
. Research on the corn poppy
Corn poppy
Papaver rhoeas is a species of flowering plant in the poppy family, Papaveraceae...
(Papaver rhoeas) has revealed that protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s in the pistil on which the pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
lands, interact with pollen and trigger PCD in incompatible (ie. self) pollen. The researchers, Steven G. Thomas and Veronica E. Franklin-Tong, also found that the response involves rapid inhibition of pollen-tube
Pollen tube
The pollen tubes is the male gametophyte of seed plants that acts as a conduit to transport the male sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms .After pollination, the pollen tube...
growth, followed by PCD.
Programmed cell death in slime molds
The social slime moldSlime mould
Slime mold or mould is a broad term describing protists that use spores to reproduce. Slime molds were formerly classified as fungi, but are no longer considered part of this kingdom....
Dictyostelium discoideum
Dictyostelium discoideum
Dictyostelium discoideum is a species of soil-living amoeba belonging to the phylum Mycetozoa. D. discoideum, commonly referred to as slime mold, is a eukaryote that transitions from a collection of unicellular amoebae into a multicellular slug and then into a fruiting body within its lifetime. D...
has the peculiarity of adopting either a predatory amoeba
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
-like behavior in its unicellular
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
form, or coalescing into a mobile slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-like form when dispersing the spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s which will give birth to the next generation
Generation
Generation , also known as procreation in biological sciences, is the act of producing offspring....
.
The stalk is composed of dead cells which have undergone a type of PCD that shares many features of an autophagic cell-death: massive vacuoles forming inside cells, a degree of chromatin
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
condensation, but no DNA fragmentation
Restriction digest
A restriction digest is a procedure used in molecular biology to prepare DNA for analysis or other processing. It is sometimes termed DNA fragmentation...
. The structural role of the residues left by the dead cells is reminiscent of the products of PCD in plant tissue.
D. discoideum is a slime mold, part of a branch which may have emerged from eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
ancestors about a billion years
Timeline of evolution
This timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
before the present. They apparently emerged after the ancestors of green plants
Viridiplantae
Viridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional Plant Kingdom to include the green algae...
and the ancestors of fungi and animals had differentiated. But in addition to their place in the evolutionary tree
Phylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
, the fact that PCD has been observed in the humble, simple, six-chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
D. discoideum has additional significance: it permits the study of a developmental PCD path which does not depend on the caspases which are characteristic of apoptosis.
Evolutionary origin of PCD
Biologists had long suspected that mitochondria originated from bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
which had been incorporated as endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s ("living together inside") of larger eukaryotic cells. It was Lynn Margulis
Lynn Margulis
Lynn Margulis was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted...
who from 1967 on championed this theory
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...
, which has since become widely accepted. The most convincing evidence
Evidence
Evidence in its broadest sense includes everything that is used to determine or demonstrate the truth of an assertion. Giving or procuring evidence is the process of using those things that are either presumed to be true, or were themselves proven via evidence, to demonstrate an assertion's truth...
for this theory is the fact that mitochondria possess their own DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and are equipped with gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s and replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
apparatus.
This evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary step would have been more than risky for the primitive eukaryotic cells which began to engulf the energy-producing
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
bacteria and conversely, a perilous step for the ancestors of mitochondria which began to invade their proto-eukaryotic hosts
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
. This process is still evident today, between human
Human
Humans are the only living species in the Homo genus...
white blood cells
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...
and bacteria. Most of the time, invading bacteria are destroyed by the white blood cells; however, it is not uncommon for the chemical warfare
Chemical warfare
Chemical warfare involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from Nuclear warfare and Biological warfare, which together make up NBC, the military acronym for Nuclear, Biological, and Chemical...
waged by prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s to succeed, with the consequence known as infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
by its resulting damage.
One of these rare evolutionary events, about two billion years
Timeline of evolution
This timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
before the present, made it possible for certain eukaryotes and energy-producing prokaryotes not only to coexist, but to mutually benefit from their symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
.
Mitochondriate eukaryotic cells live poised between life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
and death, because mitochondria still retain their repertoire of molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s which can trigger cell suicide. This process has now been evolved to happen only when programmed. Given certain signals
Cell signaling
Cell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue...
to cells (such as feedback from neighbors, stress or DNA damage
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
), mitochondria release caspase
Caspase
Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases that play essential roles in apoptosis , necrosis, and inflammation....
activators which trigger the cell-death inducing biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
cascade. As such, the cell suicide mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
is now crucial to all of our lives.