biology

biology

[bahy-ol-uh-jee]
biology, the science that deals with living things. It is broadly divided into zoology, the study of animal life, and botany, the study of plant life. Subdivisions of each of these sciences include cytology (the study of cells), histology (the study of tissues), anatomy or morphology, physiology, and embryology (the study of the embryonic development of an individual animal or plant). Also included in biological studies are the sciences of genetics, evolution, paleontology, and taxonomy or systematics, the study of classification. The methods and attitudes of other sciences are brought to the study of biology in such fields as biochemistry (physiological chemistry), biophysics (the physics of life processes), bioclimatology and biogeography (ecology), bioengineering (the design of artificial organs), biometry or biostatistics, bioenergetics, and biomathematics. Evidences of early human observations of nature are seen in prehistoric cave art. Biological concepts began to develop among the early Greeks. The biological works of Aristotle include his observations and classification of his large collections of animals. The invention of the microscope in the 16th cent. gave a great stimulus to biology, broadening and deepening its scope and creating the sciences of microbiology, the study of microscopic forms of life, and microscopy, the microscopic study of living cells. Among the many who contributed to the science are Claude Bernard, Cuvier, Darwin, T. H. Huxley, Lamarck, Linnaeus, Mendel, and Pasteur. See marine biology.

See T. Lenoir, The Strategy of Life (1989); C. A. Villee et al., Biology (3d ed. 1989); N. A. Campbell, Biology (3d ed. 1993).

Field of science concerned with the chemical structures and processes of biological phenomena at the molecular level. Having developed out of the related fields of biochemistry, genetics, and biophysics, the discipline is particularly concerned with the study of proteins, nucleic acids, and enzymes. In the early 1950s, growing knowledge of the structure of proteins enabled the structure of DNA to be described. The discovery in the 1970s of certain types of enzymes that can cut and recombine segments of DNA (see recombination) in the chromosomes of certain bacteria made recombinant-DNA technology possible. Molecular biologists use that technology to isolate and modify specific genes (see genetic engineering).

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Science that deals with the animals and plants of the sea and estuaries and with airborne and terrestrial organisms that depend directly on bodies of saltwater for food and other necessities. Marine biologists study the relations between ocean phenomena and the distribution and adaptations of organisms. Of particular interest are adaptations to the chemical and physical properties of seawater, the movements and currents of the ocean, the availability of light at various depths, and the composition of the sea floor. Other important areas of study are marine food chains, the distribution of economically important fish and crustaceans, and the effects of pollution. In the later 19th century, the emphasis was on collecting and cataloging marine organisms, for which special nets, dredges, and trawls were developed. In the 20th century, improved diving equipment, submersible craft, and underwater cameras and television have made direct observation possible.

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Study of the relationships between organisms and their environment. Physiological ecology focuses on the relationships between individual organisms and the physical and chemical features of their environment. Behavioral ecologists study the behaviours of individual organisms as they react to their environment. Population ecology is the study of processes that affect the distribution and abundance of animal and plant populations. Community ecology studies how communities of plant and animal populations function and are organized; it frequently concentrates on particular subsets of organisms such as plant communities or insect communities. Ecosystem ecology examines large-scale ecological issues, ones that often are framed in terms of measures such as biomass, energy flow, and nutrient cycling. Applied ecology applies ecological principles to the management of populations of crops and animals. Theoretical ecologists provide simulations of particular practical problems and develop models of general ecological relevance. Seealso systems ecology.

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Study of cells. Its earliest phase began with Robert Hooke's microscopic investigations of cork in 1665, during which he introduced the term cell to describe dead cork cells. Mathias Jacob Schleiden (in 1838) and Theodor Schwann (1839) were among the first to state clearly that cells are the fundamental units of both plants and animals. This pronouncement (the cell theory) was confirmed and elaborated by a series of discoveries and interpretations. In 1892 Oscar Hertwig (1849–1922) suggested that processes at the organism's level are reflections of cellular processes, thus establishing cytology as a separate branch of biology. Seealso physiology.

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Study of living things and their vital processes. An extremely broad subject, biology is divided into branches. The current approach is based on the levels of biological organization involved (e.g., molecules, cells, individuals, populations) and on the specific topic under investigation (e.g., structure and function, growth and development). According to this scheme, biology's main subdivisions include morphology, physiology, taxonomy, embryology, genetics, and ecology, each of which can be further subdivided. Alternatively, biology can be divided into fields especially concerned with one type of living thing; for example, botany (plants), zoology (animals), ornithology (birds), entomology (insects), mycology (fungi), microbiology (microorganisms), and bacteriology (bacteria). Seealso biochemistry; molecular biology.

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The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell.

In 1837 before the final cell theory was developed, a Czech Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

The word cell comes from the Latin cellula, meaning, a small room. The descriptive name for the smallest living biological structure was chosen by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.

General principles

Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.

All cells have several different abilities:

Some prokaryotic cells contain important internal membrane-bound compartments, but eukaryotic cells have a specialized set of internal membrane compartments. Material is moved between these compartments by regulated traffic and transport of small spheres of membrane-bound material called vesicles.

Anatomy of cells

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

Prokaryotic cells

Prokaryotes differ from eukaryotes since they lack a nuclear envelope and a cell nucleus. Prokaryotes also lack most of the intracellular organelles and structures that are seen in eukaryotic cells. There are two kinds of prokaryotes, bacteria and archaea, but these are similar in the overall structures of their cells. Most functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic cell's plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili — proteins attached to the cell surface; a cell envelope - consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:

  • The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
  • Most prokaryotes have a cell wall (some exceptions are Mycoplasma (bacteria) and Thermoplasma (archaea)). This wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" (cytolysis) from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like plants (cellulose) and fungi, but has a different chemical composition.
  • A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.

Eukaryotic cells

Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus." Other differences include:

  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
  • The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.
  • Eukaryotes can move using cilia or flagella. The flagella are more complex than those of prokaryotes.

Table 1: Comparison of features of prokaryotic and eukaryotic cells
  Prokaryotes Eukaryotes
Typical organisms bacteria, archaea protists, fungi, plants, animals
Typical size ~ 1-10 µm ~ 10-100 µm (sperm cells, apart from the tail, are smaller)
Type of nucleus nucleoid region; no real nucleus real nucleus with double membrane
DNA circular (usually) linear molecules (chromosomes) with histone proteins
RNA-/protein-synthesis coupled in cytoplasm RNA-synthesis inside the nucleus
protein synthesis in cytoplasm
Ribosomes 50S+30S 60S+40S
Cytoplasmatic structure very few structures highly structured by endomembranes and a cytoskeleton
Cell movement flagella made of flagellin flagella and cilia containing microtubules; lamellipodia and filopodia containing actin
Mitochondria none one to several thousand (though some lack mitochondria)
Chloroplasts none in algae and plants
Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells
Cell division Binary fission (simple division) Mitosis (fission or budding)
Meiosis

Table 2: Comparison of structures between animal and plant cells
Typical animal cell Typical plant cell
Organelles

  • Nucleus
  • Rough ER
  • Smooth ER
  • Ribosomes
  • Cytoskeleton
  • Golgi apparatus (dictiosomes)
  • Cytoplasm
  • Mitochondria
  • Vesicles
  • Chloroplast and other plastids
  • Central vacuole(large)
  • Peroxisome (e.g. Glyoxysome)
  • Vacuoles
  • Additional structures

  • Plasma membrane
  • Flagellum (only in gametes)
  • Cell wall
  • Plasmodesmata
  • Subcellular components

    All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell, then briefly describe their function.

    Cell membrane: A cell's defining boundary

    The cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. The plasma membrane in plants and prokaryotes is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signalling molecules such as hormones.

    Cytoskeleton: A cell's scaffold

    The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.

    Genetic material

    Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but some viruses (e.g., retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA) in organisms that use DNA for the genetic code itself. Transfer RNA (tRNA) molecules are used to add specific amino acids during the process of protein translation.

    Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).

    A human cell has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 23 pairs of linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production as well as specific tRNAs.

    Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.

    Organelles

    The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions.

    There are several types of organelles within an animal cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

    {| |- | Cell nucleus (a cell's information center) : The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical in shape and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm. | |- Mitochondria and Chloroplasts (the power generators) : Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by the process of oxidative phosphorylation, utilizing oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by splitting in two.

    Organelles that are modified chloroplasts are broadly called plastids, and are involved in energy storage through the process of photosynthesis, which utilizes solar energy to generate carbohydrates and oxygen from carbon dioxide and water.

    Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell. Both of these organelles contain this DNA in circular plasmids, much like prokaryotic cells, strongly supporting the evolutionary theory of endosymbiosis; since these organelles contain their own genomes and have other similarities to prokaryotes, they are thought to have developed through a symbiotic relationship after being engulfed by a primitive cell.
    |- | Endoplasmic reticulum (eukaryotes only) : The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release. |- | Golgi apparatus (eukaryotes only) : The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. It is particularly important in the processing of proteins for secretion. The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells. Vesicles that enter the Golgi apparatus are processed in a cis to trans direction, meaning they coalesce on the cis side of the apparatus and after processing pinch off on the opposite (trans) side to form a new vesicle in the animal cell. | |- Ribosomes : The ribosome is a large complex of RNA and protein molecules. This is where proteins are produced. Ribosomes can be found either foating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). |- | colspan="2" | Lysosomes and Peroxisomes (eukaryotes only) : Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. These organelles are often called a "suicide bag" because of their ability to detonate and destroy the cell. |- | colspan="2" | Centrosome (the cytoskeleton organiser) : The centrosome produces the microtubules of a cell - a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells. |- | colspan="2" | Vacuoles : Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which are able to pump water out of the cell if there is too much water.

    Structures outside the cell wall

    Capsule

    It is present only in some bacteria outside the cell wall. It is gelatinous in nature. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as bacillus anthracis or hyaluronic acid as in streptococci. Capsules not stained by ordinary stain and can detected by special stain. The capsule is antigenic. The capsule has antiphagocytic function so it determines the virulence of many bacteria. It also plays a role in attachment of the organism to mucous membranes.

    Flagella

    Flagella are the organelles of mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread like appendages, protein in nature, formed of flagellin protein (antigenic). They can not be stained by gram stain. They have a special stain. According to their arrangement they may be monotrichate, amphitrichate, lophotrichate, peritrichate.

    Fimbriae (pili)

    They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachement of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in the process of conjunction.

    Cell functions

    Cell growth and metabolism

    Between successive cell divisions, cells grow through the functioning of cellular metabolism.

    Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into a less chemically-complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways.

    The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.

    The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.

    Creation of new cells

    Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms.

    Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

    DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.

    Protein synthesis

    Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

    Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

    Cell movement or motility

    Cells can move during many processes: such as wound healing, the immune response and cancer metastasis. For wound healing to occur, white blood cells and cells that ingest bacteria move to the wound site to kill the microorganisms that cause infection. At the same time fibroblasts (connective tissue cells) move there to remodel damaged structures. In the case of tumor development, cells from a primary tumor move away and spread to other parts of the body. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. The process is divided into three steps - protrusion of the leading edge of the cell, adhesion of the leading edge and deadhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each of these steps is driven by physical forces generated by unique segments of the cytoskeleton.

    Origins of cells

    The origin of cells has to do with the origin of life, which began the history of life on Earth. The birth of the cell marked the passage from prebiotic chemistry to life.

    Origin of the first cell

    For more information, RNA world hypothesis
    For more information, Last universal ancestor

    The unit of selection in modern organisms and populations of organisms is not clear, with natural selection being proposed to work at the level of genes, cells, individual organisms, groups of organisms and even species. None of these models are mutually-exclusive and selection may act on multiple levels simultaneously. However, in a gene-centered view of evolution, life is regarded in terms of replicators—that is the DNA molecules in the organism. If freely-floating DNA molecules that code for enzymes are not enclosed in cells, the enzymes that benefit a given replicator (for example, by producing nucleotides) may do so less efficiently, and may in fact benefit competing replicators. If the entire DNA molecule of a replicator is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The replicator will directly benefit from its encoded enzymes.

    Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear many of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules may have been the first cellular life forms on Earth. Some amphiphiles have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane composed of proteins and phospholipid bilayer membranes.

    Origin of eukaryotic cells

    The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory.

    There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells.

    Sex, as the stereotyped choreography of meiosis and syngamy that persists in nearly all extant eukaryotes, may have played a role in the transition from prokaryotes to eukaryotes. An 'origin of sex as vaccination' theory suggests that the eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer. Sex-as-syngamy (fusion sex) arose when infected hosts began swapping nuclearized genomes containing coevolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.

    History

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