Cell (biology)

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Not to be confused with Cell biology. Cell (biology)_sentence_0

Cell (biology)_table_infobox_0

CellCell (biology)_header_cell_0_0_0
IdentifiersCell (biology)_header_cell_0_1_0
MeSHCell (biology)_header_cell_0_2_0 Cell (biology)_cell_0_2_1
THCell (biology)_header_cell_0_3_0 Cell (biology)_cell_0_3_1
FMACell (biology)_header_cell_0_4_0 Cell (biology)_cell_0_4_1

The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cell (biology)_sentence_1

A cell is the smallest unit of life. Cell (biology)_sentence_2

Cells are often called the "building blocks of life". Cell (biology)_sentence_3

The study of cells is called cell biology, cellular biology, or cytology. Cell (biology)_sentence_4

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Cell (biology)_sentence_5

Most plant and animal cells are only visible under a light microscope, with dimensions between 1 and 100 micrometres. Cell (biology)_sentence_6

Electron microscopy gives a much higher resolution showing greatly detailed cell structure. Cell (biology)_sentence_7

Organisms can be classified as unicellular (consisting of a single cell such as bacteria) or multicellular (including plants and animals). Cell (biology)_sentence_8

Most unicellular organisms are classed as microorganisms. Cell (biology)_sentence_9

The number of cells in plants and animals varies from species to species; it has been estimated that humans contain somewhere around 40 trillion (4×10) cells. Cell (biology)_sentence_10

The human brain accounts for around 80 billion of these cells. Cell (biology)_sentence_11

Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. Cell (biology)_sentence_12

Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells. Cell (biology)_sentence_13

Cells emerged on Earth at least 3.5 billion years ago. Cell (biology)_sentence_14

Cell types Cell (biology)_section_0

Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic, which do not. Cell (biology)_sentence_15

Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Cell (biology)_sentence_16

Prokaryotic cells Cell (biology)_section_1

Main article: Prokaryote Cell (biology)_sentence_17

Prokaryotes include bacteria and archaea, two of the three domains of life. Cell (biology)_sentence_18

Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling. Cell (biology)_sentence_19

They are simpler and smaller than eukaryotic cells, and lack a nucleus, and other membrane-bound organelles. Cell (biology)_sentence_20

The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. Cell (biology)_sentence_21

The nuclear region in the cytoplasm is called the nucleoid. Cell (biology)_sentence_22

Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter. Cell (biology)_sentence_23

A prokaryotic cell has three regions: Cell (biology)_sentence_24

Cell (biology)_unordered_list_0

  • Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.Cell (biology)_item_0_0
  • Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.Cell (biology)_item_0_1
  • On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.Cell (biology)_item_0_2

Eukaryotic cells Cell (biology)_section_2

Main article: Eukaryote Cell (biology)_sentence_25

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. Cell (biology)_sentence_26

These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. Cell (biology)_sentence_27

The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific activities take place. Cell (biology)_sentence_28

Most important among these is a cell nucleus, an organelle that houses the cell's DNA. Cell (biology)_sentence_29

This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Cell (biology)_sentence_30

Other differences include: Cell (biology)_sentence_31

Cell (biology)_unordered_list_1

  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.Cell (biology)_item_1_3
  • 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.Cell (biology)_item_1_4
  • Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Each cilium may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."Cell (biology)_item_1_5
  • Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants. Eukaryotic flagella are more complex than those of prokaryotes.Cell (biology)_item_1_6

Cell (biology)_table_general_1

Comparison of features of prokaryotic and eukaryotic cellsCell (biology)_table_caption_1
Cell (biology)_header_cell_1_0_0 ProkaryotesCell (biology)_header_cell_1_0_1 EukaryotesCell (biology)_header_cell_1_0_2
Typical organismsCell (biology)_header_cell_1_1_0 bacteria, archaeaCell (biology)_cell_1_1_1 protists, fungi, plants, animalsCell (biology)_cell_1_1_2
Typical sizeCell (biology)_header_cell_1_2_0 ~ 1–5 μmCell (biology)_cell_1_2_1 ~ 10–100 μmCell (biology)_cell_1_2_2
Type of nucleusCell (biology)_header_cell_1_3_0 nucleoid region; no true nucleusCell (biology)_cell_1_3_1 true nucleus with double membraneCell (biology)_cell_1_3_2
DNACell (biology)_header_cell_1_4_0 circular (usually)Cell (biology)_cell_1_4_1 linear molecules (chromosomes) with histone proteinsCell (biology)_cell_1_4_2
RNA/protein synthesisCell (biology)_header_cell_1_5_0 coupled in the cytoplasmCell (biology)_cell_1_5_1 RNA synthesis in the nucleus

protein synthesis in the cytoplasmCell (biology)_cell_1_5_2

RibosomesCell (biology)_header_cell_1_6_0 50S and 30SCell (biology)_cell_1_6_1 60S and 40SCell (biology)_cell_1_6_2
Cytoplasmic structureCell (biology)_header_cell_1_7_0 very few structuresCell (biology)_cell_1_7_1 highly structured by endomembranes and a cytoskeletonCell (biology)_cell_1_7_2
Cell movementCell (biology)_header_cell_1_8_0 flagella made of flagellinCell (biology)_cell_1_8_1 flagella and cilia containing microtubules; lamellipodia and filopodia containing actinCell (biology)_cell_1_8_2
MitochondriaCell (biology)_header_cell_1_9_0 noneCell (biology)_cell_1_9_1 one to several thousandCell (biology)_cell_1_9_2
ChloroplastsCell (biology)_header_cell_1_10_0 noneCell (biology)_cell_1_10_1 in algae and plantsCell (biology)_cell_1_10_2
OrganizationCell (biology)_header_cell_1_11_0 usually single cellsCell (biology)_cell_1_11_1 single cells, colonies, higher multicellular organisms with specialized cellsCell (biology)_cell_1_11_2
Cell divisionCell (biology)_header_cell_1_12_0 binary fission (simple division)Cell (biology)_cell_1_12_1 mitosis (fission or budding)

meiosisCell (biology)_cell_1_12_2

ChromosomesCell (biology)_header_cell_1_13_0 single chromosomeCell (biology)_cell_1_13_1 more than one chromosomeCell (biology)_cell_1_13_2
MembranesCell (biology)_header_cell_1_14_0 cell membraneCell (biology)_cell_1_14_1 Cell membrane and membrane-bound organellesCell (biology)_cell_1_14_2

Subcellular components Cell (biology)_section_3

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Cell (biology)_sentence_32

Inside the membrane, the cytoplasm takes up most of the cell's volume. Cell (biology)_sentence_33

All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) 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. Cell (biology)_sentence_34

There are also other kinds of biomolecules in cells. Cell (biology)_sentence_35

This article lists these primary cellular components, then briefly describes their function. Cell (biology)_sentence_36

Membrane Cell (biology)_section_4

Main article: Cell membrane Cell (biology)_sentence_37

The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. Cell (biology)_sentence_38

In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. Cell (biology)_sentence_39

This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Cell (biology)_sentence_40

Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Cell (biology)_sentence_41

Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. Cell (biology)_sentence_42

The membrane is semi-permeable, and selectively 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 (biology)_sentence_43

Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones. Cell (biology)_sentence_44

Cytoskeleton Cell (biology)_section_5

Main article: Cytoskeleton Cell (biology)_sentence_45

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. Cell (biology)_sentence_46

The eukaryotic cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. Cell (biology)_sentence_47

In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments. Cell (biology)_sentence_48

There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. Cell (biology)_sentence_49

The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. Cell (biology)_sentence_50

The subunit protein of microfilaments is a small, monomeric protein called actin. Cell (biology)_sentence_51

The subunit of microtubules is a dimeric molecule called tubulin. Cell (biology)_sentence_52

Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Cell (biology)_sentence_53

But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M). Cell (biology)_sentence_54

Genetic material Cell (biology)_section_6

Main articles: DNA and RNA Cell (biology)_sentence_55

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cell (biology)_sentence_56

Cells use DNA for their long-term information storage. Cell (biology)_sentence_57

The biological information contained in an organism is encoded in its DNA sequence. Cell (biology)_sentence_58

RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Cell (biology)_sentence_59

Transfer RNA (tRNA) molecules are used to add amino acids during protein translation. Cell (biology)_sentence_60

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Cell (biology)_sentence_61

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). Cell (biology)_sentence_62

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). Cell (biology)_sentence_63

In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. Cell (biology)_sentence_64

The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Cell (biology)_sentence_65

Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs. Cell (biology)_sentence_66

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. Cell (biology)_sentence_67

This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Cell (biology)_sentence_68

Certain viruses also insert their genetic material into the genome. Cell (biology)_sentence_69

Organelles Cell (biology)_section_7

Main article: Organelle Cell (biology)_sentence_70

Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function). Cell (biology)_sentence_71

Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound. Cell (biology)_sentence_72

There are several types of organelles in a cell. Cell (biology)_sentence_73

Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). Cell (biology)_sentence_74

The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles. Cell (biology)_sentence_75

Eukaryotic Cell (biology)_section_8

Cell (biology)_unordered_list_2

  • 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 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 messenger RNA (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.Cell (biology)_item_2_7
  • Mitochondria and chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.Cell (biology)_item_2_8

Cell (biology)_unordered_list_3

  • Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. The smooth ER plays a role in calcium sequestration and release.Cell (biology)_item_3_9
  • Golgi apparatus: 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.Cell (biology)_item_3_10
  • Lysosomes and peroxisomes: 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.Cell (biology)_item_3_11
  • 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.Cell (biology)_item_3_12
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.Cell (biology)_item_3_13

Eukaryotic and prokaryotic Cell (biology)_section_9

Cell (biology)_unordered_list_4

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).Cell (biology)_item_4_14

Structures outside the cell membrane Cell (biology)_section_10

Many cells also have structures which exist wholly or partially outside the cell membrane. Cell (biology)_sentence_76

These structures are notable because they are not protected from the external environment by the semipermeable cell membrane. Cell (biology)_sentence_77

In order to assemble these structures, their components must be carried across the cell membrane by export processes. Cell (biology)_sentence_78

Cell wall Cell (biology)_section_11

Further information: Cell wall Cell (biology)_sentence_79

Many types of prokaryotic and eukaryotic cells have a cell wall. Cell (biology)_sentence_80

The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Cell (biology)_sentence_81

Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan. Cell (biology)_sentence_82

Prokaryotic Cell (biology)_section_12

Capsule Cell (biology)_section_13

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. Cell (biology)_sentence_83

The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Cell (biology)_sentence_84

Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue; which allows for higher contrast between the cells for observation. Cell (biology)_sentence_85

Flagella Cell (biology)_section_14

Flagella are organelles for cellular mobility. Cell (biology)_sentence_86

The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. Cell (biology)_sentence_87

They are long and thick thread-like appendages, protein in nature. Cell (biology)_sentence_88

A different type of flagellum is found in archaea and a different type is found in eukaryotes. Cell (biology)_sentence_89

Fimbriae Cell (biology)_section_15

A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Cell (biology)_sentence_90

Fimbriae are formed of a protein called pilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). Cell (biology)_sentence_91

There are special types of pili involved in bacterial conjugation. Cell (biology)_sentence_92

Cellular processes Cell (biology)_section_16

Replication Cell (biology)_section_17

Main article: Cell division Cell (biology)_sentence_93

Cell division involves a single cell (called a mother cell) dividing into two daughter cells. Cell (biology)_sentence_94

This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Cell (biology)_sentence_95

Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. Cell (biology)_sentence_96

A diploid cell may also undergo meiosis to produce haploid cells, usually four. Cell (biology)_sentence_97

Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells. Cell (biology)_sentence_98

DNA replication, or the process of duplicating a cell's genome, always happens when a cell divides through mitosis or binary fission. Cell (biology)_sentence_99

This occurs during the S phase of the cell cycle. Cell (biology)_sentence_100

In meiosis, the DNA is replicated only once, while the cell divides twice. Cell (biology)_sentence_101

DNA replication only occurs before meiosis I. Cell (biology)_sentence_102

DNA replication does not occur when the cells divide the second time, in meiosis II. Cell (biology)_sentence_103

Replication, like all cellular activities, requires specialized proteins for carrying out the job. Cell (biology)_sentence_104

DNA repair Cell (biology)_section_18

Main article: DNA repair Cell (biology)_sentence_105

In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out repair processes when damages are detected. Cell (biology)_sentence_106

Diverse repair processes have evolved in organisms ranging from bacteria to humans. Cell (biology)_sentence_107

The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damages that could lead to mutation. Cell (biology)_sentence_108

E. Cell (biology)_sentence_109 coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. Cell (biology)_sentence_110

These include: (1) nucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) recombinational repair and (5) light-dependent repair (photoreactivation). Cell (biology)_sentence_111

Growth and metabolism Cell (biology)_section_19

Main articles: Cell growth and Metabolism Cell (biology)_sentence_112

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell (biology)_sentence_113

Cell metabolism is the process by which individual cells process nutrient molecules. Cell (biology)_sentence_114

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. Cell (biology)_sentence_115

Complex sugars consumed by the organism can be broken down into simpler sugar molecules called monosaccharides such as glucose. Cell (biology)_sentence_116

Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a molecule that possesses readily available energy, through two different pathways. Cell (biology)_sentence_117

Protein synthesis Cell (biology)_section_20

Main article: Protein biosynthesis Cell (biology)_sentence_118

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. Cell (biology)_sentence_119

This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Cell (biology)_sentence_120

Protein synthesis generally consists of two major steps: transcription and translation. Cell (biology)_sentence_121

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. Cell (biology)_sentence_122

This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. Cell (biology)_sentence_123

mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. Cell (biology)_sentence_124

The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. Cell (biology)_sentence_125

The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. Cell (biology)_sentence_126

The new polypeptide then folds into a functional three-dimensional protein molecule. Cell (biology)_sentence_127

Motility Cell (biology)_section_21

Main article: Motility Cell (biology)_sentence_128

Unicellular organisms can move in order to find food or escape predators. Cell (biology)_sentence_129

Common mechanisms of motion include flagella and cilia. Cell (biology)_sentence_130

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. Cell (biology)_sentence_131

For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell (biology)_sentence_132

Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. Cell (biology)_sentence_133

The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Cell (biology)_sentence_134

Each step is driven by physical forces generated by unique segments of the cytoskeleton. Cell (biology)_sentence_135

Navigation, control and communication Cell (biology)_section_22

See also: Cybernetics § In biology Cell (biology)_sentence_136

In August 2020, scientists described one way cells – in particular cells of a slime mold and mouse pancreatic cancer–derived cells – are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners. Cell (biology)_sentence_137

Multicellularity Cell (biology)_section_23

Main article: Multicellular organism Cell (biology)_sentence_138

Cell specialization/differentiation Cell (biology)_section_24

Main article: Cellular differentiation Cell (biology)_sentence_139

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Cell (biology)_sentence_140

In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions. Cell (biology)_sentence_141

In mammals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell (biology)_sentence_142

Cell types differ both in appearance and function, yet are genetically identical. Cell (biology)_sentence_143

Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain. Cell (biology)_sentence_144

Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development. Cell (biology)_sentence_145

Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division). Cell (biology)_sentence_146

Origin of multicellularity Cell (biology)_section_25

Main article: Multicellular organism Cell (biology)_sentence_147

Multicellularity has evolved independently at least 25 times, including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. Cell (biology)_sentence_148

However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants. Cell (biology)_sentence_149

It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae. Cell (biology)_sentence_150

Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships. Cell (biology)_sentence_151

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago. Cell (biology)_sentence_152

Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon. Cell (biology)_sentence_153

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure. Cell (biology)_sentence_154

Origins Cell (biology)_section_26

Main article: Evolutionary history of life Cell (biology)_sentence_155

The origin of cells has to do with the origin of life, which began the history of life on Earth. Cell (biology)_sentence_156

Origin of the first cell Cell (biology)_section_27

Further information: Abiogenesis and Evolution of cells Cell (biology)_sentence_157

There are several theories about the origin of small molecules that led to life on the early Earth. Cell (biology)_sentence_158

They may have been carried to Earth on meteorites (see Murchison meteorite), created at deep-sea vents, or synthesized by lightning in a reducing atmosphere (see Miller–Urey experiment). Cell (biology)_sentence_159

There is little experimental data defining what the first self-replicating forms were. Cell (biology)_sentence_160

RNA is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see RNA world hypothesis), but some other entity with the potential to self-replicate could have preceded RNA, such as clay or peptide nucleic acid. Cell (biology)_sentence_161

Cells emerged at least 3.5 billion years ago. Cell (biology)_sentence_162

The current belief is that these cells were heterotrophs. Cell (biology)_sentence_163

The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Cell (biology)_sentence_164

Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form. Cell (biology)_sentence_165

Origin of eukaryotic cells Cell (biology)_section_28

Further information: Evolution of sexual reproduction Cell (biology)_sentence_166

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. Cell (biology)_sentence_167

DNA-bearing organelles like the mitochondria and the chloroplasts are descended from ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, which were endosymbiosed by an ancestral archaean prokaryote. Cell (biology)_sentence_168

There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells. Cell (biology)_sentence_169

History of research Cell (biology)_section_29

Main article: Cell theory Cell (biology)_sentence_170

Cell (biology)_unordered_list_5

See also Cell (biology)_section_30

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