4.0+The+Cell

the cell A eukaryotic cell cannot divide into two, the two into four, etc. unless two processes alternate: The period between M and S is called **G1**; that between S and M is **G2**.
 * **doubling** of its [|genome] (**DNA**) in **S phase** (synthesis phase) of the cell cycle;
 * **halving** of that genome during [|mitosis] (**M phase**).

So, the cell cycle consists of: When a cell is in any phase of the cell cycle other than mitosis, it is often said to be in **interphase**.
 * **G1** = growth and preparation of the chromosomes for replication;
 * **S** = synthesis of DNA [see [|DNA Replication]] and duplication of the [|centrosome];
 * **G2** =
 * **M** = [|mitosis].

The passage of a cell through the cell cycle is controlled by proteins in the cytoplasm. Among the main players in animal cells are: Their levels in the cell rise and fall with the stages of the cell cycle. Their levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. They add phosphate groups to a variety of protein substrates that control processes in the cell cycle.
 * 
 * a **G1 cyclins** (D cyclins)
 * **S-phase cyclins** (cyclins E and A)
 * **mitotic cyclins** (B cyclins)
 * **<span style="font-family: Arial,Helvetica,sans-serif;">Cyclin-dependent [|kinases] (Cdks) **
 * <span style="font-family: Arial,Helvetica,sans-serif;">a **G1 Cdk** (Cdk4)
 * <span style="font-family: Arial,Helvetica,sans-serif;">an **S-phase Cdk** (Cdk2)
 * <span style="font-family: Arial,Helvetica,sans-serif;">an **M-phase Cdk** (Cdk1)
 * <span style="font-family: Arial,Helvetica,sans-serif;">The **anaphase-promoting complex (APC)**. (The APC is also called the **cyclosome**, and the complex is often designated as the **APC/C**.) The APC/C
 * <span style="font-family: Arial,Helvetica,sans-serif;">triggers the events leading to destruction of the **cohesins** (as described below) thus allowing the sister chromatids to separate;
 * <span style="font-family: Arial,Helvetica,sans-serif;">degrades the mitotic (B) cyclins.

<span style="font-family: Arial,Helvetica,sans-serif;"> in the cycle
//<span style="display: block; font-family: Tahoma,Geneva,sans-serif; font-size: 80%; text-align: right;">taken from:http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellCycle.html // >> >> >> >> >> >> >> The **cell** is the functional basic unit of life. It was discovered by Robert Hooke and is the 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.[1] Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have about 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 unfertilised ostrich egg cell.[//clarification needed How large?//][2] >> In 1835, before the final cell theory was developed, 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, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[3] >> The word //cell// comes from the Latin //cellula//, meaning, a small room. The descriptive term for the smallest living biological structure was coined 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.
 * <span style="font-family: Arial,Helvetica,sans-serif;">A rising level of **G1-cyclins** bind to their Cdks and signal the cell to prepare the chromosomes for replication.
 * <span style="font-family: Arial,Helvetica,sans-serif;">A rising level of **S-phase promoting factor** (**SPF**) — which includes A cyclins bound to Cdk2 — enters the nucleus and prepares the cell to duplicate its DNA (and its centrosomes).
 * <span style="font-family: Arial,Helvetica,sans-serif;">As DNA replication continues, cyclin E is destroyed, and the level of mitotic cyclins begins to rise (in G2).
 * <span style="font-family: Arial,Helvetica,sans-serif;">**M-phase promoting factor** (the complex of mitotic [B] cyclins with the M-phase Cdk [Cdk1]) initiates assembly of the mitotic spindle breakdown of the nuclear envelope cessation of all gene transcription condensation of the chromosomes
 * <span style="font-family: Arial,Helvetica,sans-serif;">These events take the cell to **[|metaphase]** of mitosis.
 * <span style="font-family: Arial,Helvetica,sans-serif;">At this point, the M-phase promoting factor activates the **anaphase-promoting complex** (**APC/C**) which
 * <span style="font-family: Arial,Helvetica,sans-serif;">allows the sister chromatids at the metaphase plate to separate and move to the poles (= anaphase), completing mitosis.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Separation of the sister chromatids depends on the breakdown of the **cohesins** that have been holding them together. It works like this.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Cohesin breakdown is caused by a [|protease] called **separase** (also known as separin).
 * <span style="font-family: Arial,Helvetica,sans-serif;">Separase is kept **inactive** until late metaphase by an inhibitory [|chaperone] called **securin**.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Anaphase begins when the anaphase promoting complex (APC/C) destroys securin (by tagging it with ubiquitin for deposit in a [|proteasome]) thus ending its inhibition of separase and allowing
 * <span style="font-family: Arial,Helvetica,sans-serif;">separase to break down the cohesins. ||
 * <span style="font-family: Arial,Helvetica,sans-serif;">destroys **B cyclins**. This is also done by attaching them to **ubiquitin** which targets them for destruction by **proteasomes**.
 * <span style="font-family: Arial,Helvetica,sans-serif;">[|Link to discussion of proteasomes.] ||
 * <span style="font-family: Arial,Helvetica,sans-serif;">turns on synthesis of G1 cyclins (D) for the next turn of the cycle.
 * <span style="font-family: Arial,Helvetica,sans-serif;">degrades geminin, a protein that has kept the freshly-synthesized DNA in S phase from being re-replicated before mitosis.
 * <span style="font-family: Arial,Helvetica,sans-serif;">This is only one mechanism by which the cell ensures that every portion of its genome is copied once — and only once — during S phase. [|Link to discussion.] ||

<span style="font-family: Arial,Helvetica,sans-serif;">Anatomy of cells
<span style="font-family: Arial,Helvetica,sans-serif;">There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

<span style="font-family: Arial,Helvetica,sans-serif;">Prokaryotic cells
<span style="font-family: Arial,Helvetica,sans-serif;">Main article: ProkaryoteDiagram of a typical prokaryotic cell The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar structure. A prokaryotic cell has three architectural regions:
 * <span style="font-family: Arial,Helvetica,sans-serif;">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;
 * <span style="font-family: Arial,Helvetica,sans-serif;">Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as //Mycoplasma// (bacteria) and //Thermoplasma// (archaea). 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 finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cells and fungi cells) also have a cell wall;
 * <span style="font-family: Arial,Helvetica,sans-serif;">Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium //Borrelia burgdorferi//, which causes Lyme disease). Though not forming a //nucleus//, the DNA is condensed in a //nucleoid//. Prokaryotes can carry extrachromosomal DNA elements called //plasmids//, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance.

<span style="font-family: Arial,Helvetica,sans-serif;">Eukaryotic cells
Main article: EukaryoteDiagram of a typical animal (eukaryotic) cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles within centrosome Eukaryotic cells are about 15 times wider than 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 a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Other differences include: > <span style="font-family: Arial,Helvetica,sans-serif;">, 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. protein synthesis in cytoplasm || Meiosis ||
 * 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
 * Many eukaryotic cells are ciliated with //primary cilia//. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[5]
 * Eukaryotes can move using //motile 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
 * ~ 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)


 * Table 2: Comparison of structures between animal and plant cells**|| ||~ Typical animal cell ||~ Typical plant cell ||
 * ~ Organelles || * Nucleus
 * Nucleolus (within nucleus)
 * Rough endoplasmic reticulum (ER)
 * Smooth ER
 * Ribosomes
 * Cytoskeleton
 * Golgi apparatus
 * Cytoplasm
 * Mitochondria
 * Vesicles
 * Lysosomes
 * Centrosome
 * Centrioles || * Nucleus
 * Nucleolus (within nucleus)
 * Rough ER
 * Smooth ER
 * Ribosomes
 * Cytoskeleton
 * Golgi apparatus (dictiosomes)
 * Cytoplasm
 * Mitochondria
 * Plastids and its derivatives
 * Vacuole(s)
 * Cell wall ||

Subcellular components
The cells of eukaryotes (left) and prokaryotes (right) 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
Main article: Cell membrane 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 signaling molecules such as hormones.

Cytoskeleton: A cell's scaffold
Main article: CytoskeletonBovine Pulmonary Artery Endothelial cell: nuclei stained blue, mitochondria stained red, and F-actin, an important component in microfilaments, stained green. Cell imaged on a fluorescent microscope. 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.[6]

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 amino acids during 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 contained in the cell 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 and 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
Main article: Organelle 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. 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 oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by splitting in two. Respiration occurs in the cell mitochondria.Organelles that are modified chloroplasts are broadly called plastids, and are involved in energy storage through photosynthesis, which uses solar energy to generate carbohydrates and oxygen from carbon dioxide and water.[//citation needed//]Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell. Both 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.[//citation needed//]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).[7]
 * 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 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. || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Diagram_human_cell_nucleus_no_text.png/220px-Diagram_human_cell_nucleus_no_text.png width="220" height="210" link="http://alcascience6b.wikispaces.com/page/edit/wiki/File:Diagram_human_cell_nucleus_no_text.png"]][[image:http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png width="15" height="11" link="http://alcascience6b.wikispaces.com/page/edit/wiki/File:Diagram_human_cell_nucleus_no_text.png"]]Diagram of a cell nucleus ||
 * 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.[//citation needed//] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/Endomembrane_system_diagram_no_text_nucleus.png/220px-Endomembrane_system_diagram_no_text_nucleus.png width="220" height="237" link="http://alcascience6b.wikispaces.com/page/edit/wiki/File:Endomembrane_system_diagram_no_text_nucleus.png"]][[image:http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png width="15" height="11" link="http://alcascience6b.wikispaces.com/page/edit/wiki/File:Endomembrane_system_diagram_no_text_nucleus.png"]]Diagram of an endomembrane system ||
 * 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.[//citation needed//] ||
 * 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.[//citation needed//] ||
 * 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 can pump water out of the cell if there is too much water. The vacuoles of eukaryotic cells are usually larger in those of plants than animals. ||

Capsule
A gelatinous capsule is present in some bacteria outside the cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as //Bacillus anthracis// or hyaluronic acid as in streptococci.[//citation needed//] Capsules are not marked by ordinary stain and can be 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.[//citation needed//]

Flagella
Flagella are the organelles of cellular mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread-like appendages, protein in nature. Are most commonly found in bacteria cells but are found in animal cells as well.

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

Cell growth and metabolism
Main articles: 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, through 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 can generate enough ATP to run all the cell functions. An overview of protein synthesis. Within the nucleus of the cell (//light blue//), genes (DNA, //dark blue//) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (//red//) that is then transported out of the nucleus and into the cytoplasm (//peach//), where it undergoes translation into a protein. mRNA is translated by ribosomes (//purple//) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (//black//) are often further modified, such as by binding to an effector molecule (//orange//), to become fully active.

Creation of new cells
Main article: Cell division 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
Main article: Protein biosynthesis 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.[8] 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. Each step is driven by physical forces generated by unique segments of the cytoskeleton.[9][10]

Evolution
Main article: Evolutionary history of life The origin of cells has to do with the origin of life, which began the history of life on Earth.

Origin of the first cell
Further information: Abiogenesis There are several theories about the origin of small molecules that could lead to life in an early Earth. One is that they came from meteorites (//see Murchison meteorite//). Another is that they were created at deep-sea vents. A third is that they were synthesized by lightning in a reducing atmosphere (//see Miller–Urey experiment//); althougyze chemical reactions (//see RNA world hypothesis//). But some other entity with the potential to self-replicate could have preceded RNA, like clay or peptide nucleic acid.[11] Cells emerged at least 4.0–4.3 billion years ago. The current belief is that these cells were heterotrophs. An important characteristic of cells is the cell membrane, composed of a bilayer of lipids. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. 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.[12]

Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are almost certainly 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 co-evolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.[13]

History

 * 1632 – 1723: Antonie van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as //Vorticella// from rain water, and bacteria from his own mouth.
 * 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.[4]
 * 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
 * The belief that life forms can occur spontaneously (//generatio spontanea//) is contradicted by Louis Pasteur (1822 – 1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
 * 1855: Rudolph Virchow states that cells always emerge from cell divisions (//omnis cellula ex cellula//).
 * 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously-unresolvable organelles.
 * 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
 * 1981: Lynn Margulis published //Symbiosis in Cell Evolution// detailing the endosymbiotic theory.

takem from:http://en.wikipedia.org/wiki/Cell_(biology)



//amoeba proteus// || of the type: //E. coli// || of a poplar tree || The cell is one of the most basic units of life. There are millions of different types of cells. There are cells that are organisms onto themselves, such as microscopic amoeba and bacteria cells. And there are cells that only function when part of a larger organism, such as the cells that make up your body. The cell is the smallest unit of life in our bodies. In the body, there are brain cells, skin cells, liver cells, stomach cells, and the list goes on. All of these cells have unique functions and features. And all have some recognizable similarities. All cells have a 'skin', called the **plasma membrane**, protecting it from the outside environment. The cell membrane regulates the movement of water, nutrients and wastes into and out of the cell. Inside of the cell membrane are the working parts of the cell. At the center of the cell is the cell **nucleus**. The cell nucleus contains the cell's [|DNA], the genetic code that coordinates protein synthesis. In addition to the nucleus, there are many **organelles** inside of the cell - small structures that help carry out the day-to-day operations of the cell. One important cellular organelle is the **ribosome**. Ribosomes participate in protein synthesis. The [|transcription] phase of protein synthesis takes places in the cell nucleus. After this step is complete, the mRNA leaves the nucleus and travels to the cell's ribosomes, where [|translation] occurs. Another important cellular organelle is the**mitochondrion**. Mitochondria (many mitochondrion) are often referred to as the power plants of the cell because many of the reactions that produce energy take place in mitochondria. Also important in the life of a cell are the **lysosomes**. Lysosomes are organelles that contain enzymes that aid in the digestion of nutrient molecules and other materials. Below is a labelled diagram of a cell to help you identify some of these structures.
 * [[image:http://web.jjay.cuny.edu/%7Eacarpi/NSC/images/amoebaproteus.gif width="75" height="120" align="center"]] ||
 * The one-celled organism
 * [[image:http://web.jjay.cuny.edu/%7Eacarpi/NSC/images/e_coli.gif width="75" height="53" align="center"]] ||
 * A single-celled bacteria
 * [[image:http://web.jjay.cuny.edu/%7Eacarpi/NSC/images/redbloodcells.gif width="72" height="67" align="center"]] ||
 * A human red blood cell ||
 * [[image:http://web.jjay.cuny.edu/%7Eacarpi/NSC/images/poplarleaf.gif width="75" height="74" align="center"]] ||
 * A plant cell from the leaf



<span style="font-family: Arial,Helvetica,sans-serif; font-size: 80%;"> There are many different types of cells. One major difference in cells occurs between plant cells and animal cells. While both plant and animal cells contain the structures discussed above, plant cells have some additional specialized structures. Many animals have skeletons to give their body structure and support. Plants do not have a skeleton for support and yet plants don't just flop over in a big spongy mess. This is because of a unique cellular structure called the **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 80%;">cell wall **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 80%;">. The cell wall is a rigid structure outside of the cell membrane composed mainly of the polysaccharide [|cellulose]. As pictured at left, the cell wall gives the plant cell a defined shape which helps support individual parts of plants. In addition to the cell wall, plant cells contain an organelle called the **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 80%;">chloroplast **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 80%;">. The chloroplast allow plants to harvest energy from sunlight. Specialized pigments in the chloroplast (including the common green pigment chlorophyll) absorb sunlight and use this energy to complete the chemical reaction: 6 CO2 + 6 H2O + energy (from sunlight)

C6H12O6 + 6 O2 In this way, plant cells manufacture glucose and other [|carbohydrates] that they can store for later use. Organisms contain many different types of cells that perform many different functions. In the next lesson, we will examine how individual cells come together to form larger structures in the human body. taken from: <span style="font-family: 'Arial Black',Gadget,sans-serif; font-size: 80%;">[|**//http://web.jjay.cuny.edu/~acarpi/NSC/13-cells.htm//**]

<span style="color: #ff0000; font-family: Tahoma,Geneva,sans-serif; font-size: 130%;">the cell

For the scientific journal, see [|The Plant Cell]. Plant cell structure <span style="font-family: Arial,Helvetica,sans-serif;">**Plant cells** are [|eukaryotic] cells that differ in several key respects from the [|cells] of other [|eukaryotic] [|organisms]. Their distinctive features include:
 * <span style="font-family: Arial,Helvetica,sans-serif;">A large central [|vacuole], a water-filled volume enclosed by a membrane known as the //[|tonoplast]//[|[1]][|[2]] maintains the cell's [|turgor], controls movement of [|molecules] between the [|cytosol] and [|sap], stores useful material and digests waste [|proteins] and [|organelles].
 * <span style="font-family: Arial,Helvetica,sans-serif;">A [|cell wall] composed of [|cellulose] and [|hemicellulose], [|pectin] and in many cases [|lignin], and secreted by the [|protoplast] on the outside of the [|cell membrane]. This contrasts with the cell walls of [|fungi] (which are made of [|chitin]), and of [|bacteria], which are made of [|peptidoglycan].
 * <span style="font-family: Arial,Helvetica,sans-serif;">Specialised cell-cell communication pathways known as [|plasmodesmata][|[3]], pores in the primary cell wall through which the [|plasmalemma] and [|endoplasmic reticulum][|[4]] of adjacent cells are continuous.
 * <span style="font-family: Arial,Helvetica,sans-serif;">[|Plastids], notably the [|chloroplasts] which contain [|chlorophyll] and the biochemical systems for light harvesting and [|photosynthesis], but also [|amyloplasts] specialized for [|starch] storage, [|elaioplasts] specialized for [|fat] storage and [|chromoplasts] specialized for synthesis and storage of [|pigments]. As in [|mitochondria], which have a genome encoding 37 genes[|[5]] plastids have their own [|genomes] of about 100-120 unique [|genes][|[6]] and probably arose as [|prokaryotic] [|endosymbionts] living in the cells of an early [|eukaryotic] ancestor of the [|land plants] and [|algae].[|[7]]

take for [[http://en.wikipedia.org/wiki/|http://en.wikipedia.org/wiki/

]]
 * Cells** are the structural and functional units of all living organisms. Some organisms, such as bacteria, are **unicellular**, consisting of a single cell. Other organisms, such as humans, are **multicellular**, or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.

//taken from:http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html//