Molecular biology is a branch of biology that concerns the molecular basis of biological activity between biomolecules in the various systems of a cell, including the interactions between DNA, RNA, proteins and their biosynthesis, as well as the regulation of these interactions. Writing in Nature in 1961, William Astbury described molecular biology as:...not so much a technique as an approach, an approach from the viewpoint of the so-called basic sciences with the leading idea of searching below the large-scale manifestations of classical biology for the corresponding molecular plan. It is concerned with the forms of biological molecules and is predominantly three-dimensional and structural – which does not mean, that it is a refinement of morphology, it must at the same time inquire into function. Researchers in molecular biology use specific techniques native to molecular biology but combine these with techniques and ideas from genetics and biochemistry. There is not a defined line between these disciplines.
This is shown in the following schematic that depicts one possible view of the relationships between the fields: Biochemistry is the study of the chemical substances and vital processes occurring in live organisms. Biochemists focus on the role and structure of biomolecules; the study of the chemistry behind biological processes and the synthesis of biologically active molecules are examples of biochemistry. Genetics is the study of the effect of genetic differences in organisms; this can be inferred by the absence of a normal component. The study of "mutants" – organisms which lack one or more functional components with respect to the so-called "wild type" or normal phenotype. Genetic interactions can confound simple interpretations of such "knockout" studies. Molecular biology is the study of molecular underpinnings of the processes of replication, transcription and cell function; the central dogma of molecular biology where genetic material is transcribed into RNA and translated into protein, despite being oversimplified, still provides a good starting point for understanding the field.
The picture has been revised in light of emerging novel roles for RNA. Much of molecular biology is quantitative, much work has been done at its interface with computer science in bioinformatics and computational biology. In the early 2000s, the study of gene structure and function, molecular genetics, has been among the most prominent sub-fields of molecular biology. Many other areas of biology focus on molecules, either directly studying interactions in their own right such as in cell biology and developmental biology, or indirectly, where molecular techniques are used to infer historical attributes of populations or species, as in fields in evolutionary biology such as population genetics and phylogenetics. There is a long tradition of studying biomolecules "from the ground up" in biophysics. One of the most basic techniques of molecular biology to study protein function is molecular cloning. In this technique, DNA coding for a protein of interest is cloned using polymerase chain reaction, and/or restriction enzymes into a plasmid.
A vector has 3 distinctive features: an origin of replication, a multiple cloning site, a selective marker antibiotic resistance. Located upstream of the multiple cloning site are the promoter regions and the transcription start site which regulate the expression of cloned gene; this plasmid can be inserted into either bacterial or animal cells. Introducing DNA into bacterial cells can be done by transformation via uptake of naked DNA, conjugation via cell-cell contact or by transduction via viral vector. Introducing DNA into eukaryotic cells, such as animal cells, by physical or chemical means is called transfection. Several different transfection techniques are available, such as calcium phosphate transfection, electroporation and liposome transfection; the plasmid may be integrated into the genome, resulting in a stable transfection, or may remain independent of the genome, called transient transfection. DNA coding for a protein of interest is now inside a cell, the protein can now be expressed.
A variety of systems, such as inducible promoters and specific cell-signaling factors, are available to help express the protein of interest at high levels. Large quantities of a protein can be extracted from the bacterial or eukaryotic cell; the protein can be tested for enzymatic activity under a variety of situations, the protein may be crystallized so its tertiary structure can be studied, or, in the pharmaceutical industry, the activity of new drugs against the protein can be studied. Polymerase chain reaction is an versatile technique for copying DNA. In brief, PCR allows a specific DNA sequence to be modified in predetermined ways; the reaction is powerful and under perfect conditions could amplify one DNA molecule to become 1.07 billion molecules in less than two hours. The PCR technique can be used to introduce restriction enzyme sites to ends of DNA molecules, or to mutate particular bases of DNA, the latter is a method referred to as site-directed mutagenesis. PCR can be used to determine whether a particular DNA fragment is found in a cDNA library.
PCR has many variations, like reverse transcription PCR for amplification of RNA, more quantitative PCR which allow for quantitative measurement of DNA or RNA molecules. Gel electrophoresis is one of the principal tools of molecular biology; the basic principle is that DNA, RNA, proteins can all be separated by means of an electric field and size. In agarose gel electrophoresis, DNA and RNA can be separated on th
Auxotrophy is the inability of an organism to synthesize a particular organic compound required for its growth. An auxotroph is an organism. Auxotrophy is the opposite of prototrophy, characterized by the ability to synthesize all the compounds needed for growth. Prototrophic cells are self sufficient producers of required amino acids, while auxotrophs require to be on medium with the amino acid that they cannot produce, it is a term used in relation to something, for example saying a cell is methionine auxotrophic means that it would need to be on a medium containing methionine or else it would not be able to replicate. In this example this is. However, a prototroph or a methionine prototrophic cell would be able to function and replicate on a medium with or without methionine. Replica plating is a technique that transfers colonies from one plate to another in the same spot as the last plate so the different media plates can be compared side by side, it is used to compare the growth of the same colonies on different plates of media to determine which environments the bacterial colony can or cannot grow in (this gives insight to possible auxotrophic characteristics.
The method of replica plating implemented by Esther Lederberg included auxotrophs that were temperature-sensitive. It is possible that an organism is auxotrophic to more than just one organic compound that it requires for growth. In genetics, a strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesize an essential compound. For example, a yeast mutant with an inactivated uracil synthesis pathway gene is a uracil auxotroph; such a strain is unable to synthesize uracil and will only be able to grow if uracil can be taken up from the environment. This is the opposite of a uracil prototroph, or in this case a wild-type strain, which can still grow in the absence of uracil. Auxotrophic genetic markers are used in molecular genetics; this allows for biosynthetic or biochemical pathway mapping that can help determine which enzyme or enzymes are mutated and dysfunctional in the auxotrophic strains of bacteria being studied. Researchers have used strains of E. coli auxotrophic for specific amino acids to introduce non-natural amino acid analogues into proteins.
For instance cells auxotrophic for the amino acid phenylalanine can be grown in media supplemented with an analogue such as para-azido phenylalanine. Many living things, including humans, are auxotrophic for large classes of compounds required for growth and must obtain these compounds through diet; the complex pattern of evolution of vitamin auxotrophy across the eukaryotic tree of life is intimately connected with the interdependence between organisms. The Salmonella Mutagenesis test uses multiple strains of Salmonella typhimurium that are auxotrophic to histidine to test whether a given chemical can cause mutations by observing its auxotrophic property in response to an added chemical compound; the mutation a chemical substance or compound causes is measured by applying it to the bacteria on a plate containing histidine moving the bacteria to a new plate without sufficient histidine for continual growth. If the substance does not mutate the genome of the bacteria from auxotrophic to histidine back to prototrophic to histidine the bacteria would not show growth on the new plate.
So by comparing the ratio of the bacteria on the new plate to the old plate and the same ratio for the control group, it is possible to quantify how mutagenic a substance is, or rather, how it is to cause mutations in DNA. A chemical is considered positive for Ames test if it causes mutations increasing the observed reversion rate and negative if presents similar to the control group. There is a normal, but small, number of revertant colonies expected when an auxotrophic bacteria is plated on a media without the metabolite it needs because it could mutate back to prototrophy; the chances of this are low and therefore cause small colonies to be formed. If a mutagenic substance is added, the number of revertants would be visibly higher than without the mutagenic substance; the Ames test is considered positive if a substance increases chance of mutation in the DNA of the bacteria enough to cause a quantifiable difference in the revertants of the mutagen plate and the control group plate. Negative Ames test means the possible mutagen DID not cause increase in revertants and positive Ames test signifies that the possible mutagen DID increase the chance of mutation.
These mutagenic effects on bacteria are researched as a possible indicator of the same effects on larger organisms, like humans. It is suggested that if a mutation can arise in bacterial DNA under presence of a mutagen the same effect would occur for larger organisms causing cancer. A negative Ames test result could suggest that the substance is not a mutagen and would not cause tumor formation in living organisms; however only few of the positive Ames Test resulting chemicals were considered insignificant when tested in larger organisms but t
In molecular biology, a reporter gene is a gene that researchers attach to a regulatory sequence of another gene of interest in bacteria, cell culture, animals or plants. Certain genes are chosen as reporters because the characteristics they confer on organisms expressing them are identified and measured, or because they are selectable markers. Reporter genes are used as an indication of whether a certain gene has been taken up by or expressed in the cell or organism population. To introduce a reporter gene into an organism, scientists place the reporter gene and the gene of interest in the same DNA construct to be inserted into the cell or organism. For bacteria or prokaryotic cells in culture, this is in the form of a circular DNA molecule called a plasmid, it is important to use a reporter gene, not natively expressed in the cell or organism under study, since the expression of the reporter is being used as a marker for successful uptake of the gene of interest. Used reporter genes that induce visually identifiable characteristics involve fluorescent and luminescent proteins.
Examples include the gene that encodes jellyfish green fluorescent protein, which causes cells that express it to glow green under blue light, the enzyme luciferase, which catalyzes a reaction with luciferin to produce light, the red fluorescent protein from the gene dsRed. The GUS gene has been used in plants but luciferase and GFP are becoming more common. A common reporter in bacteria is the E. coli lacZ gene, which encodes the protein beta-galactosidase. This enzyme causes bacteria expressing the gene to appear blue when grown on a medium that contains the substrate analog X-gal. An example of a selectable-marker, a reporter in bacteria is the chloramphenicol acetyltransferase gene, which confers resistance to the antibiotic chloramphenicol. Many methods of transfection and transformation – two ways of expressing a foreign or modified gene in an organism – are effective in only a small percentage of a population subjected to the techniques. Thus, a method for identifying those few successful gene uptake.
Reporter genes used in this way are expressed under their own promoter independent from that of the introduced gene of interest. As a result, the reporter gene's expression is independent of the gene of interest's expression, an advantage when the gene of interest is only expressed under certain specific conditions or in tissues that are difficult to access. In the case of selectable-marker reporters such as CAT, the transfected population of bacteria can be grown on a substrate that contains chloramphenicol. Only those cells that have taken up the construct containing the CAT gene will survive and multiply under these conditions. Reporter genes can be used to assay for the expression of the gene of interest, which may produce a protein that has little obvious or immediate effect on the cell culture or organism. In these cases, the reporter is directly attached to the gene of interest to create a gene fusion; the two genes are under the same promoter elements and are transcribed into a single messenger RNA molecule.
The mRNA is translated into protein. In these cases it is important that both proteins be able to properly fold into their active conformations and interact with their substrates despite being fused. In building the DNA construct, a segment of DNA coding for a flexible polypeptide linker region is included so that the reporter and the gene product will only minimally interfere with one another. Reporter gene assay have been used in high throughput screening to identify small molecule inhibitors and activators of protein targets and pathways for drug discovery and chemical biology; because the reporter enzymes themselves can be direct targets of small molecules and confound the interpretation of HTS data, novel coincidence reporter designs incorporating artifact suppression have been developed. Reporter genes can be used to assay for the activity of a particular promoter in a organism. In this case there is no separate "gene of interest"; the results are reported relative to the activity under a "consensus" promoter known to induce strong gene expression.
A more complex use of reporter genes on a large scale is in two-hybrid screening, which aims to identify proteins that natively interact with one another in vivo. GUS reporter system Research updated information on reporter genes. Staining Whole Mouse Embryos for β-Galactosidase Activity
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are proteins, but in non-protein coding genes such as transfer RNA or small nuclear RNA genes, the product is a functional RNA; the process of gene expression is used by all known life—eukaryotes and utilized by viruses—to generate the macromolecular machinery for life. Several steps in the gene expression process may be modulated, including the transcription, RNA splicing and post-translational modification of a protein. Gene regulation gives the cell control over structure and function, is the basis for cellular differentiation and the versatility and adaptability of any organism. Gene regulation may serve as a substrate for evolutionary change, since control of the timing and amount of gene expression can have a profound effect on the functions of the gene in a cell or in a multicellular organism. In genetics, gene expression is the most fundamental level at which the genotype gives rise to the phenotype, i.e. observable trait.
The genetic code stored in DNA is "interpreted" by gene expression, the properties of the expression give rise to the organism's phenotype. Such phenotypes are expressed by the synthesis of proteins that control the organism's shape, or that act as enzymes catalysing specific metabolic pathways characterising the organism. Regulation of gene expression is thus critical to an organism's development. A gene is a stretch of DNA. Genomic DNA consists of two antiparallel and reverse complementary strands, each having 5' and 3' ends. With respect to a gene, the two strands may be labeled the "template strand," which serves as a blueprint for the production of an RNA transcript, the "coding strand," which includes the DNA version of the transcript sequence.. The production of the RNA copy of the DNA is called transcription, is performed in the nucleus by RNA polymerase, which adds one RNA nucleotide at a time to a growing RNA strand as per the complementarity law of the bases; this RNA is complementary to the template 3' → 5' DNA strand, itself complementary to the coding 5' → 3' DNA strand.
Therefore, the resulting 5' → 3' RNA strand is identical to the coding DNA strand with the exception that Thymines are replaced with uracils in the RNA. A coding DNA strand reading "ATG" is indirectly transcribed through the “TAC” in the non-coding template strand as "AUG" in the mRNA. In prokaryotes, transcription is carried out by a single type of RNA polymerase, which needs a DNA sequence called a Pribnow box as well as a sigma factor to start transcription. In eukaryotes, transcription is performed by three types of RNA polymerases, each of which needs a special DNA sequence called the promoter and a set of DNA-binding proteins—transcription factors—to initiate the process. RNA polymerase. RNA polymerase II transcribes all protein-coding genes but some non-coding RNAs. Pol II includes a C-terminal domain, rich in serine residues; when these residues are phosphorylated, the CTD binds to various protein factors that promote transcript maturation and modification. RNA polymerase III transcribes 5S rRNA, transfer RNA genes, some small non-coding RNAs.
Transcription ends. While transcription of prokaryotic protein-coding genes creates messenger RNA, ready for translation into protein, transcription of eukaryotic genes leaves a primary transcript of RNA, which first has to undergo a series of modifications to become a mature mRNA; these include 5' capping, set of enzymatic reactions that add 7-methylguanosine to the 5' end of pre-mRNA and thus protect the RNA from degradation by exonucleases. The m7G cap is bound by cap binding complex heterodimer, which aids in mRNA export to cytoplasm and protect the RNA from decapping. Another modification is 3' polyadenylation, they occur if polyadenylation signal sequence is present in pre-mRNA, between protein-coding sequence and terminator. The pre-mRNA is first cleaved and a series of ~200 adenines are added to form poly tail, which protects the RNA from degradation. Poly tail is bound by multiple poly-binding proteins necessary for mRNA export and translation re-initiation. A important modification of eukaryotic pre-mRNA is RNA splicing.
The majority of eukaryotic pre-mRNAs consist of alternating segments called introns. During the process of splicing, an RNA-protein catalytical complex known as spliceosome catalyzes two transesterification reactions, which remove an intron and release it in form of lariat structure, splice neighbouring exons together. In certain cases, some introns or exons can be either removed or retained in mature mRNA; this so-called alternative splicing creates series of different transcripts originating from a single gene. Because these transcripts can be translated into different proteins, splicing extends the complexity of eukaryotic gene expression. Extensive RNA processing may be an evolutionary advantage made possible by the nucleus of eukaryotes. In prokaryotes and translation happen together, whilst in eukaryotes, the nuclear membrane separates the two processes, giving time for RNA processing to
Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids; the two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, a phosphate group; the nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA; the complementary nitrogenous bases are divided into two groups and purines. In DNA, the pyrimidines are cytosine. Both strands of double-stranded DNA store the same biological information.
This information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences; the two strands of DNA are thus antiparallel. Attached to each sugar is one of four types of nucleobases, it is the sequence of these four nucleobases along the backbone. RNA strands are created using DNA strands as a template in a process called transcription. Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms store most of their DNA inside the cell nucleus as nuclear DNA, some in the mitochondria as mitochondrial DNA, or in chloroplasts as chloroplast DNA. In contrast, prokaryotes store their DNA only in circular chromosomes.
Within eukaryotic chromosomes, chromatin proteins, such as histones and organize DNA. These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA was first isolated by Friedrich Miescher in 1869, its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, a post-graduate student of Rosalind Franklin. DNA is used by researchers as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity; the unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a long polymer made from repeating units called nucleotides.
The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, have the same pitch of 34 angstroms; the pair of chains has a radius of 10 angstroms. According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide, one nucleotide unit measured 3.3 Å long. Although each individual nucleotide is small, a DNA polymer can be large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with 220 million base pairs, would be 85 mm long if straightened. DNA does not exist as a single strand, but instead as a pair of strands that are held together; these two long strands coil in the shape of a double helix. The nucleotide contains both a segment of the backbone of a nucleobase. A nucleobase linked to a sugar is called a nucleoside, a base linked to a sugar and to one or more phosphate groups is called a nucleotide.
A biopolymer comprising multiple linked nucleotides is called a polynucleotide. The backbone of the DNA strand is made from alternating sugar residues; the sugar in DNA is 2-deoxyribose, a pentose sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings; these are known as the 3′-end, 5′-end carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. When imagining DNA, each phosphoryl is considered to "belong" to the nucleotide whose 5′ carbon forms a bond therewith. Any DNA strand therefore has one end at which there is a phosphoryl attached to the 5′ carbon of a ribose and another end a
Cell culture is the process by which cells are grown under controlled conditions outside their natural environment. After the cells of interest have been isolated from living tissue, they can subsequently be maintained under controlled conditions; these conditions vary for each cell type, but consist of a suitable vessel with a substrate or medium that supplies the essential nutrients, growth factors and gases, regulates the physio-chemical environment. Most cells require a surface or an artificial substrate whereas others can be grown free floating in culture medium; the lifespan of most cells is genetically determined, but some cell culturing cells have been “transformed” into immortal cells which will reproduce indefinitely if the optimal conditions are provided. In practice, the term "cell culture" now refers to the culturing of cells derived from multicellular eukaryotes animal cells, in contrast with other types of culture that grow cells, such as plant tissue culture, fungal culture, microbiological culture.
The historical development and methods of cell culture are interrelated to those of tissue culture and organ culture. Viral culture is related, with cells as hosts for the viruses; the laboratory technique of maintaining live cell lines separated from their original tissue source became more robust in the middle 20th century. The 19th-century English physiologist Sydney Ringer developed salt solutions containing the chlorides of sodium, potassium and magnesium suitable for maintaining the beating of an isolated animal heart outside the body. In 1885, Wilhelm Roux removed a portion of the medullary plate of an embryonic chicken and maintained it in a warm saline solution for several days, establishing the principle of tissue culture. Ross Granville Harrison, working at Johns Hopkins Medical School and at Yale University, published results of his experiments from 1907 to 1910, establishing the methodology of tissue culture. Cell culture techniques were advanced in the 1940s and 1950s to support research in virology.
Growing viruses in cell cultures allowed preparation of purified viruses for the manufacture of vaccines. The injectable polio vaccine developed by Jonas Salk was one of the first products mass-produced using cell culture techniques; this vaccine was made possible by the cell culture research of John Franklin Enders, Thomas Huckle Weller, Frederick Chapman Robbins, who were awarded a Nobel Prize for their discovery of a method of growing the virus in monkey kidney cell cultures. Cells can be isolated from tissues for ex vivo culture in several ways. Cells can be purified from blood. Cells can be isolated from solid tissues by digesting the extracellular matrix using enzymes such as collagenase, trypsin, or pronase, before agitating the tissue to release the cells into suspension. Alternatively, pieces of tissue can be placed in growth media, the cells that grow out are available for culture; this method is known as explant culture. Cells that are cultured directly from a subject are known as primary cells.
With the exception of some derived from tumors, most primary cell cultures have limited lifespan. An established or immortalized cell line has acquired the ability to proliferate indefinitely either through random mutation or deliberate modification, such as artificial expression of the telomerase gene. Numerous cell lines are well established as representative of particular cell types. For the majority of isolated primary cells, they undergo the process of senescence and stop dividing after a certain number of population doublings while retaining their viability. Cells are maintained at an appropriate temperature and gas mixture in a cell incubator. Culture conditions vary for each cell type, variation of conditions for a particular cell type can result in different phenotypes. Aside from temperature and gas mixture, the most varied factor in culture systems is the cell growth medium. Recipes for growth media can vary in pH, glucose concentration, growth factors, the presence of other nutrients.
The growth factors used to supplement media are derived from the serum of animal blood, such as fetal bovine serum, bovine calf serum, equine serum, porcine serum. One complication of these blood-derived ingredients is the potential for contamination of the culture with viruses or prions in medical biotechnology applications. Current practice is to minimize or eliminate the use of these ingredients wherever possible and use human platelet lysate; this eliminates the worry of cross-species contamination. HPL has emerged as a safe and reliable alternative as a direct replacement for FBS or other animal serum. In addition, chemically defined media can be used to eliminate any serum trace, but this cannot always be accomplished with different cell types. Alternative strategies involve sourcing the animal blood from countries with minimum BSE/TSE risk, such as The United States and New Zealand, using purified nutrient concentrates derived from serum in place of whole animal serum for cell culture.
Plating density plays a critical role for some cell types. For example, a lower plating density makes granulosa cells exhibit estrogen production, while a higher plating density
The cell is the basic structural and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are called the "building blocks of life"; the study of cells is called cellular biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as multicellular; the number of cells in plants and animals varies from species to species, it has been estimated that humans contain somewhere around 40 trillion cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres. Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. 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, that all cells come from pre-existing cells.
Cells emerged on Earth at least 3.5 billion years ago. Cells are of two types: eukaryotic, which contain a nucleus, prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Prokaryotes include two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling, they are simpler and smaller than eukaryotic cells, lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single chromosome, in direct contact with the cytoplasm; the nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. A prokaryotic cell has three architectural regions: Enclosing the cell is the cell envelope – 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 and Thermoplasma 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, acts as an additional barrier against exterior forces. It prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment; some eukaryotic cells have a cell wall. Inside the cell is the cytoplasmic region that contains the genome and various sorts of inclusions; the genetic material is found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are 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. On the outside and pili project from the cell's surface; these are structures made of proteins that facilitate communication between cells. Plants, fungi, slime moulds and algae are all eukaryotic; these cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles in which specific activities take place. Most important among these is a cell nucleus, an organelle that houses the cell's DNA; this nucleus gives the eukaryote its name, which means "true kernel". Other differences include: The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls 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 contain some DNA. Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation and thermosensation. Cilia 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." Motile eukaryotes can move using motile flagella. Motile cells are absent in flowering plants. Eukaryotic flagella are more complex than those of prokaryotes. All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out, maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells possess DNA, the hereditary material of genes, RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery.
There are other kinds of biomolecules in cells. This article lists these primary cellular components briefly