Ribonucleic acid is a polymeric molecule essential in various biological roles in coding, decoding and expression of genes. RNA and DNA are nucleic acids, along with lipids and carbohydrates, constitute the four major macromolecules essential for all known forms of life. Like DNA, RNA is assembled as a chain of nucleotides, but unlike DNA it is more found in nature as a single-strand folded onto itself, rather than a paired double-strand. Cellular organisms use messenger RNA to convey genetic information that directs synthesis of specific proteins. Many viruses encode their genetic information using an RNA genome; some RNA molecules play an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis, a universal function in which RNA molecules direct the assembly of proteins on ribosomes; this process uses transfer RNA molecules to deliver amino acids to the ribosome, where ribosomal RNA links amino acids together to form proteins.
Like DNA, most biologically active RNAs, including mRNA, tRNA, rRNA, snRNAs, other non-coding RNAs, contain self-complementary sequences that allow parts of the RNA to fold and pair with itself to form double helices. Analysis of these RNAs has revealed that they are structured. Unlike DNA, their structures do not consist of long double helices, but rather collections of short helices packed together into structures akin to proteins. In this fashion, RNAs can achieve chemical catalysis. For instance, determination of the structure of the ribosome—an RNA-protein complex that catalyzes peptide bond formation—revealed that its active site is composed of RNA; each nucleotide in RNA contains a ribose sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, in general, cytosine, guanine, or uracil. Adenine and guanine are purines and uracil are pyrimidines. A phosphate group is attached to the 5' position of the next; the phosphate groups have a negative charge each. The bases form hydrogen bonds between cytosine and guanine, between adenine and uracil and between guanine and uracil.
However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge, or the GNRA tetraloop that has a guanine–adenine base-pair. An important structural component of RNA that distinguishes it from DNA is the presence of a hydroxyl group at the 2' position of the ribose sugar; the presence of this functional group causes the helix to take the A-form geometry, although in single strand dinucleotide contexts, RNA can also adopt the B-form most observed in DNA. The A-form geometry results in a deep and narrow major groove and a shallow and wide minor groove. A second consequence of the presence of the 2'-hydroxyl group is that in conformationally flexible regions of an RNA molecule, it can chemically attack the adjacent phosphodiester bond to cleave the backbone. RNA is transcribed with only four bases, but these bases and attached sugars can be modified in numerous ways as the RNAs mature. Pseudouridine, in which the linkage between uracil and ribose is changed from a C–N bond to a C–C bond, ribothymidine are found in various places.
Another notable modified base is hypoxanthine, a deaminated adenine base whose nucleoside is called inosine. Inosine plays a key role in the wobble hypothesis of the genetic code. There are more than 100 other occurring modified nucleosides; the greatest structural diversity of modifications can be found in tRNA, while pseudouridine and nucleosides with 2'-O-methylribose present in rRNA are the most common. The specific roles of many of these modifications in RNA are not understood. However, it is notable that, in ribosomal RNA, many of the post-transcriptional modifications occur in functional regions, such as the peptidyl transferase center and the subunit interface, implying that they are important for normal function; the functional form of single-stranded RNA molecules, just like proteins requires a specific tertiary structure. The scaffold for this structure is provided by secondary structural elements that are hydrogen bonds within the molecule; this leads to several recognizable "domains" of secondary structure like hairpin loops and internal loops.
Since RNA is charged, metal ions such as Mg2+ are needed to stabilise many secondary and tertiary structures. The occurring enantiomer of RNA is D-RNA composed of D-ribonucleotides. All chirality centers are located in the D-ribose. By the use of L-ribose or rather L-ribonucleotides, L-RNA can be synthesized. L-RNA is much more stable against degradation by RNase. Like other structured biopolymers such as proteins, one can define topology of a folded RNA molecule; this is done based on arrangement of intra-chain contacts within a folded RNA, termed as circuit topology. Synthesis of RNA is catalyzed by an enzyme—RNA polymerase—using DNA as a template, a process known as transcription. Initiation of transcription begins with the binding of the enzyme to a promoter sequence in the DNA; the DNA double helix is unwound by the helicase activity of the enzyme. The enzyme progresses along the template strand in the 3’ to 5’ direction, synthesizing a complementary RNA molecule with elongation occ
A cofactor is a non-protein chemical compound or metallic ion, required for an enzyme's activity. Cofactors can be considered "helper molecules"; the rates at which these happen are characterized by enzyme kinetics. Cofactors can be subclassified as either inorganic ions or complex organic molecules called coenzymes, the latter of, derived from vitamins and other organic essential nutrients in small amounts. A coenzyme, or covalently bound is termed a prosthetic group. Cosubstrates are transiently bound to the protein and will be released at some point get back in; the prosthetic groups, on the other hand, are bound permanently to the protein. Both of them have the same function, to facilitate the reaction of enzymes and protein. Additionally, some sources limit the use of the term "cofactor" to inorganic substances. An inactive enzyme without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is called a holoenzyme; some enzymes or enzyme complexes require several cofactors.
For example, the multienzyme complex pyruvate dehydrogenase at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion: loosely bound thiamine pyrophosphate, covalently bound lipoamide and flavin adenine dinucleotide, cosubstrates nicotinamide adenine dinucleotide and coenzyme A, a metal ion. Organic cofactors are vitamins or made from vitamins. Many contain the nucleotide adenosine monophosphate as part of their structures, such as ATP, coenzyme A, FAD, NAD+; this common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world. It has been suggested that the AMP part of the molecule can be considered to be a kind of "handle" by which the enzyme can "grasp" the coenzyme to switch it between different catalytic centers. Cofactors can be divided into two major groups: organic Cofactors, such as flavin or heme, inorganic cofactors, such as the metal ions Mg2+, Cu+, Mn2+, or iron-sulfur clusters. Organic cofactors are sometimes further divided into prosthetic groups.
The term coenzyme refers to enzymes and, as such, to the functional properties of a protein. On the other hand, "prosthetic group" emphasizes the nature of the binding of a cofactor to a protein and, refers to a structural property. Different sources give different definitions of coenzymes and prosthetic groups; some consider bound organic molecules as prosthetic groups and not as coenzymes, while others define all non-protein organic molecules needed for enzyme activity as coenzymes, classify those that are bound as coenzyme prosthetic groups. These terms are used loosely. A 1980 letter in Trends in Biochemistry Sciences noted the confusion in the literature and the arbitrary distinction made between prosthetic groups and coenzymes group and proposed the following scheme. Here, cofactors were defined as an additional substance apart from protein and substrate, required for enzyme activity and a prosthetic group as a substance that undergoes its whole catalytic cycle attached to a single enzyme molecule.
However, the author could not arrive at a single all-encompassing definition of a "coenzyme" and proposed that this term be dropped from use in the literature. Metal ions are common cofactors; the study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list includes iron, manganese, copper and molybdenum. Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified. Iodine is an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor. Calcium is another special case, in that it is required as a component of the human diet, it is needed for the full activity of many enzymes, such as nitric oxide synthase, protein phosphatases, adenylate kinase, but calcium activates these enzymes in allosteric regulation binding to these enzymes in a complex with calmodulin.
Calcium is, therefore, a cell signaling molecule, not considered a cofactor of the enzymes it regulates. Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter, tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus, cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii. In many cases, the cofactor includes both an organic component. One diverse set of examples is the heme proteins, which consist of a porphyrin ring coordinated to iron. Iron-sulfur clusters are complexes of iron and sulfur atoms held within proteins by cysteinyl residues, they play both structural and functional roles, including electron transfer, redox sensing, as structural modules. Organic cofactors are small organic molecules that can be either loosely or bound to the enzyme and directly participate in the reaction. In the latter case, when it is difficult to remove without denaturing the enzyme, it can be called a prosthetic group.
It is important to emphasize that there is no sharp division between loosely and bound cofactors. Indeed, many such as NAD+ can be bound in some enzymes, while it is loosely bound in others. Another example is thiamine pyrophosphate, bound in transketolase or pyruvate decarboxylase, while it is less tightly
A macromolecule is a large molecule, such as protein created by the polymerization of smaller subunits. They are composed of thousands of atoms or more; the most common macromolecules in biochemistry are large non-polymeric molecules. Synthetic macromolecules include common plastics and synthetic fibers as well as experimental materials such as carbon nanotubes; the term macromolecule was coined by Nobel laureate Hermann Staudinger in the 1920s, although his first relevant publication on this field only mentions high molecular compounds. At that time the phrase polymer, as introduced by Berzelius in 1833, had a different meaning from that of today: it was another form of isomerism for example with benzene and acetylene and had little to do with size. Usage of the term to describe large molecules varies among the disciplines. For example, while biology refers to macromolecules as the four large molecules comprising living things, in chemistry, the term may refer to aggregates of two or more molecules held together by intermolecular forces rather than covalent bonds but which do not dissociate.
According to the standard IUPAC definition, the term macromolecule as used in polymer science refers only to a single molecule. For example, a single polymeric molecule is appropriately described as a "macromolecule" or "polymer molecule" rather than a "polymer," which suggests a substance composed of macromolecules; because of their size, macromolecules are not conveniently described in terms of stoichiometry alone. The structure of simple macromolecules, such as homopolymers, may be described in terms of the individual monomer subunit and total molecular mass. Complicated biomacromolecules, on the other hand, require multi-faceted structural description such as the hierarchy of structures used to describe proteins. In British English, the word "macromolecule" tends to be called "high polymer". Macromolecules have unusual physical properties that do not occur for smaller molecules. Another common macromolecular property that does not characterize smaller molecules is their relative insolubility in water and similar solvents, instead forming colloids.
Many require particular ions to dissolve in water. Many proteins will denature if the solute concentration of their solution is too high or too low. High concentrations of macromolecules in a solution can alter the rates and equilibrium constants of the reactions of other macromolecules, through an effect known as macromolecular crowding; this comes from macromolecules excluding other molecules from a large part of the volume of the solution, thereby increasing the effective concentrations of these molecules. All living organisms are dependent on three essential biopolymers for their biological functions: DNA, RNA and proteins; each of these molecules is required for life since each plays a distinct, indispensable role in the cell. The simple summary is that DNA makes RNA, RNA makes proteins. DNA, RNA, proteins all consist of a repeating structure of related building blocks. In general, they are all unbranched polymers, so can be represented in the form of a string. Indeed, they can be viewed as a string of beads, with each bead representing a single nucleotide or amino acid monomer linked together through covalent chemical bonds into a long chain.
In most cases, the monomers within the chain have a strong propensity to interact with other amino acids or nucleotides. In DNA and RNA, this can take the form of Watson-Crick base pairs, although many more complicated interactions can and do occur; because of the double-stranded nature of DNA all of the nucleotides take the form of Watson-Crick base pairs between nucleotides on the two complementary strands of the double-helix. In contrast, both RNA and proteins are single-stranded. Therefore, they are not constrained by the regular geometry of the DNA double helix, so fold into complex three-dimensional shapes dependent on their sequence; these different shapes are responsible for many of the common properties of RNA and proteins, including the formation of specific binding pockets, the ability to catalyse biochemical reactions. DNA is an information storage macromolecule that encodes the complete set of instructions that are required to assemble and reproduce every living organism. DNA and RNA are both capable of encoding genetic information, because there are biochemical mechanisms which read the information coded within a DNA or RNA sequence and use it to generate a specified protein.
On the other hand, the sequence information of a protein molecule is not used by cells to functionally encode genetic information. DNA has three primary attributes that allow it to be far better than RNA at encoding genetic information. First, it is double-stranded, so that there are a minimum of two copies of the information encoding each gene in every cell. Second, DNA has a much greater stability against breakdown than does RNA, an attribute associated with the absence of the 2'-hydroxyl group within every nucleotide of DNA. Third sophisticated DNA surveillance and repair systems are present which monitor damage to the DNA and repair the sequence when necessary. Analogous systems have not evolved for repairing damaged RNA molecules. Chromosomes can contain many billions of atoms, arranged in a specific chemical structure. Proteins are functional macromolecules responsible for catalysing the biochemical reactions that sustain life. Proteins carry out all functions of an organism, for example p
In chemistry in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, either saturated or unsaturated. Most occurring fatty acids have an unbranched chain of an number of carbon atoms, from 4 to 28. Fatty acids are not found in organisms, but instead as three main classes of esters: triglycerides and cholesterol esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and they are important structural components for cells; the concept of fatty acid was introduced by Michel Eugène Chevreul, though he used some variant terms: graisse acide and acide huileux. Fatty acids differ by length categorized as short to long. Short-chain fatty acids are fatty acids with aliphatic tails of five or fewer carbons. Medium-chain fatty acids are fatty acids with aliphatic tails of 6 to 12 carbons, which can form medium-chain triglycerides. Long-chain fatty acids are fatty acids with aliphatic tails of 13 to 21 carbons. Long chain fatty acids are fatty acids with aliphatic tails of 22 or more carbons.
Saturated fatty acids have no C=C double bonds. They have the same formula CH3nCOOH, with variations in "n". An important saturated fatty acid is stearic acid, which when neutralized with lye is the most common form of soap. Unsaturated fatty acids have one or more C=C double bonds; the C=C double bonds can give either cis or trans isomers. Cis A cis configuration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain; the rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the cis configuration, the less flexibility it has; when a chain has many cis bonds, it becomes quite curved in its most accessible conformations. For example, oleic acid, with one double bond, has a "kink" in it, whereas linoleic acid, with two double bonds, has a more pronounced bend. Α-Linolenic acid, with three double bonds, favors a hooked shape.
The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer, or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be packed, therefore can affect the melting temperature of the membrane or of the fat. Trans A trans configuration, by contrast, means that the adjacent two hydrogen atoms lie on opposite sides of the chain; as a result, they do not cause the chain to bend much, their shape is similar to straight saturated fatty acids. In most occurring unsaturated fatty acids, each double bond has three n carbon atoms after it, for some n, all are cis bonds. Most fatty acids in the trans configuration are not found in nature and are the result of human processing; the differences in geometry between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, in the construction of biological structures. The position of the carbon atoms in a fatty acid can be indicated from the −COOH end, or from the −CH3 end.
If indicated from the −COOH end the C-1, C-2, C-3, …. Notation is used. If the position is counted from the other, −CH3, end the position is indicated by the ω-n notation; the positions of the double bonds in a fatty acid chain can, therefore, be indicated in two ways, using the C-n or the ω-n notation. Thus, in an 18 carbon fatty acid, a double bond between C-12 and C-13 is reported either as Δ12 if counted from the −COOH end, or as ω-6 if counting from the −CH3 end; the "Δ" is the Greek letter delta. Omega is the last letter in the Greek alphabet, is therefore used to indicate the “last” carbon atom in the fatty acid chain. Since the ω-n notation is used exclusively to indicate the positions of the double bonds close to the −CH3 end in essential fatty acids, there is no necessity for an equivalent “Δ”-like notation - the use of the “ω-n” notation always refers to the position of a double bond. Fatty acids with an odd number of carbon atoms are called odd-chain fatty acids, whereas the rest are even-chain fatty acids.
The difference is relevant to gluconeogenesis. The following table describes the most common systems of naming fatty acids; when circulating in the plasma are not in their ester, fatty acids are known as non-esterified fatty acids or free fatty acids. FFAs are always bound to a transport protein, such as albumin. Fatty acids are produced industrially by the hydrolysis of triglycerides, with the removal of glycerol. Phospholipids represent another source; some fatty acids are produced synthetically by hydrocarboxylation of alkenes. Template:Says whom? In animals, fatty acids are formed from carbohydrates predominantly in the liver, adipose tissue, the mammary glands during lactation. Carbohydrates are converted into pyruvate by glycolysis as the first important step in the conversion of carbohydrates into fatty acids. Pyruvate is decarboxylated to form acetyl-CoA in the mitochondrion. However, this acetyl CoA needs to be transported into cytosol where the synthesis of fatty acids occurs; this cannot occur directly.
To obtain cytosol
A peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 of one alpha-amino acid and N2 of another along a peptide or protein chain. It can be called an eupeptide bond to separate it from an isopeptide bond, a different type of amide bond between two amino acids; when two amino acids form a dipeptide through a peptide bond it is type of condensation reaction. In this kind of condensation, two amino acids approach each other, with the non-side chain carboxylic acid moiety of one coming near the non-side chain amino moiety of the other. One loses a hydrogen and oxygen from its carboxyl group and the other loses a hydrogen from its amino group; this reaction produces two amino acids joined by a peptide bond. The two joined; the amide bond is synthesized when the carboxyl group of one amino acid molecule reacts with the amino group of the other amino acid molecule, causing the release of a molecule of water, hence the process is a dehydration synthesis reaction.
The formation of the peptide bond consumes energy, which, in organisms, is derived from ATP. Peptides and proteins are chains of amino acids held together by peptide bonds. Organisms use enzymes to produce nonribosomal peptides, ribosomes to produce proteins via reactions that differ in details from dehydration synthesis; some peptides, like alpha-amanitin, are called ribosomal peptides as they are made by ribosomes, but many are nonribosomal peptides as they are synthesized by specialized enzymes rather than ribosomes. For example, the tripeptide glutathione is synthesized in two steps from free amino acids, by two enzymes: glutamate–cysteine ligase and glutathione synthetase. A peptide bond can be broken by hydrolysis. In the presence of water they will release 8 -- 16 kilojoule/mol of Gibbs energy; this process is slow, with the half life at 25 °C of between 350 and 600 years per bond. In living organisms, the process is catalyzed by enzymes known as peptidases or proteases, although there are reports of peptide bond hydrolysis caused by conformational strain as the peptide/protein folds into the native structure.
This non-enzymatic process is thus not accelerated by transition state stabilization, but rather by ground state destabilization. The wavelength of absorption A for a peptide bond is 190–230 nm. Significant delocalisation of the lone pair of electrons on the nitrogen atom gives the group a partial double bond character; the partial double bond renders the amide group planar, occurring in either the cis or trans isomers. In the unfolded state of proteins, the peptide groups are free to adopt both isomers; the trans form is preferred overwhelmingly in most peptide bonds. However, X-Pro peptide groups tend to have a 30:1 ratio because the symmetry between the C α and C δ atoms of proline makes the cis and trans isomers nearly equal in energy; the dihedral angle associated with the peptide group is denoted ω. Amide groups can isomerize about the C' - N bond between the trans forms, albeit slowly; the transition states ω = ± 90 ∘ requires that the partial double bond be broken, so that the activation energy is 80 kilojoule/mol.
However, the activation energy can be lowered by changes that favor the single-bonded form, such as placing the peptide group in a hydrophobic environment or donating a hydrogen bond to the nitrogen atom of an X-Pro peptide group. Both of these mechanisms for lowering the activation energy have been observed in peptidyl prolyl isomerases, which are occurring enzymes that catalyze the cis-trans isomerization of X-Pro peptide bonds. Conformational protein folding is much faster than cis-trans isomerization. A nonnative isomer of some peptide groups can disrupt the conformational folding either slowing it or preventing it from occurring until the native isomer is reached. However, not all peptide groups have the same effect on folding. Due to its resonance stabilization, the peptide bond is unreactive under physiological conditions less than similar compounds such as esters. Peptide bonds can undergo chemical reactions through an attack of an electronegative atom on the carbonyl carbon, breaking the carbonyl double bond and forming a tetrahedral intermedia
Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen and nitrogen, although other elements are found in the side chains of certain amino acids. About 500 occurring amino acids are known and can be classified in many ways, they can be classified according to the core structural functional groups' locations as alpha-, beta-, gamma- or delta- amino acids. In the form of proteins, amino acid residues form the second-largest component of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first carbon atom have particular importance, they are known as α-amino acids. They include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins.
These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, in some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids; the other two are selenocysteine, pyrrolysine. Pyrrolysine and selenocysteine are encoded via variant codons. N-formylmethionine is considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon–tRNA combinations not found in nature can be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the human brain and gamma-amino-butyric acid are the main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. Others may be conditionally essential for medical conditions. Essential amino acids may differ between species; because of their biological significance, amino acids are important in nutrition and are used in nutritional supplements, fertilizers and food technology. Industrial uses include the production of drugs, biodegradable plastics, chiral catalysts; the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus, subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, remained undiscovered until 1884. Glycine and leucine were discovered in 1820; the last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. Usage of the term "amino acid" in the English language is from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". In the structure shown at the top of the page, R represents a side chain specific to each amino acid; the carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids; these include amino acids such as proline which contain secondary amines, which used to be referred to as "imino acids". The alpha amino acids are the most common form found in nature, but only when occurring in the L-isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon. Therefore, all alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amin
A unicellular organism known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Prokaryotes include bacteria and archaea. Many eukaryotes are multicellular, but the group includes the protozoa, unicellular algae, unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early protocells emerging 3.8–4 billion years ago. Although some prokaryotes live in colonies, they are not specialised into cells with differing functions; these organisms live together, each cell must carry out all life processes to survive. In contrast the simplest multicellular organisms have cells that depend on each other to survive. Most multicellular organisms have a unicellular life-cycle stage. Gametes, for example, are reproductive unicells for multicellular organisms. Additionally, multicellularity appears to have evolved independently many times in the history of life.
Some organisms are unicellular, like Dictyostelium discoideum. Additionally, unicellular organisms can be multinucleate, like Plasmodium. Primitive protocells were the precursors to today's unicellular organisms. Although the origin of life is still a mystery, in the prevailing theory, known as the RNA world hypothesis, early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication; the RNA world hypothesis assumes that RNA molecules could form in abiotic conditions, which would require nucleic acids and ribose to be present. Theoretical and experimental findings show that nucleic acids and sugars could have been synthesized in early prebiotic conditions. Compartmentalization was necessary for chemical reactions to be more as well as to differentiate reactions with the external environment. For example, an early RNA replicator ribozyme may have replicated other replicator ribozymes of different RNA sequences if not kept separate; when amphiphiles like lipids are placed in water, the hydrophobic tails aggregate to form micelles and vesicles, with the hydrophilic ends facing outwards.
Primitive cells used self-assembling fatty-acid vesicles to separate chemical reactions and the environment. Because of their simplicity and ability to self-assemble in water, it's that these simple membranes predated other forms of early biological molecules. Prokaryotes lack membrane-bound organelles, such as a nucleus. Instead, most prokaryotes have an irregular region. Most prokaryotes have a single, circular chromosome, in contrast to eukaryotes, which have linear chromosomes. Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, ammonia, or nitrite. Prokaryotes as a whole exist in extreme environments as well. Bacteria are one of the world’s oldest forms of life, are found everywhere in nature. Many common bacteria have plasmids, which are short, self-replicating DNA molecules that are separate from the bacterial chromosome. Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance.
Bacteria predominantly reproduce asexually through a process called binary fission. However, about 80 different species can undergo a sexual process referred to as natural genetic transformation. Transformation is a bacterial process for transferring DNA from one cell to another, is an adaptation for repairing DNA damage in the recipient cell. In addition, plasmids can be exchanged through the use of a pilus in a process known as conjugation; the photosynthetic cyanobacteria are arguably the most successful bacteria, changed the early atmosphere of the earth by oxygenating it. Stromatolites, structures made up of layers of calcium carbonate and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records; the existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the Archaean and Phanerozoic eons. Much of the fossilized stromatolites of the world can be found in Western Australia.
There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago. Hydrothermal vents release heat and hydrogen sulfide, allowing extremophiles to survive using chemolithotrophic growth. Archaea are similar in appearance to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA. By sequencing the ribosomal RNA, it was found that the Archaea most split from bacteria and were the precursors to modern eukaryotes, are more phylogenetically related to eukaryotes; as their name suggests, Archaea comes from a Greek word archaios, meaning original, ancient, or primitive. Some archaea inhabit the most biologically inhospitable environments on earth, this is believed to in some ways mimic the early, harsh conditions that life was exposed to. Examples of these Archaean extremophiles are as follows: Thermophiles, optimum growth temperature of 50 °C-110 °C, including the genera Pyrobaculum, Pyrodictium and Melanopyrus.
Psychrophiles, optimum growth temperature of less than 15 °C, including the genera Methanogenium and Halorubrum. Alkaliphiles, optimum growth pH of greater