Cell Biology 2

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What are Mitochondrion?: In order to understand the mechanism by which the energy released during respiration is conserved as ATP, it is necessary to appreciate the structural features of mitochondria. These are organelles in animal and plant cells in which oxidative phosphorylation takes place. There are many mitochondria in animal tissues; for example, in heart and skeletal muscle, which require large amounts of energy for mechanical work, in the pancreas, where there is biosynthesis, and in the kidney, where the process of excretion begins. Mitochondria have an outer membrane, which allows the passage of most small molecules and ions, and a highly folded inner membrane (cristae), which does not even allow the passage of small ions and so maintains a closed space within the cell. The electron-transferring molecules of the respiratory chain and the enzymes responsible for ATP synthesis are located in and on this inner membrane, while the space inside (matrix) contains the enzymes of the TCA cycle. The enzyme systems primarily responsible for the release and subsequent oxidation of reducing equivalents are thus closely related so that the reduced coenzymes formed during catabolism (NADH and FADH) are available as substrates for respiration.
Define the term Mitochondrion: In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. Mitochondria are sometimes described as "cellular power plants" because their primary purpose is to manufacture adenosine triphosphate (ATP), which is used as a source of energy.

The number of mitochondria found in different types of cells varies widely. At one end of the spectrum, the Trypanosome protozoan has one large mitochondrion; by contrast, human liver cells normally have between one and two thousand each. Mitochondria can occupy up to 25% of cell cytosol.
What is the structure of mitochondria?: Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.

* The outer membrane encloses the entire organelle and contains channels made of protein complexes called porins through which molecules and ions can move in and out of the mitochondrion. It is composed of about 50% lipids and 50% proteins. Large molecules are excluded from traversing this membrane.
* The inner membrane, folded into cristae, encloses the matrix (the internal fluid of the mitochondrion). It contains several protein complexes, and is about 20% lipid and 80% protein. Stalked particles are found on the cristae: these are the ATP synthase enzyme molecules, which produce ATP.
* The intermembrane space between the two membranes contains enzymes that use ATP to phosphorylate other nucleotides and that catalyze other reactions.

"Mitochondrion" literally means 'thread granule', which is what they look like under a light microscope: tiny rod-like structures present in the cytoplasm of all cells. The matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules. Parts of the Krebs cycle occur within mitochondria. The matrix also contains several copies of the mitochondrial DNA (usually 5-10 circular DNA molecules per mitochondrion), as well as special mitochondrial ribosomes, tRNAs, and proteins needed for DNA replication.

When the cell divides, mitochondria replicate by fission. They also replicate if the long-term energy demands of a cell increase. For example, fat storage cells, which require little energy, have very few mitochondria, but energy-demanding muscle cells tend to have many. Mitochondria are generally theorised to be highly adapted symbiotic bacteria, probably belonging to the alpha-proteo bacteria (with the closest known candidate being Rickettsia, the causative agent of typhus), and are believed to have been incorporated only once (compare chloroplast).

The mitochondrial proteins are found on the outer membrane, the inner membrane, or the intermembrane space. Stop-transfer sequences anchor proteins to the outer membrane. Matrix-targeting sequences target the protein for the mitochondrial matrix.
How do mitochondria acomplish energy conversion?: Mitochondria convert the potential energy of food molecules into ATP. The production of ATP is achieved by the Krebs cycle (see citric acid cycle), electron transport and oxidative phosphorylation. Without oxygen, these processes cannot occur.

The energy from food molecules (e.g., glucose) is used to produce NADH and FADH2 molecules, via glycolysis and the Krebs cycle. This energy is transferred to oxygen (O2) in several steps. The protein complexes in the inner membrane (NADH dehydrogenase, cytochrome c reductase, cytochrome c oxidase) that perform the transfer use the released energy to pump protons (H+) against a gradient (the concentration of protons in the intermembrane space is higher than that in the matrix). An active transport system (energy requiring) pumps the protons against their physical tendency (in the "wrong" direction) from the matrix into the intermembrane space.

As the proton concentration increases in the intermembrane space, a strong diffusion gradient is built up. The only exit for these protons is through the ATP synthase complex. By transporting protons from the intermembrane space back into the matrix, the ATP synthase complex can make ATP from ADP and inorganic phosphate (Pi). This process is called chemiosmosis and is an example of facilitated diffusion. Part of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their clarification of the working mechanism of ATP synthase.
What non-mainstream, or lesser known funtions are mitochondria known for?: Mitochondria have several important functions besides the production of ATP. This variety of functions corresponds to the variety of mitochondrial diseases.

Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. These enzymes are not made in the mitochondria of cardiac cells.

Mitochondria also play a role in the following:

* apoptosis
* glutamate-mediated excitotoxic neuronal injury
* cellular proliferation
* regulation of the cellular redox state
* heme synthesis
* steroid synthesis
* heat production (enabling the organism to stay warm)
What is the role of mitochondria in population genetic studies?: Because eggs destroy the mitochondria of the sperm that fertilize them, the mitochondrial DNA of an individual derives exclusively from the mother. Individuals inherit the other kinds of genes and DNA from both parents jointly. Because of the unique matrilineal transmission of mitochondrial DNA, scientists in population genetics and evolutionary biology often use data from mitochondrial DNA sequences to draw conclusions about genealogy and evolution. See: mitochondrial Eve.
What is The endosymbiotic theory?: Mitochondria are unusual among organelles in that they contain ribosomes and their own genetic material. Mitochondrial DNA is circular and employs characteristic variants of the standard eukaryotic genetic code.

These and similar pieces of evidence motivate the endosymbiotic theory Â— that mitochondria originated as prokaryotic endosymbionts. Essentially this widely accepted hypothesis postulates that the ancestors of modern mitochondria were independent bacteria that colonized the interior of the ancient precursor of all eukaryotic life.
What is Adenosine triphosphate?: Adenosine triphosphate (ATP) is the nucleotide known in biochemistry as the "molecular currency" of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ATP also plays an important role in the synthesis of nucleic acids.
What are crista?: Cristae are the infoldings of the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes, and function in cellular respiration.
What is the function of the mitochondrial outter membrane?: The outer membrane encloses the entire organelle and contains channels made of protein complexes called porins through which molecules and ions can move in and out of the mitochondrion. It is composed of about 50% lipids and 50% proteins. Large molecules are excluded from traversing this membrane.
What is the function of the mitochondrial inner membrane?: The inner membrane, folded into cristae, encloses the matrix (the internal fluid of the mitochondrion). It contains several protein complexes, and is about 20% lipid and 80% protein. Stalked particles are found on the cristae: these are the ATP synthase enzyme molecules, which produce ATP.
What is the function of the mitochondrial intermembrane space?: The intermembrane space between the two membranes contains enzymes that use ATP to phosphorylate other nucleotides and that catalyze other reactions.
What are porins?: Porins are transmembrane proteins that are large enough to facilitate passive diffusion. They are prevelant in the outter membrane of the mitochondria.
What is a matrix in biology?: In biology, the word matrix is used for the material between animal or plant cells, or generally the material (or "tissue") in which more specialized structures are embedded, and also specifically for one part of the mitochondrion. The internal structure of connective tissues is a extracellular matrix.

* The term is also used for the "medium" in which bacteria are grown (or "cultured"), so a Petri dish of agar may be the matrix for culturing a sample swabbed from someone's sore throat.
What is ATP synthase?: An ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate by utilizing some form of energy. The overall reaction sequence is:

ADP + Pi ¨ ATP
These enzymes are of crucial importance in almost all organisms, because ATP is the common "energy currency" of cells.

In mitochondria, the FOF1 ATP synthase has a long history of scientific study. The F1 portion of the ATP synthase is above the membrane, the FO portion is within the membrane. It's easy to visualize the FOF1 particle as resembling the fruiting body of a common mushroom, with the head being the F1 particle, the stalk being the gamma subunit of F1, and the base and "roots" being the FO particle embedded in the membrane. The F1 particle was first isolated by Ephraim Racker in 1961.

The F1 particle is large and can be seen in the transmission electron microscope by negative staining (1962, Fernandez-Moran et al., Journal of Molecular Biology, Vol 22, p 63). These are particles of 9 nm diameter that pepper the inner mitochondrial membrane. They were originally called elementary particles and were thought to contain the entire respiratory apparatus of the mitochondrion, but through a long series of experiments, Ephraim Racker and his colleagues were able to show that this particle is correlated with ATPase activity in uncoupled mitochondria and with the ATPase activity in submitochondrial particles created by exposing mitochondria to ultrasound. This ATPase activity was further associated with the creation of ATP by yet another long series of experiments in many laboratories.

In the 1960s through the 1970s, Paul Boyer developed his binding change, or flip-flop, mechanism, which postulated that ATP synthesis is coupled with a conformational change in the ATP synthase generated by rotation of the gamma subunit. John E. Walker crystallized the ATP synthase and was able to determine that Boyer's conformational model was essentially correct. In the crystal structure, the F1 particle can be seen to be composed of a cylinder of 6 subunits, alternating alpha and beta subunits, that form a ring around an asymmetrical gamma subunit. Facilitated diffusion of protons causes the FO particle to rotate, rotating the gamma subunit of F1, while the major F1 subunits are fixed in place. This rotation forces a conformational change in the F1 particle, eventually leading to the synthesis of ATP. For elucidating this Boyer and Walker shared in the 1997 Nobel Prize in Chemistry.

The F1FO ATP synthase is a reversible enzyme. Large enough quantities of ATP cause it to create a transmembrane proton gradient, this is used by fermenting bacteria which do not have an electron transport chain, and hydrolyze ATP to make a proton gradient, which they use for flagella and transport of nutrients into the cell.

In respiring bacteria under physiological conditions, ATP synthase generally runs in the opposite direction, creating ATP while using the protonmotive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. Same process takes place in mitochondria, were ATP synthase is located in the inner mitochondrial membrane (so that F1-part sticks into mitochondrial matrix, were ATP synthesis takes place).

In plants ATP synthase is also present in chloroplasts (CFOF1-ATP synthase). The enzyme is integrated into thylakoid membrane; the CF1-part sticks into stroma, were dark reactions of photosynthesis (Calvin cycle) and ATP synthesis take place. The chloroplast ATP synthase overall general structure and the catalytic mechanism is almost the same as in mitochondria, but protonmotive force is generated not by respiratory electron transport chain, but by primary photosynthetic proteins - photosystems I and II and cytochrome b6f (http://www.arc.unm.edu/~aroberts/main/photosyn.htm).

E. coli ATP synthase is the simplest known form of ATP synthase, with 8 different subunit types. The most complex known is from yeast and have 20 different subunit types.
What is Phosphorylation?: ATP, the "high-energy" exchange medium in the cell, is synthesized in the mitochondrion by addition of a third phosphate group to ADP in a process referred to as oxidative phosphorylation. ATP is also synthesized by substrate level phosphorylation during glycolysis.

ATP is synthesized at the expense of solar energy by photophosphorylation in the chloroplasts of plant cells.

Phosphorylation of sugars is often the stage of their catabolism. It allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across their transporter.
In biochemistry, phosphorylation is the addition of a phosphate (PO4) group to a protein or a small molecule. Its prominent role in biochemistry is the subject of a very large body of research (the Medline database returns over 100,000 articles on the subject, largely on protein phosphorylation. See also protein kinase for more details on the different types of phosphorylation

Within a protein, phosphorylation can occur on several amino acids. Phosphorylation on serine is the most common, followed by threonine. Tyrosine phosphorylation is relatively rare. However, since tyrosine phosphorylated proteins are relatively easy to purify using antibodies, tyrosine phosphorylation sites are relatively well understood. Histidine and aspartate phosphorylation occurs in prokaryotes as part of two-component signalling.
What's in the matrix?: The matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules. Parts of the Krebs cycle occur within mitochondria. The matrix also contains several copies of the mitochondrial DNA (usually 5-10 circular DNA molecules per mitochondrion), as well as special mitochondrial ribosomes, tRNAs, and proteins needed for DNA replication.
What is the citric acid cycle?: The sum of all reactions in the citric acid cycle is:

Acetyl-CoA + 3 NAD+ + FAD + ADP + Pi --->
CoA-SH + 3 NADH + H+ + FADH2 + ATP + 2 CO2

The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. In these aerobic organisms, the citric acid cycle is a metabolic pathway that forms part of the break down of carbohydrates, fats and proteins into carbon dioxide and water in order to generate energy. It is the second of three metabolic pathways that are involved in fuel molecule catabolism and ATP production.

The citric acid cycle also provides precursors for many compounds such as certain amino acids, and some of its reactions are therefore important even in cells performing fermentation.
he citric acid cycle takes place within the mitochondria in eukaryotes, and within the cytoplasm in prokaryotes.

Fuel molecule catabolism (including glycolysis) produces acetyl-CoA, a two-carbon acetyl group bound to coenzyme A. Acetyl-CoA is the main input to the citric acid cycle. Citrate is both the first and the last product of the cycle (Fig 1), and is regenerated by the condensation of oxaloacetate and acetyl-CoA.
What is the TCA Cycle?: The citric acid cycle is the second step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA and enters the citric acid cycle.

In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. These amino acids are brought into the cells and can be a source of energy by being funnelled into the citric acid cycle.

In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart, fatty acids are broken down through a process known as beta oxidation which results in acetyl-CoA which can be used in the citric acid cycle. Sometimes beta oxidation can yield propionyl CoA which can result in further glucose production by gluconeogenesis in liver.

The citric acid cycle is always followed by oxidative phosphorylation. This process extracts the energy from NADH and FADH2, recreating NAD+ and FAD, so that the cycle can continue. The citric acid cycle itself does not use oxygen, but oxidative phosphorylation does.

The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the citric acid cycle and oxidative phosphorylation equals about 38 ATP molecules. The citric acid cycle is called an amphibolic pathway because it participates in both catabolism and anabolism.
Cytochromes are what?: A heme-containing protein involved in electron-transfer reactions. Cytochrome is a chemical compound consisting of a protein linked to heme (the oxygen-carrier of the blood). Cytochromes are involved in many types of biological chemical reactions that support the life of cells. cytochromes are enzymes which transfer electrons from flavoproteins or other carrier enzymes to cytochrome oxidase; the cytochromes, of which there are a number in aerobic cells, are cell pigments with a prosthetic group resembling the heme of hemoglobin. A class of protein, found in mitochondrial membranes, whose main function is oxidative phosphorylation of ADP to form ATP. ron-containing porphyrin ring (e.g., heme) complexed with proteins which act as electron carriers in an electron-transport chain. Â— any of several intracellular respiratory pigments that are enzymes functioning in electron transport as carriers of electrons, which respond to light and color a class of hemoprotein whose principle biological function is as carriers of electrons.
What is Reduction?: any process in which electrons are added to an atom or ion (as by removing oxygen or adding hydrogen); always occurs accompanied by oxidation of the reducing agent The gain of an ELECTRON by an ATOM; takes place simultaneously with OXIDATION (loss of an electron by an atom), because an electron that is lost by one atom is accepted by another. (Latin reductio, a bringing back; originally "bringing back" a metal from its oxide). reduction is the addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (no dissolved oxygen present), sulfur compounds are reduced to odor-producing hydrogen sulfide (H2S and other com- pounds. The opposite of oxidation. The electromotive force exhibited at an electrode by 1 M concentrations of a reducing agent and its oxidized form at 25 oC and pH 7.0; a measure of the relative tendency of the reducing agent to lose electrons.
What is Electrochemical potential?: Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. It represents one of many interchangeable forms of potential energy through which energy may be conserved.

In electrochemistry, electrochemical potential (also called electrode potential) is the mechanical work done in bringing 1 mole of an ion from a standard state to a specified concentration and electrical potential.

The term is typically applied in contexts where a chemical reaction is to take place, such as one involving the transfer of an electron at a battery electrode. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential of that reaction (see also electrode potential and Table of standard electrode potentials). In instances pertaining specifically to the movement of electrically charged solutes, the potential is often expressed in units of volts.

In biology too the term is sometimes used in the context of a chemical reaction, in particular to describe the energy source for the chemical synthesis of ATP. More generally, however it used to characterize the propensity of solutes to simply diffuse across a membrane (i.e., a process involving no chemical transformation).

With respect to a cell, organelle, or other subcellular compartment, the propensity of an electrically charged solute, such as a potassium ion, to move across the membrane is decided by the difference in its electrochemical potential on either side of the membrane, which arises from three factors:

* the difference in the concentration of the solute between the two sides of the membrane
* the charge or "valence" of the solute molecule
* the difference in voltage between the two sides of the membrane (i.e. the transmembrane potential).

A solute's electrochemical potential difference is zero at its "reversal potential", the transmembrane voltage at which the solute's net flow across the membrane is zero. This potential is predicted theoretically either by the Nernst equation (for systems of one permeant ion species) or the Goldman-Hodgkin-Katz equation (for more than one permeant ion species).

The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential (see chemiosmotic hypothesis). In this context protons are often considered separately, using units either of concentration or pH.
What is the Electron transfer chain?: The electron transfer chain (also called the electron transport chain, or simply electron transport), is a complex sequence found in the mitochondrial membrane that accepts electrons from electron donors such as NADH or succinate, shuttles these electrons across the mitochondrial membrane creating an electrical and chemical gradient, and through the proton driven chemistry of the ATP synthase (aka the F0F1 particle), generates adenosine triphosphate (ATP).

There are five complexes normally associated with the electron transfer chain.

* Complex I - NADH dehydrogenase, also called NADH coenzyme Q reductase.
* Complex II - Succinate - coenzyme Q reductase.
* Complex III - Coenzyme Q - cytochrome c reductase.
* Complex IV - Cytochrome c oxidase.
* Complex V - ATP synthase, also known as the F0F1 particle.

All of these are proteolipid complexes, with the first four containing either flavins, iron-sulfur clusters, copper centers, or heme moieties. Complexes I, III, and IV are proton pumps. Complex II is part of the Krebs cycle and does not pump protons, and Complex V uses the electrochemical potential generated to create ATP. Complex IV is the terminus of the electron transfer chain, consuming oxygen and making water.

* Cytochrome c

Cytochrome c is also an essential part of the electron transfer chain. It is a soluble protein loosely associated with the inner mitochondrial membrane, and transfers electrons between Complexes III and IV.

The electron transfer chain can be inhibited by various poisons. Among them we can cite carbon monoxide, cyanide, azide, antimycin, amytal and rotenone.
What is complex 1?: The NADH dehydrogenase (EC 1.6.5.3) complex, also called NADH coenzyme Q oxidoreductase, is the first complex in the electron transfer chain of mitochondria and catalyzes the transfer of electrons from NADH to coenzyme Q (CoQ):

NADH + CoQ + H+ ¨ NAD+ + CoQH2

In the process of reducing coenzyme Q, this complex also translocates protons, helping to provide the electrochemical potential used to produce ATP. It is a large complex, the mammalian enzyme containing 43 separate protein subunits, as well as a flavin prosthetic group and several iron-sulfur clusters.

Seven of the 43 mammalian protein subunits are encoded in the mitochondrial DNA. Defects in the subunits of Complex I are associated with various mitochondrial diseases.
What is complex 2?: Succinate - coenzyme Q reductase.
What is complex 3?: The Coenzyme Q - cytochrome c reductase complex, sometimes called the cytochrome bc1 complex, and at other times Complex III, is the third complex in the electron transfer chain (PDB 1KYO (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=97471034356082&page=0&pdbId=1KYO), EC 1.10.2.2). It is a transmembrane lipoprotein, and it catalyzes the reduction of cytochrome c by accepting reducing equivalents from Coenzyme Q (CoQ):

CoQH2+ 2 Fe+3-cytochrome c ¨ CoQ + 2 Fe+2-cytochrome c

In the process, protons are translocated across the mitochondrial membrane. Therefore, the bc1 complex is a proton pump.

Compared to the other major proton pumping subunits of the electron transport chain, the number of subunits found can be small, as small as three polypeptide chains. This number does increase, and as many as eleven subunits can be found in higher animals. The major prosthetic groups in the complex are a pair of cytochromes, the b cytochrome and the c1 cytochrome, and a two iron, two sulfur iron-sulfur cluster.
What is complex 4?: The enzyme cytochrome c oxidase (PDB 2OCC (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=85881034355622&page=0&pdbId=2OCC), EC 1.9.3.1 ) is a large transmembrane protein found in the mitochondrion and is the terminal electron acceptor in the electron transfer chain, taking 4 reducing equivalents from cytochrome c and converting molecular oxygen to water. In the process, it translocates protons, helping to establish a chemiosmotic potential that the ATP synthase then uses to synthesize ATP.

Summary reaction:

4 Fe+2-cytochrome c + 4H+ + O2 ¨ 4 Fe+3-cytochrome c + H2O.

The complex is a large lipoprotein composed of several metal prosthetic sites and 13 protein subunits. In mammals, 10 subunits are nuclear in origin and 3 are synthesized mitochondrially. The complex contains 2 cytochromes, the a and a3 cytochromes, and two copper centers, the CuA and CuB centers. In fact, the cytochrome a3 and CuB are a binuclear center that is the site of oxygen reduction. The mechanism of action of this large complex is still an active research topic.
What is chemiosmotic coupling?: The Chemiosmotic Hypothesis is the proposal in 1961, by Peter D. Mitchell, that the mitochondrion functioned as a kind of electrochemical capacitor, using the energy of NADH and FADH2 to create a proton gradient across the mitochondrial membrane and that this energy was used by a reversible proton pump, the ATP synthase, to create ATP. This was a radical proposal at the time, and not well accepted. The prevailing view was that the energy of electron transfer was stored as a stable high potential intermediate, a chemically more conservative concept.

The problem with the older paradigm is that no high energy intermediate was ever found, and the evidence for proton pumping by the complexes of the electron transfer chain grew too great to be ignored. Eventually the weight of evidence began to favor the chemiosmotic hypothesis, and in 1978, Peter Mitchell was awarded the Nobel Prize in Chemistry.
What is DNA?: Deoxyribonucleic acid (DNA) is a nucleic acid which carries genetic instructions for the biological development of all cellular forms of life and many viruses. DNA is sometimes referred to as the molecule of heredity as it is inherited and used to propagate traits. During reproduction, it is replicated and transmitted to offspring.

In bacteria and other simple cell organisms, DNA is distributed more or less throughout the cell. In the complex cells that make up plants, animals and in other multi-celled organisms, most of the DNA is found in the chromosomes, which are located in the cell nucleus. The energy-generating organelles known as chloroplasts and mitochondria also carry DNA, as do many viruses. * Genes are the organism's cookbook;
* DNA is made of genes, areas that regulate genes, and areas that either have no function, or a function we don't know;
* DNA is organized as two complementary strands, head-to-toe, with bonds between them that can be "unzipped" like a zipper, separating the strands;
* DNA is encoded with four interchangeable "building blocks", called "bases", which can be abbreviated A, T, C, and G; each base "pairs up" with only one other base: A+T, T+A, C+G and G+C; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand;
* The order does matter: A+T is not the same with T+A, just as C+G is not the same with G+C;
* However, since there are just four possible combinations, naming only one base on the conventionally chosen side of the strand is enough to describe the sequence;
* The order of the bases along the length of the DNA is what it's all about, the sequence itself is the description for genes;
* Replication is done not by some magical copy machine, but rather by splitting (unzipping) the double strand down the middle via relatively trivial chemical reactions, and recreating the "other half" of each new single strand by drowning each half in a "soup" made of the four bases. Since each of the "bases" can only combine with one other base, the base on the old strand dictates which base will be on the new strand. This way, each split half of the strand plus the bases it collects from the soup will ideally end up as a complete replica of the original, unless a mutation occurs;
* Mutations are simply chemical imperfections in this process: a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to; all other basic mutations can be described as combinations of these accidental "operations".
What are Chargaff's rules?: Chargaff's rules state that double-stranded DNA from any cell of all organinisms have a 1:1 ratio of pyrimidine and purine bases but more specifically that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. They were discovered by Austrian chemist Erwin Chargaff.

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