A restriction enzyme cleaves DNA at or near recognized sequence patterns. Many type II systems produce reproducible fragments useful in genetic engineering, while a corresponding DNA modification system protects the host sequence. Different recognition patterns and cut positions can produce blunt ends or single-stranded overhangs. Thus recognition specificity and the physical end structure are separate properties important for assembling DNA fragments. ^[ref]

An enzyme inhibitor lowers an enzyme's reaction rate. In reversible enzyme inhibition, reducing free inhibitor concentration can restore activity; slow release can make this difficult to distinguish experimentally from irreversible inactivation. Competitive inhibition and mixed inhibition describe different changes in enzyme kinetics; pure noncompetitive inhibition is a special case of mixed inhibition, not a name for every inhibitor that binds away from an active site.^[ref]

The useful question is how adding substrate changes the inhibition. The model below connects binding to a measurable reaction rate, predicts when more substrate can overcome inhibition, and supplies examples for comparison with an enzyme inhibition initial-rate assay. Its inhibition constants are explicitly defined; neither a binding cartoon nor one reduced reaction rate by itself determines a mechanism.

A protease catalyses hydrolysis of peptide bonds in proteins or peptides. Different proteases recognize different sequences or structures, and some cut within a chain while others remove terminal residues. Digestion, signalling activation and controlled protein degradation all use this chemistry. The sequence specificity of chymotrypsin, for example, differs from indiscriminate breakdown of every peptide bond. ^[ref]

Trypsin is a serine protease: an enzyme that performs proteolysis by catalysing peptide bond hydrolysis. It preferentially cuts on the C-terminus side of lysine and arginine amino acid residues, leaving the selected amino acid residue in the preceding fragment.^[ref]

The bond being broken is the backbone amide linkage, rather than the lysine or arginine amino acid side chain. A formal connectivity equation for hydrolysis isHere $R^1$ and $R^2$ stand for the connected portions of the original peptide. Actual terminal protonation and electric charge depend on pH; the equation displays connectivity. Water supplies the elements needed to form the two new ends, while the enzyme is regenerated.

Selective cutting turns an unwieldy protein into shorter peptides, making peptide mapping useful for comparing proteins. In cell culture, trypsin also helps detach adherent biological cells: the HeLa culture viability experiment provides a concrete example of its use and subsequent stopping of proteolysis. The same enzyme can therefore serve both chemical analysis and preparation of a biological cell suspension.

Papain is an enzyme used for proteolysis, including limited Papain digestion of IgG.

It is a cysteine protease: a catalytic cysteine participates directly in peptide-bond hydrolysis. Limited digestion can cleave accessible regions while preserving folded domains, which is why carefully interpreted fragment patterns can reveal aspects of antibody architecture. ^[ref]

Lactate dehydrogenase couples pyruvate-to-lactate reduction to NADH-to-NAD+ oxidation.

Pyruvate kinase catalyzes phosphate transfer from phosphoenolpyruvate to ADP, producing pyruvate and ATP in glycolysis.

A scientist who studies insects. An entomologist can connect observations of development, behavior or population change with questions in physiology, ecology, agriculture or disease transmission.

Electroneutrality means that positive and negative charges balance on the length scale under consideration. For a bulk electrolyte with ionic concentrations $c_i$ and charge numbers $z_i$, the approximation is ${\sum }_i{z}_i{c}_i=0$.

A solution containing sodium and sulfate ions therefore needs twice as many singly charged sodium ions as doubly charged sulfate ions if these are its only ions. Local charge separation still occurs near an electrode in an electric double layer. Thus bulk electroneutrality does not mean that every microscopic region is neutral, and it should not be imposed pointwise when modeling a charged interface.

An ecologist studies ecology.

A solar eclipse occurs when the Moon passes in front of the Sun for an observer on Earth. The same event appears different from different locations: an observer in the umbra sees totality, while an observer in the penumbra sees only part of the solar disk covered.

If the Moon's apparent angular diameter is too small to cover the Sun, the central alignment produces an annular eclipse instead. The comparison is between apparent sizes, which depend on both physical size and Euclidean distance. During totality the normally overwhelming solar disk is hidden, exposing the much fainter solar corona and allowing observations of nearby background stars.^[ref]

An azeotrope has equal coexisting liquid and vapor compositions: $y=x$. Ordinary fractional distillation cannot cross this composition boundary at fixed pressure; more packing does not remove it. A constant head temperature therefore does not establish a pure chemical substance.^[ref] The stipulated $\alpha =2$ model has $y>x$ throughout $0<x<1$, so it has no interior azeotrope and cannot represent this limit.

McMaster University's 1997/98 laboratory manual compares simple distillation and fractional distillation using charges of $25\mathrm{mL}$ each of cyclohexane and p-xylene. Part B uses a $100\mathrm{mL}$ round-bottom flask and a fractionating column. The protocol specifies a boiling chip, a heating mantle on a ring support, and the top of the thermometer bulb midway in the opening toward the laboratory condenser. Heating is adjusted for roughly one drop per second. Students record head temperature every $2\mathrm{mL}$ collected, up to $45\mathrm{mL}$, and plot both runs against collected volume on the same axes.^[ref]

Charge, atmospheric operation, and collection rate provide comparison conditions; the fractionating column is changed. A sharper separation of temperature regions would support improved fractionation. It would not directly measure product mole fractions or prove purity. The manual distinguishes collected volume from composition and states that this pair forms no azeotrope. No student dataset is supplied here, and the constant-relative volatility example is not assigned to these liquids.^[ref]

Consider a nonreacting two-component mixture processed into just two products, with no losses or unaccounted inventory. Let $F,D,B$ be feed, distillate, and bottoms amount of substance values in moles, and let $z,x_D,x_B$ be their component-A mole fractions. These are whole-stream compositions; $x_D$ is not automatically the instantaneous equilibrium vapor composition $y$ above the pot. Total and component balances giveThe component-B balance follows by subtracting the second equation from the first. Eliminating $B$ gives, for $x_D\ne x_B$,Nonnegative product amounts require $z$ between $x_B$ and $x_D$. For an original stipulated feed $F=1\mathrm{mol}$, $z=0.35$, $x_D=0.80$, and $x_B=0.10$,Check the component inventories:The distillate contains a fraction $(D{x}_D)/(Fz)=40/49\approx 0.816327$ of the original component A, despite its higher mole fraction $x_D=0.80$. This distinction between composition and recovery is useful when deciding how much enriched product to collect. These balances determine neither temperatures, heat input, nor the number of equilibrium stages; they do not prove that the specified products are attainable in a particular fractionating column.

For a two-component mixture, let $x$ be component A's mole fraction in the liquid and $y$ its mole fraction in the coexisting vapor. At interior compositions, define the relative volatility of A with respect to B by the ratio of composition odds:Thus $\alpha >1$ means A is enriched in the vapor, while $\alpha =1$ means $y=x$. Solving this definition givesHere a constant relative volatility is a stipulated model of vapor-liquid equilibrium, not a measured property of a named chemical solution. Real relative volatility can vary with composition, temperature, and pressure. The relation specifies no absolute temperature scale: even fixing $\alpha =2$ does not determine a boiling-temperature curve or a heat requirement.

Fractional distillation brings rising vapor into contact with descending liquid in a fractionating column. Packing supplies contact area; it does not act as a sieve for individual molecules. Repeated exchange can enrich the rising vapor in the more volatile component. Some condensed liquid returns downward as reflux, rather than all being withdrawn as distillate.^[ref]

An equilibrium stage is an idealization of this contact. A real fractionating column has finite contact time and imperfect wetting and mixing; its length is not automatically a count of equilibrium stages. Product withdrawal and reflux affect the separation together. Thus the repeated ideal-stage enrichment example illustrates the composition advantage of repeated equilibration, but supplies neither a column design nor a specified reflux requirement.^[ref]

In simple distillation, pot vapor passes directly to the laboratory condenser. An initial small condensate sample can approximate vapor-liquid equilibrium with the current pot liquid. Removing enriched distillate depletes the pot's more volatile component. A collected fraction averages the changing condensate composition, rather than preserving the first-drop mole fraction.^[ref]

A laboratory condenser removes heat from vapor so that it becomes liquid. In distillation, the downstream laboratory condenser makes collectable distillate. Complete condensation, without loss or reaction, preserves the entering vapor's overall component mole fractions; cooling alone does not give another enrichment step. Partial condensation can instead leave two streams of different composition, requiring both vapor-liquid equilibrium and a distillation material balance.

Vacuum distillation carries out distillation at reduced pressure.