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A deoxyribozyme, or DNAzyme, is a DNA molecule with catalytic activity. Its nucleotide sequence permits a functional folded structure that accelerates a specified chemical reaction. Catalytic function is distinct from merely recognizing or binding a target, as an aptamer can do. The term identifies the catalytic material; it does not imply that every DNAzyme catalyzes the same reaction.

A motivating primary experiment by Ronald R. Breaker and Gerald F. Joyce selected DNA molecules that promoted cleavage of an RNA linkage. Their 1994 paper started with a large randomized DNA population and used repeated selection and amplification to enrich activity. The selected catalyst required lead ions under the tested conditions, and an intermolecular version achieved repeated substrate turnover. This established catalytic DNA in a laboratory selection system, rather than demonstrating a naturally occurring cellular enzyme.^[ref]

A DNAzyme turnover experiment can distinguish a catalyst from a one-use reagent. For a constructed molecular count, suppose a preparation contains $10\mathrm{nM}$ active DNA catalyst and produces $100\mathrm{nM}$ specifically cleaved product. The ratiomeans ten reactions per active catalyst molecule on average. This interpretation requires controls for uncatalyzed product formation and an appropriate count of active molecules. The ratio alone gives neither a reaction rate nor a turnover number per unit time.

A time-resolved experiment additionally measures initial reaction rates while varying substrate concentration and holding other conditions fixed. A selected sequence, its inactive or altered controls and a substrate-only reference answer different questions about sequence dependence and background cleavage. Yingfu Li and Ronald R. Breaker's primary measurements of chemically driven RNA degradation illustrate why that baseline matters when quantifying catalytic enhancement.^[ref]

A catalyst changes the rate of approach to a chemical equilibrium without supplying unlimited chemical free energy. Its performance depends on folding, molecular recognition, reaction chemistry and product release; observing binding alone does not establish all of these functions.

RNA degradation is the chemical or enzymatic breakdown of RNA, including cleavage of its phosphodiester bonds into shorter strands. Fragmentation can remove the continuity needed for an RNA molecule's function even while much of its nucleotide sequence remains present. Ribonucleases catalyze RNA cleavage; chemical cleavage also occurs without an enzyme.

A chemical route exploits the hydroxyl group at the $2^{\prime }$ position of ribose. Its oxygen can act as a nucleophile toward the adjacent backbone phosphorus, producing an internal transesterification that breaks the chain. In their 1999 primary study, Yingfu Li and Ronald R. Breaker used chimeric DNA/RNA substrates to measure hydroxide-dependent cleavage of RNA phosphodiester bonds. Such a substrate isolates a susceptible linkage within a defined molecular context, allowing chemical instability to be measured separately from a complex sample's overall fragmentation pattern.^[ref]

An illustrative kinetic model follows the number $N$ of molecules that remain intact under constant conditions. If each intact molecule has a constant probability per unit time $k$ of undergoing its first destructive cleavage, thenThe last quantity is an RNA half-life in this model. With stipulated $k=0.020\mathrm{mi}{\mathrm{n}}^{-1}$, half the starting molecules remain intact after about $34.7\min$, and $e^{-1.2}\approx 30.1\%$ after an hour. The numerical rate is an example, not a universal value for RNA or a measurement from the cited experiment.

A cleavage need not remove material from the tube: breaking one long RNA into two pieces initially preserves its total number of nucleotides. Consequently a concentration measurement can stay almost unchanged while RNA integrity declines. An electropherogram can reveal redistribution toward shorter fragments, while a short target region may remain detectable by reverse transcription and polymerase chain reaction. The relevant question is which molecular continuity the intended assay requires.

Yingfu Li is a biochemist at McMaster University who studies functional nucleic acids, including catalytic DNA and RNA and their use in molecular sensing. His institutional profile and laboratory biography verify the identity and research connection with Ronald R. Breaker.^[ref]

Li is first author, with Ronald R. Breaker, of the 1999 primary study measuring chemically driven cleavage of RNA phosphodiester bonds. Defined chimeric DNA/RNA substrates allowed the investigators to examine hydroxide-dependent transesterification involving the adjacent $2^{\prime }$-hydroxyl group. This addresses a necessary reference question for studying ribozymes and deoxyribozymes: how rapidly does the corresponding linkage break without the proposed catalyst?^[ref]

An original kinetic example illustrates the comparison. Suppose an uncatalyzed linkage has observed rate constant $k_0=10^{-5}\mathrm{mi}{\mathrm{n}}^{-1}$ and a tested catalytic condition gives $k_{\mathrm{obs}}=0.10\mathrm{mi}{\mathrm{n}}^{-1}$. The rate ratio is $k_{\mathrm{obs}}/k_0=10^4$. For a simple first-order reaction, the corresponding half-lives are $\ln 2/k_0$ and $\ln 2/k_{\mathrm{obs}}$, approximately 69,315 minutes and 6.93 minutes. These are constructed values, not the study's measured constants. Changing pH, temperature or substrate structure between the two measurements can change the baseline and therefore the meaning of that ratio.

The connection to molecular sensing is that molecular recognition and reaction are distinct functions. A useful signal can depend on binding a target, changing a catalyst's activity and detecting the resulting product. Quantifying each contribution is more informative than observing an output change without identifying its origin.