Biotechnology: King of Advanced Life Sciences

DNA can be damaged by external agents and by replication errors. Since maintenance of the correct base sequence of DNA and of daughter DNA molecules is essential for hereditary fidelity, repair systems have evolved that restore the correct base sequence. Mi smatch Repair and Methylation of DNA: DNA polymerases occasionally catalyze incorporation of an "incorrect" base that cannot form a hydrogen bond with the template base in the parental strand; such errors usually are corrected by the editing function of these enzymes.  The editing process occasionally fails, so a second system, called mismatch repair, exists for correcting the errors that are not edited out. In mismatch repair, a pair of non-hydrogen-bonded bases (e.g., G . . . T) within a helix are recognized as aberrant and a polynucleotide segment of the daughter strand is excised, thereby removing one member of the unmatched pair.  The resulting gap is filled in by pol I, which presumably uses this "seco...

Problems of Replication:        The replication process needs that each double-helical molecule of DNA produce two identical molecules of DNA. This means that wherever a G-C or A-T base pair occurs in the parental molecule, the identical base pair must occur in the progeny molecules. However, many factors interfere with accurate replication of DNA. If an A should pair with C or G with T as a result of tautomerization, a point mutation (a change in one base pair) will result. Occasionally, a segment of DNA will be replicated more than once (duplication) or a segment may fail to be replicated (deletion). These and other aberrations in DNA replication do occur, but the mechanism of replication has evolved to minimize such mistakes. Semiconservative Replication:         The information in each strand of the double helix acts as the template for the construction of a new double-helical DNA molecule; this is called semiconservative replication sinc...

DNA Probes:                     The ability to isolate specific fragments of DNA containing known sequences of genes gives rise to DNA probes that can be used for a variety of diagnostic, forensic, and therapeutic purposes. DNA probes can be labeled with either radioactive or nonradioactive markers. A DNA probe has a strong interaction with (ideally) a specific DNA target and can be detected after the interaction. DNA probes consisting of 20 bases or fewer usually will have a unique target even in a large set of DNA molecules. The probability that any base will follow any other base in DNA is one in four or 0.25. Therefore, the probability of a specific sequence of 20 bases occurring in a DNA molecule by chance is 0.25 to the power of 20 a vanishingly small number. The use of DNA probes in various aspects of medical diagnostics is increasing rapidly.                       ...