Calculations for Molecular Biology and Biotechnology 3rd Edition
There are some 3,000,000,000 base pairs making up human genomic DNA within a haploid cell. If that DNA is isolated from such a cell, it will weigh approximately 0.0000000000035 g. To amplify a specific segment of that purified DNA using the polymerase chain reaction (PCR), 0.00000000001 moles of each of two primers can be added to a reaction that can produce, following some 30 cycles of PCR, over 1,000,000,000 copies of the target gene.
On a day-to-day basis, molecular biologists work with extremes of numbers far outside the experience of conventional life. To allow them to more easily cope with calculations involving extraordinary values, two shorthand methods have been adopted that bring both enormous and infinitesimal quantities back into the realm of manageability. These methods use scientific notation and metric prefixes. They require the use of exponents and an understanding of significant digits.
Certain techniques in molecular biology, as in other disciplines of science, rely on types of instrumentation capable of providing precise measurements. An indication of the level of precision is given by the number of digits expressed in the instrument’s readout. The numerals of a measurement representing actual limits of precision are referred to as significant digits.
Although a zero can be as legitimate a value as the integers 1e9, significant digits are usually nonzero numerals. Without information on how a measurement was made or on the precision of the instrument used to make it, zeros to the left of the decimal point trailing one or more nonzero numerals are assumed not to be significant. For example, in stating that the human genome is 3,000,000,000 base pairs in length, the only significant digit in the number is the 3. The nine zeros are not significant. Likewise, zeros to the right of the decimal point preceding a set of nonzero numerals are assumed not to be significant. If we determine that the DNA within a sperm cell weighs 0.0000000000035 g, only the 3 and the 5 are significant digits. The 11 zeros preceding these numerals are not significant.
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