Tips on specifying the steps of a polymerase chain reaction for thermocycles and primers

Polymerase chain reaction (PCR) is commonly used for ‘amplifying’ lengths of DNA, that is, for multiplying, or making millions of copies, of a given length of DNA. This method is particularly useful when only a small sample of DNA is available.

What concerns us here is the correct way to specify the steps of a PCR, each of which involves a combination of temperature and the duration (in seconds) over which a given temperature is maintained. Each sequence of these steps makes a cycle, and such cycles are repeated many times over (the entire process is automatic). Below are some tips of presenting steps of a PCR:

1. Temperature: Specifying temperatures is quite straightforward. They are always expressed in degrees Celsius (°C), that is, the degree symbol and a capital C. Whereas nearly all scientific style guides including Scientific Style and Format [1] and the current official recommendations [2] of the International Bureau of Weights and Measures clearly specify that a single space should separate the value and unit, as in 27 °C, the AMA Manual of Style recommends [3] that the two parts should be set closed up, as in 27°C. Follow the guidelines from the style guides recommended by the journal.

A related matter is that of the degree symbol. Make sure that you use the correct symbol for the degree i.e. ° and not a superscript o (^o) or zero (⁰). Type alt + 0176 or type 00B0 (or simply the letter b followed by a zero followed by alt x) to get the correct symbol.

2. Duration:  The symbol for second is s (a small s); do not abbreviate it to sec.

3. Prime sign: Primers are short chains of nucleotides. The ‘forward’ primer identifies the start or 5' region of DNA to be amplified and the ‘reverse’ primer identifies the ends or the 3' region of a DNA fragment. As far as the correct style is concerned, remember to use the correct prime sign and not the apostrophe: type ‘2b9’ (2, followed by b, followed by 9, followed by alt x) or use the character map utility of Windows and search for the character.

[1] CSE, Style Manual Committee. 2014. Scientific Style and Format: the CSE manual for authors, editors, and publishers, 8th edn, p. 163. Wheat Ridge, Colorado, USA: Council of Science Editors. 722 pp.

[2] Bureau International des Poids et Mesures. 2006. The International System of Units (SI), 8th edn., p. 133. Paris: BIPM. 180 pp.

[3] AMA. 2007. AMA Manual of Style: a guide for authors and editors, 10th edn., p. 792. New York: Oxford University Press [and American Medical Association]. 1010 pp.