Editing GC-content (section)

==Determination==
[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a human, showing an overview of the [[human genome]] on [[G banding]] (which includes [[Giemsa-stain]]ing), wherein GC rich regions are lighter and GC poor regions are darker.<br />{{further|Karyotype}}]]
GC-content is usually expressed as a percentage value, but sometimes as a ratio (called '''G+C ratio''' or '''GC-ratio'''). GC-content percentage is calculated as<ref>{{cite book | author =Madigan, MT. and Martinko JM. | title = Brock biology of microorganisms| edition = 10th | publisher =Pearson-Prentice Hall | year = 2003| isbn = 978-84-205-3679-8}}</ref>

:<math>\cfrac{G+C}{A+T+G+C}\times100%</math>

whereas the AT/GC ratio is calculated as<ref>{{Cite web |url=http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-A/A+T_G+C_ratio.html |title=Definition of GC-ratio on Northwestern University, IL, USA |access-date=11 June 2007 |archive-date=20 June 2010 |archive-url=https://web.archive.org/web/20100620045958/http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-A/A+T_G+C_ratio.html }}</ref>  
:<math>\cfrac{A+T}{G+C}</math> .

The GC-content percentages as well as GC-ratio can be measured by several means, but one of the simplest methods is to measure the [[DNA melting|melting temperature]] of the DNA [[double helix]] using [[spectrophotometry]]. The [[absorbance]] of DNA at a [[wavelength]] of 260 [[Nanometer|nm]] increases fairly sharply when the double-stranded DNA molecule separates into two single strands when sufficiently heated.<ref>{{cite journal |vauthors=Wilhelm J, Pingoud A, Hahn M |title=Real-time PCR-based method for the estimation of genome sizes |journal=Nucleic Acids Res. |volume=31 |issue=10 |page=e56 |date=May 2003 |pmid=12736322 |pmc=156059  |doi=10.1093/nar/gng056}}</ref> The most commonly used protocol for determining GC-ratios uses [[flow cytometry]] for large numbers of samples.<ref>{{cite journal |author=Vinogradov AE |title=Measurement by flow cytometry of genomic AT/GC ratio and genome size |journal=Cytometry |volume=16 |issue=1 |pages=34–40 |date=May 1994 |pmid=7518377 |doi=10.1002/cyto.990160106 |doi-access=free }}</ref>

In an alternative manner, if the DNA or RNA molecule under investigation has been reliably [[sequenced]], then GC-content can be accurately calculated by simple arithmetic or by using a variety of publicly available software tools, such as the [http://www.basic.northwestern.edu/biotools/oligocalc.html free online GC calculator].