Because mammalian 28S and 18S rRNAs are approximately 5 kb and 2 kb in size, the theoretical 28S:18S ratio is approximately 2.7:1; but a 2:1 ratio has long been considered the benchmark for intact RNA. While crisp 28S and 18S rRNA bands are indicative of intact RNA, it is less clear how these long-lived and abundant molecules actually reflect the quality of the underlying mRNA population, which turns over much more rapidly.
Visual assessment
of the 28S:18S rRNA ratio on agarose gels is somewhat subjective
because appearance of rRNA bands is affected by electrophoresis
conditions, amount of RNA loaded, and saturation of ethidium bromide
fluorescence.
Although the 28S/18S rRNA ratio is a reasonable way to estimate RNA integrity, it
is not ideal and in particular in the context of the Bioanalyzer, which
does not run under denaturing conditions sometimes the ratios do not
reflect the exact level of RNA degradation. Besides the ratio of
the 18S to 28S ribosomal RNAs, Agilent has introduced a software
algorithm that takes the entire electrophoretic trace into account to
help scientists estimate the integrity of total RNA samples.
S代表沉降系数，当用超速离心测定一个粒子的沉降速度，此速度与粒子的大小直接成比例。5s、18s、28s分别具有120、1900和4700个核苷酸。
RNA电泳条带中的 28s 18s 5s分别代表什么？分别代表核糖体RNA的大小。
S为沉降系数，当用超速离心测定一个粒子的沉淀速度时，此速度与粒子的大小直径成比例，可间接反映分子量的大小。
真核生物有4种rRNA，它们分子大小分别是5S、5.8S、18S和28S。
RNA电泳中可以依据核糖体RNA的大小大致判断条带的大小，RNA电泳条带上，应是28S在上，18S在下，且亮度为28S是18S的两倍。
RNA电泳条带中的 28s 18s 5s分别代表RNA电泳中核糖体RNA的大小。
Northern blot中最为常用的电泳胶是含有甲醛的琼脂糖凝胶，甲醛可以减少RNA的二级结构，电泳完成后的胶可经过EB染色后在紫外下检测RNA的质量。
对小分子的RNA 或者microRNA一般采用聚丙烯酰胺变性胶电泳。RNA电泳中可以依据核糖体RNA的大小大致判断条带的大小，28S RNA大小一般为5kb，18S RNA 大小一般为2kb。28S RNA 的亮度一般是18S 的两倍。
【？？？一般真核生物28s：18s=2左右，就是根据他们的分子量来定的。28s 3600nt左右 18s 1800nt左右。 电泳检测的时候，我们都是看亮度是2：1就符合要求，实际情况这个2：1很难界定，电泳检测时只要28s比18s亮一些就判断没有降解。 我理解的是宽度和亮度是一个意思，就是判断他们的比例的一种说法而已。】
http://muchong.com/t-9812204-1
https://zhidao.baidu.com/question/156941070.html?qbl=relate_question_1&word=rRNA%20ratio%2028S%2018S
https://watermark.silverchair.com/gar037.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAAskwggLFBgkqhkiG9w0BBwagggK2MIICsgIBADCCAqsGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMkH_hkj3rSYpLKWANAgEQgIICfGQ3LbqNJGb3Vhj7GhZ1zYB1EvdFXv_H1ZTLHJPa4Jo_TIBx29fz7PddX1aMC4BmdLm_EjspNWRO95LSyYBbE5OW0VGcAEEYBCAcnFoH-HAEIGjEfGcOQus4XdRnJebdO8KCioQLPOVCMvcTGwJ4GBs-kS37zwPSb4avFwDPv6vA4A6pztqGJL1KQ87qRui8i5e5eObqav0TLeVPhe2tKlOK6A_vH8uDOgvzU7yWmOmFnNQFyirVwzCHAREkmSHgtv7ovhgqgUjYCFsOEAhN7EeYrH9hg8lx1S9h_49tIC54LxpAwQ11xRu5dm3arCBdOPWC5HBC4hq7v86Dum0Xchg9O2LKtlcvhd7snySwwyZ2iHkXwCQcNh6Y_ROfdaSXbdeCtw195I2Pk3LPgWVGF90AFX1oS9Mmr_N2zIH5rDU9op4HmWkgSBoVLmc5SBLhkrX_y3OPf1nU89CTKCqTBVtYw20iIL99RwFF0vg6omPLntdsmDo1vaztD7jecQ2l1kK6_uSZAkMGgtZmUQkJEwnfEKYJ-4q8ZlRuevvJhuZh0mlWfPdLyd3Fe6wk88DVNxIZNM7BC-lANz1XGEfT5eNQV5J8x0I5GLAULbKSnsdfYNlsxtCDMSnR6NlqCpqqLS9R_UIUGyVwfyM1rNR_8DUG9k3JtulqniI0-te6bWOdh2v3nVLrVXUE0YmP8j-2RhJDT8M_nAYknY6x3uz1uOYJi6ofCGAlcLhs-aySJ_hk6L1ZjJ3ySoQTq4zcE2DI80ioUnPqrB2ZmX1LeYYvkz098IcG9zm49JGjCodH6BGtV8T_VSZmSN4fLJpGH9jDVT8T5HUTb8q-3FN-Iw