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代表沉降系數，當用超速離心測定一個粒子的沉降速度，此速度與粒子的大小直接成比例。5s、18s、28s分別具有120、1900和4700個核苷酸。
RNA電泳條帶中的 28s 18s 5s分別代表什么？分別代表核糖體RNA的大小。
S為沉降系數，當用超速離心測定一個粒子的沉淀速度時，此速度與粒子的大小直徑成比例，可間接反映分子量的大小。
真核生物有4種rRNA，它們分子大小分別是5S、5.8S、18S和28S。
RNA電泳中可以依據核糖體RNA的大小大致判斷條帶的大小，RNA電泳條帶上，應是28S在上，18S在下，且亮度為28S是18S的兩倍。
RNA電泳條帶中的 28s 18s 5s分別代表RNA電泳中核糖體RNA的大小。
Northern blot中最為常用的電泳膠是含有甲醛的瓊脂糖凝膠，甲醛可以減少RNA的二級結構，電泳完成后的膠可經過EB染色后在紫外下檢測RNA的質量。
對小分子的RNA 或者microRNA一般采用聚丙烯酰胺變性膠電泳。RNA電泳中可以依據核糖體RNA的大小大致判斷條帶的大小，28S RNA大小一般為5kb，18S RNA 大小一般為2kb。28S RNA 的亮度一般是18S 的兩倍。
【？？？一般真核生物28s：18s=2左右，就是根據他們的分子量來定的。28s 3600nt左右 18s 1800nt左右。 電泳檢測的時候，我們都是看亮度是2：1就符合要求，實際情況這個2：1很難界定，電泳檢測時只要28s比18s亮一些就判斷沒有降解。 我理解的是寬度和亮度是一個意思，就是判斷他們的比例的一種說法而已。】
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