DNA凝膠電泳中使用的DNA marker中標準大小的基因片段是怎麼製備的？序列一定嗎？如果一定是什麼？

把這篇文章的Introduction看了就知道了

Easy method for production of a home-made DNA ladder in every laboratory

Adv Biomed Res. 2015; 4: 70.

Easy method for production of a home-made DNA ladder in every laboratory

如果所在單位沒有買資料庫的話，那就看我下面貼過來的部分，足夠回答問題了：

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Typically, standard DNA markers are classified in two types I: Molecular weight markers and II: Molecular size markers or DNA ladders. Following reported methods of marker production are reviewed.

Generally, the mw markers have been produced by enzyme digestion of E. coli plasmids[2] or genomic DNA of bacteriophages.[1,3,4,5] However, Barvish et al. described a method for preparation of a mw standard from a completely different DNA source, Tenebrio molitor genomic DNA, using Eco RI digestion.[6] Banding pattern of such DNA markers in electrophoresis is non-uniform while the size (ranging from one hundred to several thousand of nucleotides) and number of bands depend on the frequency of recognition site(s) for the enzyme(s) used in digestion.[7]

In contrast to type I DNA markers, molecular size markers (DNA ladders) have numerous bands that regularly increase in size, even intervals. Also, the bands of DNA ladder show an equivalent intensity upon staining.[7] Commercially produced DNA ladders are routinely generated through three basic strategies:

1. Ligation of uncloned subunits into concatemers;
2. Partial restriction digestion of a vector and insert, where the insert is composed of concatemerized subunits; and
3. Partial restriction digestion of an excised insert, composed of concatemerized subunits, without the plasmid DNA.[8]

Recently, some strategies have been reported in the literature based on different PCR techniques.[9,10,11,12,13,14] Currently, DNA ladders are commercially available from different vendors including Sigma, Pharmacia, Life Technologies, Promega, Boerhinger-Mannheim, Amersham, New England Biolabs, Stratagene, fermentas and Invitrogen with varied characters.

除了PCR外，也可以是有專用的質粒經限制酶消化而成的，例如我最常用的New england biolabs。

最經典的，也是實驗室自己可以製備的marker叫λ/Hind III，λ DNA 被Hind III消化後的產物。

其實你隨便用一些已知分子量和濃度的Fragment也可以自己Mix出marker。

不過我最建議的還是買吧。。。這玩意兒現在都很便宜，省點用也就1-2塊錢一次。

商品化的marker是經過內切酶酶切獲得相應序列，再通過測序、測濃度等得到可以定性定量的產品。

還有經過PCR獲得目的條帶，混合得到的。