2013考研英语二图表作文预测:出生率和婴儿死亡率

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　　At the start of BAC library construction, an unqualified ligation reaction mainly produced 40-60 kb inserts (results not shown), the reason is probably that a number of short digested segments were wrapped with large segments together, then the short segments were preferentially ligated to BAC vector. This problem was effectively solved in subsequent ligation reactions. The recoveries of 100-300 kb segments were separated again by PFGE to remove the short segments and to recover the large ones. At the same time, other effective steps were to reduce the concentration of HMW DNA wrapped in the plugs and to break the plugs placed in the sample holes, these strategies could promote the separateness of DNA segments with different size.