等到在S期／G2期有了姊妹染色分体（sister chromatid）的加入，染色体套数倍增为4n，这时候HR机制就有更多的修复模版可供选择，因此一般认为HR修复的运作在S期／G2期比较活跃。
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此时每条染色体由一对染色单体构成，这两条联合的染色体称为四分体。
At this point esch chromosome consist of a pair of chromatids and the two associated chromosomes are termed a tetrad.
双线期时，配对的染色单体开始从四分体中分离，此时常可见染色体交叉。
In diplotene the pairs of chromatids begin to separate from the tetrad formed by the association of homologous chromosomes.
此外，观察了前期I的簇毛麦染色体形态，统计了四分体和小孢子中小麦和簇毛麦染色质微核频率。
Otherwise, the chromosome morphology involving Haynaldia chromatin was observed at preleptotene I, and the frequencies of chromatin nucleus at tetrad stage and at microspore stage were investigated.
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