DNA复制是一个不对称的过程。不对称的一个体现是复制链分为前导链和滞后链, 前者连续复制而后者的复制却不连续。这种不对称最终导致两条链上核酸组成的不对称。链特异的核酸组成偏差最先发现于棘皮类动物和脊椎动物的线粒体DNA上。随着全基因组的大量测序, 越来越多的细菌被发现具有类似的链特异的核酸组成偏差, 甚至很多真核生物的染色体基因组也被发现具有类似偏差。在某些细菌中, 链特异的组成偏差强烈到足以使前导链和滞后链的基因间具有分离的密码子使用。至今, 共有11种细菌被发现具有和复制相关的分离的密码子使用。这11种细菌无一例外都属于专性寄生(共生)菌。对于链组成偏差产生的内在机制以及特定细菌具有强烈链组成偏差的成因, 目前学术界尚没有统一的理论解释。文章对这一遗传学及基因组学的重要问题进行了综述和展望。
DNA replication includes a set of asymmetric mechanisms, which is a division into lagging and leading strands. The former is synthesized continuously whereas the synthesis for the latter is discontinuous. Such a asymmetric mechanism leads to distinct nucleotide composition of these two strands. Strands specific nucleotide composition bias was originally found in genomes of echinoderm and vertebrate mitochondria and then in several bacterial genomes. With the rapid growth in the number of sequenced genomes, many bacteria and even eukaryotes are found to have the consistent strand composition bias. In some bacteria, the extent of strand specific composition bias was so strong that genes on the two replicating strands could be separated according to their codon usages. Till now, 11 obligate intracellular bacteria have been found to have separate codon usages according to whether genes located on the leading or lagging strands. However, there is still not a well-accepted theory that could interpret the reason for the occurrence of separate codon usages in some special bacterial genomes and not in others. This paper reviews the related works and points out its open problems.
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