基因工程分子生物学第三章载体PPT.ppt

第三章 载体 第一节 载体的概念 一、基因复制的基本单位——复制子（replicon） The unit of DNA in which an individual act of replication occurs is called the replicon. Each replicon fires once and only once in each cell cycle. The replicon is defined by its possession of the control elements needed for replication. It has an origin at which replication is initiated. It may also have a terminus at which replication stops (Jacob, Brenner, and Cuzin, 1963). 某一复制行为发生所在的DNA单位称为复制子(replicon)。 每个复制子在每个细胞周期中只“引发”一次。复制子是由它具有复制作用所必需的各种控制元件而定义的。它有一个复制启动的起点(origin)，可能还有一个复制终止的终点(terminus)。 ; Any sequence attached to an origin—or, more precisely, not separated from an origin by a terminus—is replicated as part of that replicon. The origin is a cis-acting site, able to affect only that molecule of DNA on which it resides 连在起点后的任何序列(或更确切地说，没有被终点从起点上分隔开的序列)均作为这个复制子的一部分进行复制。起点是一个顺式作用部位，只能对与它相连的DNA分子起作用。 A genome in a prokaryotic cell constitutes a single replicon; 原核细胞内的一个基因组由单一复制子构成。; Bacteria may contain additional genetic information in the form of plasmids. A plasmid is an autonomous circular DNA genome that constitutes a separate replicon. A plasmid replicon may show single copy control, which means that it replicates once every time the bacterial chromosome replicates. Or it may be under multicopy control, when it is present in a greater number of copies than the bacterial chromosome. 细菌可含有以质粒形式存在的附加遗传信息。 质粒是一种自主的环状DNA基因组，它含有一个与染色体分离的复制子。 质粒可以是单拷贝控制（single copy control）的，即，每次细菌染色体复制时它复制一次。也可以是多拷贝控制（single copy control）之下的，相对于细菌染色体，它存在大量拷贝。; Each phage or virus DNA also constitutes a replicon, able to initiate many times during an infectious cycle. Perhaps a better way to view the prokaryotic replicon, therefore, is to reverse the definition: any DNA molecule that contains an origin can be replicated autonomously in the cell. 每个噬菌体或病毒也包含一个复制子， 但每次侵染周期能够