原始细胞是如何分裂的.PPT

The origin of replication Figure 3.l Evolution in a test-tube. The initial tube contains the four nucleotides from which RNA is synthesized, a replicase enzyme that copies RNA, and a primer RNA molecule. This primer molecule is copied many times, with some errors. After incubation for time T, a drop of solution is transferred to a new tube containing nucleotides and enzyme (but not, of course, evolutionary changes in the population of RNA molecules observed.?any primer). The process of incubation and transfer can be repeated indefinitely, andacid-like molecule lies in explaining the specificity with which the units, the monomers, are linked together. If they are linked by the wrong chemical bonds, replication will be blocked. At present , the best hope seems to lie in seeking a polymer with a chemically simpler backbone than RNA, thus reducing the number of ways in which the monomers can be linked. The extra specificity to be gained from metabolism on a surface may also help. For the present, we have to accept that the origin of molecules with unlimited heredity is an unsolved problem. But progress is rapid. Despite the last sentence of the Origin of species we cannot advise creationists to put their faith in the belief that only God could create a molecule with unlimited heredity. FROM HEREDITY TO SIMPLE CELLS 5细胞的起源 细胞的起源可能是多元的。细胞膜、细胞的信息遗传系统是独立发生的。后来由于生命的共生作用又形成了某些细胞器，使细胞真正成了生命和进化的主体。这样看来，细胞实际是一个嵌合体。在这个嵌合体中最重要的是体现生命本质的信息传递系统，但这个系统并没有满足于核酸本身的自我催化和自我复制。 The origin of chromosomes  The linkage of genes on chromosomes has two advantages. First, it ensures that when one gene is replicated, all are :no one can cheat. Second, it makes it easier to arrange that, when the cell divides, Existing membranes( Fig.5.4) consist of a lipid bilayer, formed of long-chain fatty acids. This sounds complicated, but isn’t. A fatty acid is a linear molecule with two chemically different ends. One end is ‘hydrophobic’: that is , like fat or oil, it will not mix with water. The other