![]() ![]() ![]() These are regions of the DNA molecule that are recognized by special origin recognition proteins that bind the DNA. In either situation, DNA replication is initiated at sites called origins of replication. Figure 5.1.1: Semi-conservative DNA replication coli, there is a single circular chromosome. In cells like our own, the vast amount of DNA is broken up into many chromosomes, each of which is composed of a linear strand of DNA. Cells, whether bacterial or eukaryotic, have to replicate all of their DNA before they can divide. Even in bacteria, the number is in the millions. The sheer number of nucleotides to be copied is enormous.įor example, in human cells, the number of nucleotides to be copied is on the order of several billion. Although the specific proteins involved are different in bacteria and eukaryotes, it is useful to understand the basic considerations that are relevant in all cells, before attempting to address the details of each system.Ī generalized account of the steps in DNA replication is presented below, focused on the challenges mentioned above. Numerous proteins involved in replication have been identified and characterized, including multiple different DNA polymerases in both prokaryotes and eukaryotes. These investigations have revealed that DNA replication is carried out by the action of a large number of proteins that act together as a complex protein machine called the replisome. Our understanding of the process of DNA replication is derived from studies using bacteria, yeast, and other systems, such as Xenopus eggs. With this in mind, we can begin to examine how cells deal with each of these challenges. Ensuring accuracy in the copying of so much information.The use of RNA primers requires that the RNA nucleotides must be removed and replaced with DNA nucleotides and the resulting DNA fragments must be joined.The 5' to 3' extension of both new strands at a single replication fork means that one of the strands is made in pieces. DNA polymerases can only extend a strand in the 5' to 3' direction.DNA polymerases cannot begin synthesis of a new DNA strand de novo and require a free 3' OH to which they can add DNA nucleotides. ![]() The unwound single strands of DNA must be kept from coming back together long enough for the new strands to be synthesized.This unwinding must be accomplished without introducing significant topological distortion into the molecule.A double-helical parental DNA molecule must be unwound to expose single strands of DNA that can serve as templates for the synthesis of new DNA strands.The sheer number of nucleotides to be copied is enormous: e.g., in human cells, on the order of several billion.Consider the challenges facing a cell in this process: Before examining the actual process of DNA replication, it is useful to think about what it takes to accomplish this task successfully. Each time a cell divides, all of its DNA must be copied faithfully so that a copy of this information can be passed on to the daughter cell. DNA, as you know, carries the genetic information that each cell needs. This means that all organisms depend on cell division for their continued existence. The only way to make new cells is by the division of pre-existing cells. ![]()
0 Comments
Leave a Reply. |