Two replication forks are formed at the origin of replication, and these get extended in both directions as replication proceeds. As the DNA opens up, Y-shaped structures called replication forks are formed ( Figure 9.10). Certain proteins bind to the origin of replication while an enzyme called helicase unwinds and opens up the DNA helix. How does the replication machinery know where on the DNA double helix to begin? It turns out that there are specific nucleotide sequences called origins of replication at which replication begins. During initiation, the DNA is made accessible to the proteins and enzymes involved in the replication process. Recall that eukaryotic DNA is bound to proteins known as histones to form structures called nucleosomes.
It occurs in three main stages: initiation, elongation, and termination. DNA Replication in Eukaryotesīecause eukaryotic genomes are very complex, DNA replication is a very complicated process that involves several enzymes and other proteins. When two DNA copies are formed, they have an identical sequence of nucleotide bases and are divided equally into two daughter cells. This is known as semiconservative replication. Each new double strand consists of one parental strand and one new daughter strand. The new strand will be complementary to the parental or “old” strand. Gray indicates the original DNA strands, and blue indicates newly synthesized DNA.ĭuring DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. Figure 9.9 The semiconservative model of DNA replication is shown. This model for replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied ( Figure 9.9). Figure 9.8 The two strands of DNA are complementary, meaning the sequence of bases in one strand can be used to create the correct sequence of bases in the other strand.īecause of the complementarity of the two strands, having one strand means that it is possible to recreate the other strand. For example, a strand of DNA with a nucleotide sequence of AGTCATGA will have a complementary strand with the sequence TCAGTACT ( Figure 9.8).
This means that the two strands are complementary to each other. Recall that adenine nucleotides pair with thymine nucleotides, and cytosine with guanine. The elucidation of the structure of the double helix provided a hint as to how DNA is copied. The replication of DNA occurs during the synthesis phase, or S phase, of the cell cycle, before the cell enters mitosis or meiosis. This is accomplished by the process of DNA replication. When a cell divides, it is important that each daughter cell receives an identical copy of the DNA. Explain the importance of telomerase to DNA replication.By the end of this section, you will be able to:
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