 Have you ever thought about how a baby becomes an adult? How do so many cell divisions occur, yet all cells still contain the same DNA? Well, the body ensures this through DNA replication. So, what is this process, when does it occur, and why is it important? DNA replication is the process of copying an entire genome in a cell to produce two exact copies. The entire genome's DNA must be replicated before cell division in order to produce functional daughter cells. This can be done relatively easily due to weak hydrogen bonds between strands that allow them to be separated. So, let's explore the way that DNA is replicated. It is replicated semi-conservatively, meaning that the resulting two strands of DNA consist of one old strand and one newly synthesized strand. We can observe this in this simplified diagram of replication, where an enzyme known as DNA polymerase III uses old strands as a template to synthesize new strands. Also, note that the strands are anti-parallel, meaning that they are running parallel to each other but are going in opposite directions. So, how is DNA anti-parallel? Well, it has to do with the sugars in the backbone of DNA, which contain five carbons, four of which are found in a ring. These carbons are assigned numbers one to five, and phosphates are attached to the third carbon and the fifth carbon. The third carbon, or the three prime end, is one of the carbons found in the ring. The fifth carbon, however, or the five prime end, is the carbon attached to the ring. We can see this in the diagram on the right. In DNA, one strand runs from the three prime to five prime end, and another runs from the five prime to three prime end. This characteristic is important to note because replication can only occur in the five prime to three prime direction, and we will investigate the impacts of this later. So, let's have a look at some of the enzymes involved in the process of DNA replication. We have DNA helicase, which helps unwind two strands of DNA, which in turn exposes the nucleotide bases. DNA polymerase III then comes in and reads the strands and synthesizes a new one. Primase actually helps the DNA polymerase locate where to start by placing down primers, which are short chains of DNA. Now remember that these are just a couple of many enzymes involved in this process. Now this slide puts together all of the things that we have learned today about replication. We can see the DNA polymerase III in yellow moving along the strands of DNA, producing two identical DNA strands. Note the semi-conservative nature of this replication, where the DNA polymerase is replicating using the old strand as a template. We can also see helicase unwinding the strands. The most important thing to notice is that both new strands are synthesized in a five prime to three prime direction, meaning that the top strand here can be replicated continuously, whereas the bottom strand has to be replicated in short fragments that are stitched together later. This is what makes a leading strand with continuous replication and a lagging strand with fragmented replication. So, in summary, today we learned about how DNA replication is semi-conservative and we learned about how DNA strands are anti-parallel. We also talked about some of the enzymes involved in this process, such as DNA helicase, DNA polymerase III, and primase. Lastly, we discussed leading and lagging strands and how these occur because of DNA polymerase's limitation in synthesizing a new strand only in a five prime to three prime direction.