DNA is a large molecule made up of two strands twisted together to form the so-called double helix. Each strand is made up of a sequence of four nitrogenous bases represented by the letters A, C, G and T. The two strands are complementary, this means that where there is a Thymine in one strand there will be an Adenine in the opposite strand and where there is a Cytosine there will be a Guanine in the other strand. Each strand has an end called the 3' and an end called the 5'. The two strands are said to be antiparallel because they are oriented in opposite directions. This is important because it determines how each strand is replicated. The first step in DNA replication is the separation of the two strands. This separation is carried out by an enzyme called helicase, which opens the double helix a bit like a zipper, generating the so-called replication fork. Each of the two strands will serve as a template for the creation of two DNA double helices. Now an enzyme called primase comes into play, which is able to produce a small stretch of RNA, called a primer, which defines the starting point for the construction of the new DNA strand. Now it's time for the enzyme that plays a key role in this process, which is called DNA polymerase. DNA polymerase binds to the primer and moves forward, producing the new DNA strand by adding one nitrogenous base at a time. But DNA polymerase can only add bases in one direction, from 5' to 3'. It is for this reason that only one of the new DNA strands, called the leader strand, can be continuously produced by DNA polymerase, advancing in the 5' 3' direction. The other strand, called the lagging strand, cannot be formed in this way because the replication fork is moving in the opposite direction. DNA polymerase can therefore only produce this strand in a series of small stretches called Okazaki fragments. Again, the process requires an RNA primer, again produced by primase. DNA polymerase then adds a certain number of bases in the 5' to 3' direction. Then a new primer and again another Okazaki fragment and the process repeats numerous times as the replication fork continues to advance. Once all the new DNA has been produced, the enzyme exonuclease comes into action, removing all the RNA primers from both strands of DNA. DNA polymerase then goes back to fill in the gaps created by the exonuclease. Finally, the DNA ligase enzyme joins the various strand fragments to give continuity to the entire structure. From what we have seen, DNA replication is therefore called semi-conservative, because each new DNA molecule is made up of an old DNA strand (the conserved strand) and a new strand, synthesized as we have seen. Simple, right? If you liked this video, hit that button and subscribe to the channel. See you in the next video of Ulysses, your guide to the world.