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3.4.1 Explain DNA Replication

3.4.1 Explain DNA ReplicationDNA Replication is the process by which two identical pieces of DNA are produced from one original piece of DNA. This process ensures that new cells that are made contain exactly the same information as the original cell.Firstly you need to recall the structure of DNA in order to understand the process of replication. Recall that the molecule is double stranded and that those strand are antiparallel to each other. You can see this on the video by the labeled prime ends. Please refer to the video regarding the structure of DNA (3.3.3/3.3.4/3.3.5) in order to familiarize yourself with its structure if you need to refresh!The two strands undergo replication in a slightly different fashion so in the video I show you first the leading strand only followed by then the lagging strand only, before finally demonstrating how the enzymes move relative to each other to build both these strands. In reality both the leading and lagging are replicated at the same time, just in different ways as we shall see now.The leading strand is the strand that is going from 3' to 5' (on the left hand side in the video). Firstly the DNA Helicase unwinds the double helix and separates the two strands by breaking the hydrogen bonds between the complimentary base pairs. Then DNA Polymerase adds complimentary DNA nucleotides to the original strand working in the 5'-3' direction, the same direction as the DNA Helicase (as it continues to unwind and separate the DNA). Therefore the leading strand is replicated continuously. Note that DNA Polymerase can only add nucleotides to the 3' end of the previously added nucleotide.In contrast to the leading strand, the lagging strand is that which goes from 5' to 3' (on the right hand side in the video). It progresses through the process of DNA replication at the same time as the leading strand. Therefore DNA Helicase is still the enzyme that unwinds the DNA and separates it by breaking the hydrogen bonds between the complimentary base pairs. DNA Polymerase also, as per the leading strand, adds complimentary DNA nucleotides. However, here is where there is a difference. As previously mentioned, DNA Polymerase can only add nucleotides to the 3' end of the previously added nucleotide. Since the lagging strand is antiparallel to the leading strand, this means that the enzyme DNA Polymerase is moving in the opposite direction to the DNA Helicase. This lead to the creation of the newly synthesized strand in fragments known as Okazaki fragments. DNA replication is completed here by joining the Okazaki fragments using the enzyme DNA ligase.In the video I show you relatively the movement of the enzymes for both the leading and lagging strands together. Note that the nucleotides are present in the image, but that they are in fact added by DNA Polymerase as it moves along the strands. The animation is trying to point out that DNA replication for both the leading and lagging strands takes place at the same time, but on the lagging strand it takes place discontinuously because the DNA Polymerase can only add DNA nucleotides in the opposite direction to the movement of the DNA Helicase (resulting from the antiparallel nature of the strands).Finally, DNA replication produces two identical 'daughter' pieces of DNA from one original piece of DNA. Each new 'daughter' piece of DNA rewinds to form a double helix
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