DNA, of course, replicates. Why? It’s a pretty crucial element in the reproduction of living things. For example, a bacterium replicates by splitting itself into 2 (binary fission). The DNA must stay intact and be copied with a high degree of accuracy in order for the two newly formed bacteria to develop and function as their parent – adequately. In multi-cellular organisms such as ourselves, DNA replication occurs as a prelude to cell division.
For such a complex molecule, past scientists have had a challenging time working out the precise mechanism by which DNA replicates. Three hypotheses were made: (for this purpose imagine one DNA molecule)
1. The DNA molecule replicates by providing itself as a template for a brand new shiny DNA molecule, and then remaining its own intact DNA molecule. This is called the conservative replication model.
2. The DNA molecule replicates by providing itself as a template and being modified itself throughout, resulting in 2 new DNA molecules with patches of the old parent DNA molecule combined with patches of brand new material. This is called the dispersive replication model.
3. The DNA molecule replicates by providing each of its strands as a template for 2 new DNA molecules, each having one entire new strand, and one entire old strand from the parent DNA molecule. This is called the semi-conservative replication model.
Here’s a visual aid for those who found the above descriptions gibberish:
How does one go about working out which one of these models is the correct one? Well, in the ’50s these two chaps by the names of Meselson and Stahl cracked the riddle by carrying out a classic experiment which the examiners are in love with (so learn it well). Such complicated affairs can only be properly depicted by a lovely video. Videos always give the impression that what you are watching, surely, must be a piece of entertainment rather than advanced biology.
Now that we’re all clear on the replication model of DNA, let’s delve deeper into the details of the process. These are the key steps involved in the semi-conservative replication of DNA:
1. The enzyme DNA helicase unwinds the double helix, causing the hydrogen bonds between the two polynucleotide strands to break.
2. DNA-binding proteins maintain the 2 strands separate during replication.
3. Enzymes called primases attach primers to the exposed strand. Primers are a few nucleotides long and constitute the site where DNA polymerase starts its action.
4. DNA polymerase binds to the aforementioned primer and begins catalysing the reaction between free nucleotides (new) and DNA-bound nucleotides (old). Of course this complies with the principle of complemetarity i.e. A-T, C-G.
5. Because one strand is replicated continuously while the complementary strand is replicated backwards, and hence in fragments (Okazaki fragments) rather than continuously, the resulting DNA fragments must be connected together to form the new strand. New phosphodiester bonds between the sugar-phosphate backbone of DNA are catalysed by the enzyme DNA ligase.
Here is a good video animation of this process.