The 3 models of DNA replication that Watson and Crick presented in Nature were conservative, semi-conservative and dispersive. The conservative model was that the daughter DNA will model itself after the parent DNA, while the parent DNA stays intact. Watson and Crick hypothesized that throughout each replication the daughter DNA will become less precise and clean each time. The dispersive method is that random parts of the parent DNA are split into two strands while nucleotides fill in the gaps for the daughter DNA. This model however induces mutation because the method would require us to break the DNA. Thus the semi-conservative method was choose, this model is where the parent DNA splits directly down the middle while nucleotides used half of the parent strand to form the daughter strands. This model was later proven by Meselson-Stahl.
2. Bubble Replication
We know DNA replicates itself with the semi-conservative method, however if helicase were to unzip the entire DNA double helix before polymerase 3 started replication, there would be an extremely high risk of mutation. Thus multiple helicases start to unzip the DNA simultaneously at different positions, while multiple polymerase 3 attaches immediately and begins replication, allowing little chance for mutation to occure.
3. mRNA caps
During the production of proteins the DNA is copied into pre mRNA, this transcription is a direct pairing from the DNA gene. Once we have our pre mRNA the ends of the strand are at a risk of mutation, thus we naturally protect these ends with caps. The 5' end is protected with what is known as a G-cap, where an extra guanine nucleotide is added. The 3' end is protected with a poly A polymerase tail, which is simply a chain of adenine nucleotides. Theses caps will help prevent any mutation on the pre mRNA because any mutation will be prevented or it will occur on the caps.
4. Introns
After capping our pre mRNA we also need to protect not only our ends but the middle too. The pre mRNA is divided into two types, exons (important codes) and introns (unimportant codes). If mutation does occur during the pre mRNA phase, there is a lower probability that the mutations will occur at the exons, if all the exons are spaced with introns. Since the introns are not important to the production of proteins, if a mutation occurs there it will be a silent mutation.
5. Wobble Position
After the pre mRNA becomes mRNA we need to translate the mRNA into a polypeptide chain. The mRNA is grouped into triplets (codons) and each codon codes for a specific amino acid. However most codons code for the same amino acids, for example AAA and AAG both code for lysine. Thus if there is a mutation in AAA and it becomes AAG there will be no difference to the overall polypeptide chain. The third nucletide in a codon is called the wobble position because its allows for mutations to occur without damaging our protein production.
3. mRNA caps
During the production of proteins the DNA is copied into pre mRNA, this transcription is a direct pairing from the DNA gene. Once we have our pre mRNA the ends of the strand are at a risk of mutation, thus we naturally protect these ends with caps. The 5' end is protected with what is known as a G-cap, where an extra guanine nucleotide is added. The 3' end is protected with a poly A polymerase tail, which is simply a chain of adenine nucleotides. Theses caps will help prevent any mutation on the pre mRNA because any mutation will be prevented or it will occur on the caps.
4. Introns
After capping our pre mRNA we also need to protect not only our ends but the middle too. The pre mRNA is divided into two types, exons (important codes) and introns (unimportant codes). If mutation does occur during the pre mRNA phase, there is a lower probability that the mutations will occur at the exons, if all the exons are spaced with introns. Since the introns are not important to the production of proteins, if a mutation occurs there it will be a silent mutation.
5. Wobble Position
After the pre mRNA becomes mRNA we need to translate the mRNA into a polypeptide chain. The mRNA is grouped into triplets (codons) and each codon codes for a specific amino acid. However most codons code for the same amino acids, for example AAA and AAG both code for lysine. Thus if there is a mutation in AAA and it becomes AAG there will be no difference to the overall polypeptide chain. The third nucletide in a codon is called the wobble position because its allows for mutations to occur without damaging our protein production.
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