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DNA replication is a multi-step process that is essential to cell division. If DNA is replicated incompletely or incorrectly, then dysfunctional proteins will be produced. Proteins that are dysfunctional or incomplete are undeniable symptoms of DNA mutations and causes of diseases and terminal cancers. As of now, there are 7 categorized mutations that can occur to DNA; point mutation, deletion, frame shift, inversion, substitution, insertion, translocation, and duplication (Dean’s Lecture #36). In the case of the first mutation, there is a substitution mutation with the first adenine nucleotide in the sequence. In place of the adenine, there is now a thymine nucleotide. So as a result, instead of the tyrosine amino acid that should be present for the UAC mRNA codon, there is an asparagine amino acid. This may indicate that whatever protein the DNA sequence was suppose to synthesize will have a deformity that may cause limited function or complete dysfunction. Similarly, in the second mutation, there is a substitution mutation that changes an adenine to a thymine. In this case, it causes a phenylalanine amino acid to change to an isoleucine.
Choice A, Both, is an INCORRECT answer choice. This answer choice is incorrect because substitution mutations do not interfere with DNA replication. DNA replication occurs during cellular mitosis. If mitosis halts, then in essence, DNA replication halts as well. Mitosis is kept in check by checkpoints that determine whether the cell is ready to proceed to the next stage of mitosis. None of the checkpoints involve checking the DNA sequence for mutations. In other words, whether there is a substitution mutation or not in DNA, DNA replication will continue and the mutation will be carried through to daughter cells.
Choice C, Only the first mutation, is an INCORRECT answer choice. This answer choice is incorrect because if both the first and second mutations are the same kind of mutation (substitution mutation), then it is illogical that the first mutation will stop DNA replication but the second mutation will not. The argument may be that the first one will stop DNA replication because it affects a different codon than the second mutation. However, it still neglects an explanation on how that mutation will affect DNA replication in any way. Replication occurs with a DNA polymerase that simply reads both the non-template and template strand and produces a complementary strand. Whether there is a base error in the sequence does not affect the ability or efficiency of DNA polymerase to perform its job (Dean’s Lecture #36).
Choice D, Only the second mutation, is an INCORRECT answer choice. This answer choice is incorrect because like the explanation before, both the first and second mutation are the same kind of mutation, a substitution mutation. A physically small mutation like a substitution mutation will not affect DNA replication because it does not change chromosome density or size (Dean’s Lecture #36). Changes in the physical size or chemistry of a chromosome may affect M checkpoint during metaphase. Other than that, a simple mutation such as in the first and second mutation will not trigger enzymes at cell checkpoints to send the cell to G0.
Choice B, Neither, is the CORRECT answer choice. This answer choice is correct because it correctly indicates that substitution mutations do not affect DNA replication. Any changes to the DNA sequence will not affect how DNA is synthesized and replicated. In a more extreme and unlikely scenario, if every since nucleotide in DNA was changed to a different nucleotide, DNA replication will still occur like usual. The only exception is that it will be impossible to synthesize viable proteins from this DNA sequence. Thus, in the end because of mutation cells will die. Cellular apoptosis because of mutations is not due to the inability for DNA to replicate but protein synthesis failure.
The study of DNA mutation is crucial in our understanding of genetic diseases. This is because a mutation in DNA is passed down generations. In other words, mutations affect heredity. When a patient is admitted into a hospital, the patient is always asked about family history because of the concept of inheriting diseases and biological reactions. Sickle cell anemia is a common but devastating example of how deadly the smallest of mutations can be. Sickle cell anemia describes the condition where the red blood cells of an affected patient are irregularly shaped like a sickle or a crescent. Red blood cells have an optimal shape that optimizes hemoglobin’s binding to oxygen. However, in sickle cell anemia, red blood cells have a harder time carrying oxygen to the body. In turn, patients suffering from the disease often has tissue damage due to the lack of oxygen. All of this trouble and pain is due to a substitution mutation in the sixth amino acid of hemoglobin. In the sixth codon an adenine is changed to a thymine. As a result, the glutamic acid amino acid that is supposed to be incorporated into the amino acid chain is now replaced by a valine. A single nucleotide change is the difference between a person having a happy and healthy life and a person having to suffer and live on drugs for the rest of their life.
Poster Cunzhen Fan was able to reach the conclusion that answer choice B is the correct answer choice. However, his explanation of his answer choice does not clarify the reason for his choices. In his explanation for answer choice A, Fan states that there is no stop codon in DNA replication. In reality, there is a stop codon in DNA replication but it is not expressed because a stop codon only signals the ribosome to stop the synthesis of an amino acid chain. A stop codon to a DNA polymerase means nothing.
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