Types of Mutations: Duplication, Deletion, Inversion, Translocation
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A mutation is a change in the nucleotide base sequence of DNA. The types of mutations are duplications, deletions, inversions, and translocations. Duplication occurs when a segment of DNA bases is repeated. Deletion occurs when a segment of DNA is removed. The segment that is removed can become attached to another chromosome or the sister chromatid, causing a duplication or a translocation. A translocation is when a segment of DNA is moved from one chromosome to a non-homologous chromosome. Inversion occurs when a segment of DNA is reversed, so the original end becomes the beginning.
Preventing Mistakes
Telomeres are noncoding, repeating strands of nucleotides at the end of linear chromosomes that protect from erosion of genes. Because each time DNA is replicated in only the 5' to 3' direction, the 3' end of the DNA strand is shortened, so the telomeres provide a buffer between coding genes and the end of the DNA strands. During DNA replication, DNA Polymerase proofreads the nucleotide bases of the new strand against those of the original strand, removing and replacing any incorrect bases.
Repairing Errors
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If a mistake is missed, mismatch repair takes place. Enzymes remove and replace incorrectly paired nucleotides that result from replication errors.
Mistakes can also occur after DNA replication is complete from a variety of causes such as exposure to harmful chemical and physical agents such as cigarette smoke and X-rays. These changes are usually changed before they become permanent mutations.
Most DNA repair systems, such as nucleotide excision repair, use the base pairing properties of DNA to fix mistakes. A nuclease removes the segment of DNA with the mistake and nucleotides fill the gap, using the undamaged DNA strand as a template. DNA polymerase and DNA ligase are involved in filling the gap left by the nuclease.
Mistakes can also occur after DNA replication is complete from a variety of causes such as exposure to harmful chemical and physical agents such as cigarette smoke and X-rays. These changes are usually changed before they become permanent mutations.
Most DNA repair systems, such as nucleotide excision repair, use the base pairing properties of DNA to fix mistakes. A nuclease removes the segment of DNA with the mistake and nucleotides fill the gap, using the undamaged DNA strand as a template. DNA polymerase and DNA ligase are involved in filling the gap left by the nuclease.