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4 1

  1. 1. MisheelTopic 4.1; Chromosomes, Genes, Alleles and Mutations Chromosomes are made of DNA and protein; they contain genes (heritable factors thatcontrol characteristics). Alleles are different forms of a specific gene. The gene locus is aspecific position of a gene on a chromosome. The same species carry the same genes at the sameloci on the same chromosome (also known as homologous); they also have the same number oschromosomes. A genome is the entire genetic information of an organism. A mutation is a permanent change in the base sequence of DNA. Organisms eventuallyevolve from their ancestors due to natural selection and the numerous cumulative mutations, fewcause diseases. Mutation is possible because there is the possibility of mistakes occurring, eventhough the DNA replication is protected by base-pairing. This is likely after contact to mutagens,like radiation and some chemicals. This mutation may cause a genetic disease. A knownmutation is base-substitution, in which one base is replaced for another. This can change a singleamino acid, but if it doesn’t, it’s known as the silent mutation. If the change has caused onedifferent amino acid to be produced, it’s labeled as missense mutation (sickle cell). For anonsense mutation, the change in base sequence results in a STOP codon – shortening thepolypeptide (cystic fibrosis). Sickle cell disease, a missense mutation, produces valine instead of glutamine, whichleads to production of abnormal red blood cells. Those with one copy of the faulty gene cancope; however it is challenging and dangerous for those with two copies. This disease is a primeexample of natural selection; which there is a variation within a species due to a mutation, andthen passes passed on to the off springs. However, this can be treated with utilization of stemcells, or so proven in mice. Retroviruses inserted into the corrected gene began to producehealthy red blood cells, after the faulty genes were repaired and replaced. This opens endlessopportunities for treatment of mutation diseases.

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