Lecture 33 I_Mutations II_Viruses notes - Copy

Lecture 33 I_Mutations II_Viruses notes - Copy - Lecture 33...

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Lecture 33 Part I: Mutations Campbell 6 th Ed. 280-281; 322-323 7 th Ed. 286; 328-330 Part II: Viruses Campbell, 6 th Ed. 328-331; 333-335 (Know Fig. 18.6) 7 th Ed. 334-337; 339-341 (Know Fig. 18.8) Today’s lecture consists of two unrelated parts. PART I: Mutations We learned earlier that mutations can affect phenotype. For instance, a mutant fly had white eyes instead of the normal, wild-type red eyes. Later work showed exactly how that occurred. Mutations are changes, or uncorrected errors, in the DNA sequence. Normally, DNA polymerase is very accurate during DNA replication, because of its proof-reading function. However, very occasionally, it can make a mistake. (Mutations can also arise from harmful agents in the environment that damage DNA). Once these are in place, they are passed down every time the DNA is replicated. An error in the DNA sequence of a gene can lead to an error in the amino acid sequence of a protein. This often makes it non-functional. There are 3 classes of mutation: Point Mutations (single base changes), Insertions or Deletions (where one or more bases is added or left out), and Large-scale Chromosomal Changes. We’ll discuss each in turn. Another name for a point mutation is a substitution . This is a change in one base in DNA; that is, a substitution of one base for another. There are three classes of substitution mutation. The first is a silent mutation . This is where substitution changes a codon to another that codes for the same amino acid. This can occur because the Genetic Code is redundant, and more than one codon can code for the same amino acid. For instance, if a point mutation occurred in the codon CCA, and it were accidentally mutated to CCG, it would still code for proline. The second kind of substitution mutation is a missense mutation . In these cases, a point mutation changes a codon so that it codes for a different amino acid. A famous example is in the beta-globin gene. This codes for one form of globin, a protein in red blood cells that’s important in carrying oxygen to the tissues. A missense mutation can occur to change one codon to a different codon that codes for a different amino acid. The globin protein that’s made with the wrong amino acid doesn’t work as well. It causes red blood cells to have a distinctive abnormal sickle-like shape, and results in sickle cell disease. This is an example of a genetic disease, because it results from a mutation that’s
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Lecture 33 I_Mutations II_Viruses notes - Copy - Lecture 33...

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