Lecture 22.ppt

The word mutation often carries a negative connotation. This is because the majority of mutations have a neutral or negative effect on the fitness of an organism. However, mutations can occasionally enhance an organism’s ability to survive and reproduce. Although rare, advantageous mutations are responsible for the very existence of life on earth. If genomes were perfectly stable, life would never have evolved beyond its earliest stages. Then, about point mutations. Mutations affecting only one base pair are called point mutations. Usually, point mutations involve substitution of one base with another base. If a pyrimidine is replaced with aother pyrimidine, or a purine by another purine it is called a transition. If a purine is replaced by a pyrimidine or vice-versa, it is called a transversion. This is a diagram to show the meanings of transition and transversion. In the upper part of the diagram, the changes are between G and A or T and C. So, they are of transition. In the lower part, the change is between G and T. It is of transversion. Mistakes during DNA replication are a common source of transitions and transversions. DNA polymerase occasionally pairs a base on the template strand with a non-complementary base. Usually, the mistake is recognized and removed using the protein’s 3’?5’ exonuclease activity. However, the error is occasionally left unfixed, creating an abnormality called a mismatched base or a DNA mismatch. Deamination is the removal of an amino group from a nucleotide. It occurs by simple reaction with water. When deamination occurs on cytosine, a uracil base is produced. As with a mismatched base, if left uncorrected each uracil will cause a mutation during the next round of replication. Because uracil is very similar to thymine, it is paired to adenine during DNA replication. In the original gene, however, cytosine is paired to guanine. Thus, a point mutation is introduced into the gene. Depurination is the detachment of a pu