alter the reading frame of all base pair triplets Mutation Frequency

Alter the reading frame of all base pair triplets

This preview shows page 19 - 22 out of 33 pages.

alter the reading frame of all base-pair triplets Mutation Frequency Spontaneous mutations are infrequent Factors influencing the rate of spontaneous mutations Accuracy of the DNA replication machinery Efficiency of the mechanisms for the repair of damaged DNA Degree of exposure to mutagenic agents in the environment Muller Demonstration: X-rays induced mutations in Drosophila Three components of CIB chromosomes o C, for cross-over suppressor o I recessive lethal mutation o B bar-eye phenotype (dominant) Male with CIB dies If lethal only female progeny Non-lethal female to mal ratio of 2:1 (sine males with CIB die) Irradiation dosage and mutation frequency Ionization radiation breaks chromosomes and can cause deletions duplications inversions, and translocations Ultraviolet irradiation UV does not possess sufficient energy to include ionizations Readily absorbed by many organic molecules such as purines and pyrimidines in DNA
Image of page 19
Two major products of UV absorption by pyrimidines are pyrimidine hydrates and pyrimidine dimers Cross linking of adjacent thymine forms thymine dimers, which block DNA replication Chemical Mutagens 1. Chemical that are mutagenic to both replicating and non-replicating DNA (alkylating agents and nitrous acid) 2. Chemical that are mutagenic to only replicating DNA (base analogs and acridine dyes) Alkylating agents Ethyl methane sulphonate causes ethylation of the bases in DNA G:C A:T transitions Alkylating agents as a class therefore exhibit less specific mutagenic affects than do base analogs, nitrous acid and acridine dyes Base Analogs 5-bromoracil is a thymine analog, bromine at the 5 position changes the charge distribution and increases the frequency of tautomeric shifts cause G:C A:T transition if it is resent in less frequent enol form A:T G:C transition if in more forewent keto form Mutations originally induced with this thymine analog can also be induced to mutate back to the wildtype with 5-bromouracil Nitrous Acid (HNO 2 ) Causes oxidative deamination of the amino groups in adenine, guanine, and cytosine Converts the amino groups to keto groups and changed hydrogen bonding potion of the modified bases Nitrous acid mutations are also induced to mutate back to wild-type by nitrous acid Intercalation of Acridine Dyes Frameshift mutations When DNA containing intercalated acrdines replicate, additions and deletions of one to a few pairs occur Hydroxylating Agents –Hydroxylamine (NH 2 OH) Hydroxylates amino group of cytosine and leads to G:C A:T
Image of page 20
Chemical Mutagens 46 structures similar to normal bases The Ames Test Screening of chemical for mutagenicity and carcinogens Ames Test uses histidine-requiring mutants of Salmonella to screen chemicals for their ability to induce mutations Phenotypic Effects of Mutations Isoalleles o Have no effect on phenotype or small effects that can be recognized only by special techniques Null alleles o Result in no gene product or totally non-functional gene products Mutations may be dominant or recessive Recessive lethal o
Image of page 21
Image of page 22

You've reached the end of your free preview.

Want to read all 33 pages?

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture