Genetics Lecture 25, 10/31; Chapter 15 DNA repair and mutation

DNA repair and mutation
Looking specifically at alterations to the chemical composition of DNA

• (1:45) Mutation is the basis of genetic study. Mutate a gene and the determine the phenotype
• Certain organisms are easier to mutate, for example: bacteria, fungi, yeast, viruses and fruit flies. THey have shorter lifespans.

(4:35) How mutations arise.

• Two models for how bacterial mutations arise
• Spontaneous: mutation occurs at any given time
• Adaptive: cell will mutate in response to environmental pressure
• EX. Antibiotic resistance: the cell will mutate in response to the presence of the antibiotic.
• 1943 - Luria and Delbruck set out to test the two models. They set up a number of cultures of e. coli. Sensitive to infection with the T1 phage. Prior to starting the counted the total number of cells. They then exposed the cells to phage. Following exposure they counted number of survivors.
  
• Survivors are cells witha mutation that allowed them to survive infection.
• start with small # of cells and grow them.
  
• Spontaneous mutation could occur at any time.
• if it happens ealy 1 cell picks up a mutation and divides. All cells from this cell have the mutation. High percentage of cells with a mutation
• if it happens late you get orginal cell with the mutation and a few rounds of cell division and a small percentage of resistant cells.
• in the experiment they looked at 10 cultures. SPontaneous model would propose that there would be variety in the number of resistant cells in the cultures. In the adaptive model mutation is the result of environmental pressure. THis suggests that with a number of different cultures, if all cells are exposed to phage at the same time then each culture should have mutation occuring at the same time. Each culrue should have about the same number of reistant cells - adaptive model
• found that 10 cultures had a wide variety of resistant cultures. Indicates the mutation is spontaneous. Reffered to as the fluctuation test.
  
• Additional work has been down to show that mutation is primarily spontaneous. There are rare occasions when mutation is adaptive.

(23:00) How to classify mutations

Spontaneous vs. induced.

• Spontaneous mutation arises naturally in a spontaneous fashion. No exposure that led to the mutation.
• Induced mutation: some envionmental exposure leads to a mutation. EX: Smoke two packs a day and you develop a mutation --> lung cancer.

(27:10) Somatic mutations vs gametic mutations

• somatic mutations arise in somatic cells. A somatic cell mutation typically leads to a small population of cells with that mutation.
• Gametic mutations - mutations in gametes. A gametic mutation in a gamete involved in a fertilization, that offspring will have that mutation in every cell.

(32:20) Broad categories of mutation

• morphological mutation. A mutation which alters the structure of a all organisms.
• Nutritional mutation: Lack the ability to synthesize an amino acid vitamin or a mutation that prevents you from breaking down some compound.
• Behavioural mutation: any mutation which alters the behaviour of an organism. EX. Fruitflies - beating of wings is important in mating. A mutation that impacts the ability of a fruit fly to Beat its wings will have behavioral consequences.
• (38:00) Regulatroy mutations: mutations in genes which produce proteins that regulate the expression of other genes. Alter the way the regulated gene is expressed.
• (40:10) Lethal Mutation: any mutation which kills an organism
• Conditional mutation: only seen under certain conditions.
• EX temperature sensitive mutation: a mutation that is scene at a particular temperature.