Mutation and Selection Exam - Monday Q&A – Friday 3 PM Higgins 300 Mutation introduces variablity Selection increases the frequency of mutant genes

Researchers believe that bacteria contribute more to earth’s biomass than the sum of animals and plants “ This is truly the age of bacteria: as it was in the beginning, is now and ever shall be ” Stephen Jay Gould How many bacteria fit on the top of a pin? prokaryotes play diverse roles in ecosystems

Nitrogen-fixing bacteria convert nitrogen to ammonia Bacteria are essential for life on earth Atmospheric nitrogen can ’ t be used for metabolism Roots of legumes have nodules colonized by rhizobial species

Cyanobacteria have chlorophyll deposits within internal membranes Carboxysomes are aggregates of carbon-fixing enzymes (dark reaction of photosynthesis) Cyanobacteria use water as an electron donor for photosynthesis, producing oxygen

Soils have complex bacterial populations: play important roles in nutrient cycling, water dynamics and disease suppression Bacteria growing on soil fungi BIOLOGICAL WARFARE!!! Bacterial and fungal populations keep each other in check - soil fungi are a major source of antibiotics USDA – estimates one teaspoon of soil contains between 100 million and 1 billion bacteria (mass equivalent to 2 cows per acre)

Bacteria have been popular models for studying mutations because they divide quickly, doubling approx. every 20 minutes under optimum conditions. Over a period of two hours, one bacterium could give rise to how many bacteria?

Mutations Changes in the DNA sequence Can occur anywhere in a genome Are rare events Can be large or small Effects vary from none to lethality Introduce variability into populations Provide the raw material for evolution

An experiment in evolution Day zero - culture is inoculated with a genetically homogeneous sample of plant bacterium Pseudomonas fluorescens The culture is incubated at 37 degrees for several days A sample is removed and grown on an agar plate

several kinds of colonies are seen after a week (each colony has arisen from a single cell) Fuzzy Spreader Wrinkly Spreader Smooth Morph

Three morphs occupy different ecological niches when they are used separately to start new cultures Stable genetic changes have occurred rapidly Different mutations are associated with each new form

Many different kinds of mutations, with different consequences Point mutation – a single nucleotide is changed Missense mutation – point mutation changes an amino acid Synonymous mutation – point mutation does not change an amino acid Nonsense – point mutation introduces a stop codon Frameshift – nucleotides are inserted or deleted – often changes the position of the stop codon, affecting the protein length

What causes mutations? Chemical changes in DNA Radiation Errors during DNA replication Errors in meiosis (structural changes in chromosomes) Mutagens have characteristic signatures that can often be identified by sequencing the mutant gene Some mutagens (particularly radiation) can cause chromosome rearrangements

Error in replication Mismatched bases Following next round of replication, mutation is passed on to one daughter strand

Chemical mutagens (substances that cause mutations) Most chemical mutagens are also carcinogens (substance that causes cancers) Many mutagens react directly or indirectly with the DNA, leading to base changes and errors in DNA replication

Body converts many pro- carcinogens into carcinogens in the liver Substances with potential mutagenic activity are all around us (and in us)

Radiation Different kinds of radiation have their own mutational signatures

Most radiation exposure occurs as a routine part of life

Long-term health studies of Hiroshima survivors show: Very high rates of cancers, especially leukemias Somatic mutation (not germ) cells Children of survivors have normal cancer rates Germ-line mutations were not significantly elevated Which kinds of mutations are inherited?

The good news: Cells have several kinds of active DNA repair systems Deinococcus radiodurans Withstands radiation doses that are 500 times more than that required to kill a human Also withstands extremes of heat, cold, vacuum, desiccation, acid World’s toughest bacterium in Guinness Book of World Records Maintains multiple copies of its genome and many DNA repair enzymes

Thousands of errors are produced and corrected in each round of cell replication Different kinds of enzymes catalyze each repair pathway

Excision repair Enzymes detect damage Incorrect bases are removed Correct bases are inserted using the undamaged strand as the template Several kinds of excision repair for different kinds/extents of damage

Mismatch repair Often due to errors in DNA replication Enzymes detect mismatched bases and correct the damage DNA repair enzymes may play a protective role against cancer

Selection Mutations the confer selective advantages will increase in frequency in a population

Antibiotic resistance: the coming plague?

Penicillium mold kills Staphylococcus and gram positive bacteria Discovered accidentally by Alexander Fleming in 1929 Penicillin - the first documented antibiotic

Fleming shared Nobel Prize with Florey and Chain, 1945 Resistance was documented even before penicillin was widely available for clinical use! Scaling up penicillin production to an industrial scale was important in WWII war effort Pharmaceutical companies have modified penicillin structure to generate additional antibiotics Produced synthethically today

The target of penicillin is an enzyme required to build the peptidoglycan layer of the cell wall

The microbes strike back Tracks 20-24