Week One Flashcards
How did mitochondria become the powerhouse of the cell? What were the consequences?
Mitochondria (aerobic bacterium) was enveloped by pre-eukaryote and started a symbiotic relationship. Mitochondrion produced Oxygen for ATP formation, but produced reactive oxygen species as a byproduct. Peroxisomes were needed to neutralize these, as the cytoplasm is a reducing environment.
Why did Eukaryotic sex originate in organisms? What are the advantages of the diploid stage?
- Repair of double stranded DNA damage from the homologous chromosome.
- Complementation of one allele by the second copy:
- -> Compensates for recessive deleterious mutations.
- ->Heterosis: two different alleles are better than two of the same.
Describe why sex may have originated before Eukaryotes. What is the mechanism of this?
Replication of genetic material allows selfish genetic elements an opportunity to propagate themselves within and between genomes. These genes could cut, copy and paste; shuffling genes and causing a recombination-like process, in extension, the origin of sex. The use of sex for repair might have been secondary.
What are the methods of genetic exchange for bacteria?
Conjugation: direct connection between cells using a pilus. Ex. the F-factor in E.coli.
Transformation: taking up DNA from the environment.
Transduction: viral transfer of DNA between cells.
What is the mechanism of conjugation?
How is it confirmed in the lab?
1) F- plasmid inserted into bacterial chromosome.
2) Plasmid drives transmission of foreign DNA.
3) Recombination between foreign and resident DNA
4) A new, competent, cell is born through gene conversion.
Use auxotrophs to confirm; if it needs a new compound for survival, then conjugation occurred.
What is an auxotroph, why is it important when studying conjugation?
What is the difference between Gal+ and Gal- genes?
A mutant organism (typically a bacterium or fungus) that requires a particular additional nutrient that the normal strain does not. It is unable to naturally synthesize this compound. When new gene combinations are produced, the cell can survive on different sources of medium; which is evidence of gene shuffling.
Gal+ : the organism can survive on minimal medium as it can synthesize its own Gal.
Gal- : the organism can survive on minimal media when Gal is supplemented.
What is the criticism of Bacterial sex? Can it really be called sex?
While eukaryotic sex is mutually beneficial, bacterial genetic exchange is non-reciprocal.
- Rosie Redfield argues that conjugation and transduction are ‘parasexual’ and driven by selfish genetic elements.
- -> She thinks that bacteria are not likely to share the same benefits from recombination that eukaryotes do. 1/billion “good” cells are not worth it for the cost of sex.
- -> Parasex is a parasite or coincidence; machinery of recombination likely to have evolved to affect repair, not exchange.
- -> Transformation; the ability to eat DNA was more likely to have evolved as a feeding mode.
So why did non-reproductive sex evolve in the first place?
When does the population benefit from it?
Started through low-cost gene parasites like F plasmids; which then capitalize on DNA rep. machinery to reproduce.
- -> Inheritance of plasmid and new genes can be dangerous to the organism, but still spread because of the freak accident of of resistance!
- -> Population serves to benefit from selfish elements especially when the environment is changing or the mutation rate is high.
What are the costs and benefits to having plasmids?
Costs: Energetic and physiological costs are estimated at 0% - 10% reduction in competitive fitness for F in E.coli.
Benefits: DNA repair
Spread of resistance genes, Stop Muller’s Ratchet, Speed up adaptation.
What are the three indicators of sexual reproduction in Eukaryotes?
1) A reduction from 2n to 1n via meiosis.
2) Recombination between homologous chromosomes.
3) The alternation diploid to haploid (gamete) phases in the lifecycle.
–> In animals, the diploid stage predominates but in some plants, the gametophyte is the thing we encounter most.
What is the “2-fold cost” of males for sex?
Making males means that 50% of your offspring are dependent on famales for propagation of genes.
–> It reduces the rate of increase of a population by half.
A sexual female also halves her genetic contribution to the next generation by 50%.
–> One theory is that males evolved as parasites.
Why is sex so costly?
Mates must find one another.
Fertilization is often inefficient.
Direct conflict and injury mating.
Missed opportunity costs; feeding, growing.
Increased predation risk.
STDs
Competition for mates; intrasexual contests, expensive ornaments.
Females and males share the same gene pool; genes that function well in one sex may not do well in the other.
What is the Muller’s Ratchet genetic theory?
Muller’s Ratchet: Sex prevents the erosion of fitness by relentless mutation accumulation in the genome.
What is the Interference: Fisher-Muller genetic theory?
Sex is beneficial because it allows the combination of beneficial alleles at different loci in the same individual.
- helps with progressive evolution (ex. adaptation to new environments) by creating new high fitness genotypes.
- reduces ‘clonal interference’ between genotypes.
What is the interference: RR genetic theory?
Interference: Ruby in the Rubbish (RR): Frees beneficial alleles from linked deleterious variation - reduces background selection.
What is the Mutational Deterministic (MD) genetic theory?
Synergism of mutational effects leads to more efficient elimination when they occur in the same individual genome.
What are the requirements of Muller’s ratchet, so it can be a proper predictor of asexual reproduction?
Finite population size creates genetic drift - gene frequencies change by chance, leading to extinction of alleles in populations: the good and the bad.
-Mutations must also occur randomly across the genome.
Why is Muller’s ratchet ultimately a “march of the living dead”?
Mutations are predominantly deleterious –> most new mutations in coding sequence are believed to be small-effect and harming, due to alteration or loss-of-function in a working gene.
Clonal lineages have no mechanism to rid themselves of new harming mutations.
Fitness decreases with the number of mutations! There is a directional effect of mutation. Lines are also lost due to genetic drift (by chance).
Why does sex stop Muller’s ratchet?
It recreates the low mutation classes which were lost due to directional mutation or drift.
-Crossing over generates both more and less mutant-bearing offspring, but critically restores the most fit class.
How does the Fisher-Muller interference genetic theory explain the benefits of sex?
Beneficial mutations, by chance, occur in separate clonal lineages.
- If two bacterial pop. are competing, they will evolve to increase fitness by displacing slower-reproducing types.
- Each interferes with the other by becoming a better competitor.
RECOMB. (stimulated by F) would allow a single individual genome to obtain both beneficial mutations.
Otherwise…they would have to “wait for mutation”, and beneficial mutations are extremely rare.
How did Cooper test FM Interference? What were the results?
F-M predicts that competition between beneficial mutations will limit the rate of adaptation.
He used E.coli to test the theory; by using F plasmids that induce conjugation.
-When mutation was frequent, recombination increased the rate of adaptation by 3x…THREE TIMES compared to clones.
Why does gene linkage reduce the effectiveness of certain genes?
Hitchhiking: low or average fitness genes can be dragged to high frequency if linked to high fitness genes.
How does sex reduce background selection?
Background selection: high fitness genes may not be able to spread if they are trapped in poor genetic backgrounds.
Recombination reduces genetic linkage.
