Molecular Evolution Flashcards
What is ‘on the origin of species’ about?
→ Theory to explain the current variety of life on earth
What are the two main concepts in ‘the origin of species’?
→ Natural selection and fitness
What is the definition of natural selection?
→ The effects of a wide range of factors on the frequency of heritable changes in a species
What is the definition of fitness?
→ How well a species is able to reproduce in its environment
What is the relationship between fitness ad selection?
→ Anything that increases fitness is selected for
→ anything that decreases fitness is selected against
What happens to neutral changes?
→ They vary randomly
What is modern synthesis?
→ unifying evolution with genetics to explain the molecular processes underlying evolution
What is the main source of heritable variation in a species?
→ Genetic variation
What 4 things are the frequency of genetic variants affected by?
→ Selection
→ Mutation
→ Migration
→ Genetic drift
What types of genetic variants are selected for?
→ variants that confer a positive advantage
What is an example of a positive advantage?
→ resistance to disease
→ an ability to metabolise a new food source
→ change in appearance to enhance mate choice
What parts of the genomes are resistant to change and why?
→ They are conserved
→ because they contain vital sequences
What is mutation?
→ process by which variation arises
What does genomic variant frequency depend on?
→ Selection
→ When the variants first occurred
What are the three possibilities of low frequency in a rare variant?
→ May have arisen recently
→ be deleterious and being selected against
→ both
What is migration and what does this result in genetically and what is this called?
→ Physical movement from a different population
→ Result in new pools of variants being introduced to an existing population
→ Admixture
Why does admixture need to be taken into account when studying populations?
→ Population frequencies of specific variants can change purely due to admixture and not be disease related
What is genetic drift?
→ How the frequency of a variant changes in a population due to chance
What are 2 reasons that variation is not passed on?
→ Not all organisms will pass on their genetic variants
→ mechanisms such as recombination will also result in not all variants being passed on
What are all variants subject to?
→ Genetic drift
What types of sequences don’t show variation?
→ DNA sequences that is vital to the survival of an organism
What happens if variants occur in conserved regions?
→ They will be selected against as they are likely to have a strong deleterious effect
Why is there some flexibility in variation in conserved regions?
→ there is flexibility in the third base of codons as some amino acids are encoded by multiple codons
Where is high conservation seen and what is the exception?
→ Coding regions
→ but not in exons as they have non coding regions
→The further you move on the tree from us, the less introns we have in common with them
Where is intermediate conservation seen?
→ promoter
→ 5’ UTR
→ Splicing sites
→ 3’ UTR
→ Terminator
Where is low conservation seen?
→ Introns
→ 3rd base of codons
→ Terminator (vague signals such as GC rich regions)
What can be used to generate an evolutionary profile and why?
→ Cross species comparison
→ To see how genes change over time
What is phylogenetics?
→ Working out the history of the genome
What is needed for phylogenetic sequencing?
→ Observe sequences
→see how are they related
What does a phylogenetic tree show?
→ Illustrate relatedness of different species
What does distance show on a phylogenetic tree?
→ The more distance the less related they are
→ distance is related to both evolutionary pressures and time
How is time estimated on phylogenetic trees?
→ measuring mutation rates
What is the theory behind the introduction of HIV to the human population?
→ a contaminated polio vaccine
→ some polio vaccines were produced with cultured chimpanzee cells which could have been infected with SIV
What is gene duplication?
→ Duplication of a DNA sequence containing a gene, can be multiple genes
What is the typical mechanism of gene duplication?
→ Unequal crossing over
What are the two options after duplication?
→ One copy can continue the original function
→ the other copy can evolve new functions by changes in the coding or control sequences
→Means possibilities of related genes arising that allows better adaptation.
What is unequal crossing over?
→ Recombination between sequences that are not the correct sequence but are very similar
→ often low copy number repeat sequences
When does unequal crossing over usually happen?
→ often low copy number repeat sequences
How many clusters of the globin gene are there?
→ 2
Where are the alpha like globin genes?
→ chromosome 16
→ 3 genes
→ 3 pseudogenes
Where are the beta like globin genes?
→ chromosomes 11
→ 5 genes and one pseudo gene
How are the globin genes arranged?
→ In order of expression during development
What are the 3 types of Hb?
→ Embryonic
→ Fetal
→ Postnatal
What is the control region of the beta globin cluster?
→ beta LCR
How have globin genes evolved?
→ Divergence
What are pseudogenes?
→ non functioning genes
What allows the expression of globin genes at different stages?
Divergence of promoters has occurred so they bind different transcription factors and allow expression of genes at different stages of development
What is fetal Hb made from?
→ Alpha and gamma subunits
What is adult Hb made from?
→ Alpha and beta subunits
How are pseudogenes formed?
→ After gene duplication one gene maintains original function and the other diverges
What do pseudogenes contain?
→ many mutations and are non functional
When do sickle cell symptoms start and why?
→ 5- 6 months
→ change between HbF and HbA
What are the symptoms of sickle cell?
→ Anaemia
→ Acute pain episodes - crises - due to oxygen deprivation
→ Increased infection
→ stroke, hypertension
What are the symptoms due to?
→ Sickling of red blood cells
How does sickle cell occur?
→ A single base change in the beta globin gene of HbA
What is the codon change in sickle cell?
→ GAG to GTG
→ Glutamine to valine at position 6
What kind of a disease is sickle cell?
→ Autosomal recessive
What is the chance of a child getting SCD if the parents have 1 copy each?
→ 1 in 4
What is the distribution of HbS?
→ Africa
→ Middle east
→ India
Why has sickle cell not been selected against?
→ with one copy of HbS it improves reproductive fitness
→ because it confers resistance to severe malaria
What does heterozygote advantage mean for HbS?
→ The variant is maintained because of the huge selection pressure
