Molecular evolution Flashcards
What are the concepts in Darwin’s “The Origin of Species” Theory?
Two concepts explain the current variety of life on Earth:
- natural selection
- fitness
Explain natural selection.
A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits
Define fitness.
How well a species is able to reproduce in its environment
What is the main source of heritable changes in a species?
Genetic variation
What are genetic variant frequencies affected by?
- selection
- mutation
- migration
- genetic drift
What is the relationship between selection and genetic variant frequencies?
genetic variants with a positive advantage will be selected for (and vice versa) because it will be more fit for its environment e.g.:
- resistance to disease
- ability to metabolise new food source
- antibiotic resistance
- change in appearance that enhances mate choice
What are conserved regions of the genome?
These regions are said to be resistant to change as they contain vital sequences
What are mutations in genetic variant frequencies?
the process by which variation in the genome arises
a rare variant may:
- arisen recently
- be deleterious
- both (arisen recently and be deleterious)
What is migration?
When there is physical movement of people from a different population resulting in new pools of variants being introduced to an existing population. This is known as admixture
What is genetic drift?
Genetic drift is how the frequency of a variant changes in a population due to chance
-all variants are subject to genetic drift
Not all organisms in a population will pass on their genetic variants
-mechanisms such as recombination will also result in variants not being passed on
Which DNA sequences do not show much variation?
Any DNA sequence that is vital to the survival of an organism does not show much evidence of variation for obvious reasons.
Most variants in these regions will be selected against as they are likely to have a strong deleterious effect
What is sequence conservation (resistance to change)?
High Conservation Regions
-coding region (not exons as exons have non-coding regions)
Intermediate Conservation Regions
-promoter, 5’UTR, 3’UTR, terminator and splice site regions
Low Conservation Regions
-introns, 3rd base of codons and terminator regions
Why is there flexibility/low conservation in the third base of codons?
Because some amino acids are encoded by multiple codons
What can be used to see how a gene changes over time?
Cross-species comparison can be used to generate an evolutionary profile for .a gene or gene family to see how genes can changes over time
What is phylogenetics?
the study of evolutionary relationships and genetic connections among species
What is a phylogenetic tree?
A family tree that shows the evolutionary relationships thought to exist among groups of organisms
Distance between entities on a tree is usually related to how similar they are
What is gene duplication?
duplication of DNA sequence containing a gene due to unequal crossing over during meiosis
What is the Consequence of gene duplication?
one copy can continue being the original functioning gene and hence functioning protein
other copy diverges and can evolve new function(s) by changes in the coding sequence and/or control sequences (can also become pseudogene)
What us unequal crossing over?
Misalignment of the two DNA molecules during crossing over, resulting in one DNA molecule with an insertion and the other with a deletion
These are typically low number repeat sequences
What are globin gene clusters?
Two clusters:
- Alpha-like are on chromosome 16, with 4 genes and 3 pseudogenes
- Beta-like are on chromosome 11, with 5 genes and 1 pseudogene
How are globin genes arranged?
in order of expression during development
How have globin genes evolved?
through duplication and accumulation of mutations (divergence)
some are functioning genes and some are not (pseudogenes)
How are globin genes expressed at different stages of development?
Divergence of promoters has occurred so they bind different transcription factors and allow expression of genes at different stages of development
Embryo>Foetus>Postnatal
What are the Predominant forms of haemoglobin?
HbF (foetal)- alpha and gamma
HbA (adult)- alpha and beta
What are pseudogenes?
former genes that have accumulated mutations as a consequence of gene duplication over a long time and no longer produce functional proteins
-pseudogenes complicate PCR/sequencing/etc.
What is sickle cell disease?
A human genetic disease caused by a recessive allele that results in the substitution of a single amino acid in the haemoglobin protein; characterised by deformed red blood cells that can lead to numerous symptoms.
“sickling” of red blood cells, which tends to block blood vessels
When do symptoms of sickle cell disease arise? And why?
start at 5-6 months of age because there is a change in haemoglobin at this stage from HbF to HbA
What are the Main symptoms of SCD?
Anaemia
-fatigue, restlessness, jaundice
Acute Pain Episode “Crisis”
-due to oxygen deprivation of tissues
Increased Frequency of Infections
-due to spleen damage
Stroke Pulmonary Hypertension Gallstones Liver and Kidney Problems Joint Problems Delayed Puberty
What is the cause of SCD?
Single base change in beta-globin gene of HbA
Codon changes from GAG (glutamic acid) to GTG (valine) at position 7 of the protein
What is an Advantage of a sickle cell CARRIER?
one copy of the HbS variant confers resistance to severe malaria
this heterozygote advantage means that HbS variant is maintained in the population when otherwise it would have been selected against and lost
