Variation and Mutation Flashcards
where does variation come from? (3)
environmental variation: individuals can possess diff phenotypes as result of exposure to diff env, despite identical genotypes
genetic variation: individuals can possess diff phenotypes as result of diff genotypes, transmitted from parents to offspring
genotype by environment interaction: individuals can possess diff phenotypes as a result of interaction of genotypes w environment
phenotypic plasticity
genetically identical individuals can have diff phenotypes in diff environmental conditions
reaction norm
pattern or range of phenotypes that same genotype can possess as result of diff environment
mutation
- ultimate source of genetic variability
- a change in DNA base sequence
- discovered by T.H. Morgan (fruit flies)
- generally classed single gene mutations OR chromosomal mutations (which affect many genes)
- initially, new mutation at diploid locus is 1/(2N), where N = pop size
mutagens
can cause mutations
DNA structure
G-C (3 H bonds)
A-T (2 H bonds)
phosphodiester linkage
sugar-phosphate backbone
flow of genetic info
DNA -> mRNA -> protein
transcription translation
64 possible codons, 20 possible AAs
stop codon
UAA/UGA/UAG
start codon
AUG (methionine)
DNA mutation
due to DNA alteration
due to DNA copying error (most common in repeating sequences)
point mutations
- single base pair changes
- results in new alleles
- transitions more common than transversions
transition and transversion
transition - purine to purine or pyrimidine to pyrimidine
transversion - purine to pyrimidine or vice versa
3 potential impacts (point mutation)
(1) synonymous (silent) mutations - no change in AA
(2) non-synonymous mutations - change in AA
(3) non sense mutations - premature stop (often produces non-functional proteins)
insertions or deletions
- like point mutations, can vary in effect
- any non-multiple of 3 (insertion/deletion) will shift codon reading frame
fitness impacts of mutation
mutation may result in phenotypic changes
- neutral - no impact
- deleterious (disease-causing) - reduces individual fitness
- beneficial - increases individual fitness
- lethal - organism death before reproduction
sickle cell anemia
- deleterious allele persistent in pop
- homozygous mutant allele: sickle-cell anemia
- homozygous normal allele: susceptible to malaria
- heterozygous: resistance to malaria (heterozygote superiority maintains mutant allele)
where do new genes come from?
gene duplication via 2 mechanisms:
(1) duplication by unequal crossing over (meiosis)
- homologous chromosomes align incorrectly
- inner chromatids cross over at non-allelic repeat loci
(2) duplication by retroposition
- introns spliced out (processed mRNA) -> reverse transcription
- this copy can be inserted into genome (same exons, no introns => new gene); often non-functional
exon/intron
exon - coding region
intron - non-coding region
pseudogene
non-functional; no regulatory sequences for transcription to RNA
comparison of unequal crossing over and retroposition
unequal crossing over - genes have same introns as original genes, typically occur in tandem with original genes on same chromosome
retroposition - genes typically lack introns, may be found far from original gene
DNA derived from scratch
derived from non-coding regions of DNA (e.g. removed stop codon)
chromosome mutations
may affect only gene order and organization and/r produce duplication or deletions that affect large amount of genetic material
chromosome mutations types (2)
inversions (loop over, breaks, reanneal)
- drosophila: suggests due to linkage, higher fitness for certain inversions
- genes closer together on chromosomes are closer linked
polyploidy
- diploid gametes
- 1st gen can self-fertilize, mate w 4n sibling, or backcross w parent
- tetraploid 2nd gen individuals
mutation rates & natural selection
- the rate at which mutations occur subject to natural selection
- heritable variation exists in accuracy of enzymes that synthesize and repair DNA
- there is variation in fitness among variants in particular environments
- individuals w higher mutation rates have higher fitness ONLY when exposed to novel, highly variable environments
