lecture 8 - sae 1 Flashcards
describe interactions between genes in a single pathway
one gene one enzyme hypothesis
genes in sam pathway
assume on diff chromosomes - not a distance factor
like amino acids - things build on each other to eventually make final product
how to identify genes involved in a biochemical pathway
forward genetic screens - xrays shine through organism so random gene mutations
describe process of identifying genes involved in biochemical pathway
isolate each spore in the ascus and grow each separately in tubes
genotype the same since starting from same
on complete medium = has everything it needs to grow, like nutrients
on minimal medium = basically carbs - sugar molecules, lacks vitamins and aas = MUTANT WONT BE ABLE TO GROW = since cannot synthesize certain metabolites
what happens if cannot grow on minimal medium
identifies nutritional mutant
start adding back stuff that mutant could need
= discovers its aas missing
then test on medium - each different aa, when arginine supplemented = will grow, so mutant cannot produce arginine from basic metabolites
describe interactions between genes in pathways - ex
difference mutants on enzymes = can affect pathway of making an amino acid
arg 1 = can growth as long as ornithine
arg 2 = can grow as long on citrulline
arg 3 = cannot grow with precursors need arginine directly
if mutation on something upstream = downstream pathway intact
describe what forward genetic screening tells us
mutants carrying mutations in diff genes can be isolated - arg1, arg2, arg3
multiple mutant alleles of the same gene can also be isolated - diff mutant alleles carrying mutations in arg1
what must we first determine when studying gene interactions
determine whether mutations are alleles of the same gene or diff genes
loss of function mutations = affected by mutation of same gene
what is complementation test
can be used for recessive mutations
to figure out if on same gene
describe is 2 diff genes - complementation test
if appears wild type
must create heterozygous
chances more likely that it carries wild type
chance it being mutated = low
no longer see deficiency since each gene complemented by wild type of other gene
describe if 2 alleles on same gene - complementation test
homozygous
for mutants = cross with another mutant
appears mutant
since failed to complement = must mean it on same gene
describe complementation testing - white flowered mutants
white recessive to blue
F1 monohybrid cross will tell if phenotype is caused by on gene or multiple genes = 3:1 ratio of blue to white - if deviated = can assume mutation caused by more than one gene
do complementation test - if failed to complement= must have mutation on same gene
what does genetic interaction result in
modified 9:3:3:1 ratio
phenotypically
describe how ratio can be changed - genetic interaction
2 enzymes make flowers blue - double mutant = acts as only have single mutant
- same if acting on same pathway
9:7 ratio = 2 genes in same pathway
same linear pathway - most of time
describe interaction between regulator gene and target gene
target gene depends on transcription factor (expression of A completely depends on transcription factor coded by R)
NORMAL = 9
NO protein produced = when mutation in gene that encodes regulatory protein and when mutation in both genes
mutant protein = when mutation in gene that encodes structural protein
THESE 3 all produce non functional = 7
if 9:7 ratio = assume act on same pathway, have regulatory relationship to each other
describe mendelian ratio of 2 genes affecting the same characteristic but work independently
act on same biological process but diff pathway
2 genes control snake colour = O (orange) and B (black)
O+B = camouflage
= typical ratio 9:3:3:1 (camo, black, orange, albino)
have nothing to do with each other functionally, double mutant diff than single mutant
what is epistasis
a situation where phenotypic manifestation of an allele is dependent on the genotype of a different gene
does not have to be on same pathway but can be
what is recessive epistatic - example, formal
recessive genotype of one gene - w - masks pink flower phenotype associated with other gene - m -
w is epistatic to m
in this context = dihybrid cross = 9:3:4 ratio (4 = white flower)
can distinguish one of single mutants
what is recessive epistatic - example, gen
same pathway
intermediate has magenta colour
can distinguish phenotype
if single mutant = end up with pink
if double mutant = looks like single mutant of white alone (so 2 mutations give same phenotype), cannot produce pigment
what is a suppressor
mutant allele of a gene that reverses effects of an original mutation
describe ex of supressor
ex = function by forming heterodimer
first mutation = changes aa sequence so cannot heterodimerize so
2nd mutation = becomes functional, new allele can interact with modified mutant gene and together suppress the phenotype associated with the gene
Suppressor provides downstream product = can suppress phenotype of mutant
what is a modifier
2nd mutation that changes the degree of expression of a mutated gene - phenotype
regulatory function
describe ex of modifier
mutations in regulatory sequences
regulates a in absence of b, 30% of gene a expressed
changes degree of phenotypic manifestation
what is synthetic lethal
mutations in 2 genes
each often has a weak mutant phenotype
Resulting in lethality
give ex of synthetic lethal
multiproptien complexes
mutant a = leads to partial binding
mutant b = leads to partial binding (weak phenotype)
mutant c = leads to impossible binding - none functional so lethal
used to screen for cancer therapy drugs = does not affect normal cells but specifically cancer cells - with cancer promoting mutation
describe modified rations between genetically interacting genes
9:3:3:1 = no interaction
9:7 = genes in same pathway
9:3:4 = recessive epistasis
12:3:1 = dominant epistasis
13:3 = supressor has no phenotype
14:2 = suppressor is like mutant
numbers vary on phenotypes and degree of suppression