Lecture 19 Flashcards
3 grape colours and the difference in pigment gene expression
purple = WT
green = LOF
red = partially-functional allele
what causes the differences in pigment gene expressioni?
insertion of transposable element
what causes the LOF allele for green colour?
insertion of Gret1 retrotransposon with flanking repeats upstream of gene –> exons intact but can’t be expressed so LOF
what causes the partially-functional allele for red colour?
Gret1 cut out but repeats remain
biology of maize
ENDOSPERM
- haploid sperm cell combines with diploid central cell in ovary to form triploid endosperm
EMBRYO
- haploid sperm cell combines with haploid egg cell in ovary to form diploid embryo
genotype of endosperm is the same as _____
genotype of endosperm is the same as the next generation bc has chromosomes from male and female
which maize cells have the same sets of chromosomes?
egg cell and central cell have the same sets of chromosomes
both sperm cells have the same sets of chromosomes
3 alleles involved in maize pigment
C = dominant, purple
c = recessive, no pigment
C1 = dominant negative, no pigment
phenotype ratio of F2 with C/C x c/c parental cross
3:1
what is the prediction if you cross C/C ovule with C1/C1 pollen? what actually happens
make C1/C
PREDICTION: predict fully colourless bc C1 dominant neg
ACTUAL: spotted kernel, therefore something allowing pigment to be made
since there was a spotted kernel, what is the hypothesis?
what is the experiment?
what is the result?
what is the conclusion?
hypothesis: C1 allele is lost in some cells of the developing endosperm
experiment: look for breakage in Chr of pollen parent
result: high rate of breakage
therefore: loss of C1 allele due to chromosome breakage
kernel phenotype without Ac
Ds is stable –> no cleavage –> yellow kernel
kernel phenotype with Ac
Ac allows Ds to cleave C1 off the chromosome –> yellow kernel with purple spots
loss of C1 means C function is no longer blocked to allow purple
kernel phenotype with recessive alleles and Ac
Ac allows Ds to cleave, allowing the recessive alleles to be uncovered and give the bronze phenotype
therefore, the break always occurs at the same place –> not random breakage
do the breaks occur in the same location in all strains? how do they know?
breaks occur in diff location in related strains, can see by which phenotypes are uncovered
kernel phenotype with recessive allele and Ac when Ds JUMPS to diff place
yellow kernel –> just jumps so it maintains WT alleles and recessive alleles not recovered bc C1 maintained
kernel phenotype with recessive allele and Ac when Ds cleaves after jumping
yellow kernel with purple spots, complementation is maintained by C1 is cleaved
what is the progeny when homozygous c is crossed with homozygous C (where one of the chromosomes has Ac)?
50% are fully purple (C/c)
50% are purple with yellow spots
*some rare ones are yellow with purple spots
why are 50% of progeny fully purple?
inherited chromosome without Ac –> no Ac = Ds is stable = maintain C = all cells C/c
why are 50% of progeny purple with yellow spots?
inherited chromosome with Ac –> Ac allows Ds to cleave C = some cells are c/c = some spots yellow
why is there a rare type of progeny where kernels are yellow with purple spots?
inherited chromosome with Ac –> Ds jumps INTO C, creating new c-m1 allele to make yellow
purple parts = Ds jumps out of c-m1 and restores ability of C to make pigment
Ds vs Ac
Ds = non-autonomous
Ac = autonomous
non-autonomous transposable element
does not make its own transposition machinery, relies on another transposable element
autonomous transposable element
encodes its own transposition machinery
what determines spot size?
depends on when Ds jumps out to make C gene become active during endosperm development
what happens if Ds jumps out EARLY in endosperm development?
large spot
what happens if Ds jumps out LATE in endosperm development?
small spot
which organisms have transposable elements?
all organisms
2 classes of transposable elements
- retrotransposons
- DNA transposons
what type of organisms have retrotransposons?
eukaryotes
what are retrotransposons similar to?
retrotransposons
describe movement of retrotransposons (6 steps)
- in DNA, flanked by LTR
- transcribed into transposon genome by cell machinery
- exported
- translated
- RT makes ds transposon genome
- goes back to nucleus to insert into DNA at terminal repeats
evolution of retrotransposons
retroviruses got into cell, integrated into genome but lost env gene so gets trapped in cell
what organisms are DNA transposons in?
prokaryotes and eukaryotes
describe movement of DNA transposons
- Ac encodes transposon
- transposon cleaves Ac/Ds
- transposon + Ac/Ds integrate into target DNA in new location
what is the P element in drosophila?
encodes 1 protein (transposase) which is flanked by 31 bp inverted repeats in DNA
