Lecture 29 Flashcards
general development of drosophila
within 1 day becomes multicellular, then at 9 days become full drosophila
describe 2 stages of cell division of drosophila
- NUCLEAR DIVISION
- CELLULARIZATION
what happens during drosophila nuclear division?
replicate DNA and divide many many times, then nuclei migrate to PM
what happens during drosophila cellularization?
individual nuclei are enclosed in PM to form cells
describe the genes used in drosophila development
at beginning: only using mRNA from mother (cytoplasmic inheritance) so there is no transcription
later: transcription of embryo’s own mRNA (mendelian inheritance)
2h to 10h to adult stages of drosophila
2h: embryo has defined anterior, posterior, back, and front regions
10h: body divided into 14 segments
adult: each segment forms specific structures
what was the heidelberg screen?
forward genetic screen to identify genes required for Drosophila embryo organization
2 types of genes investigated in heidelberg screen?
- maternal genes
- zygotic genes
what is the purpose of the screen for maternal genes?
find the mutations in the mother that prevent her offspring from completing embryonic development –> i.e. what mRNAs from cytoplasmic inheritance are required for development?
what are maternal-effect genes?
genes whose phenotype are determined by maternal genotype
experiment to find maternal-effect genes
paternal genotype doesn’t matter: if it carries mutation, offspring are normal
only maternal genotype matters: if it carries mutation, offspring are mutant
what is the purpose of the screen for zygotic genes?
find the genes from embryo that are required for embryonic development
what is drosophila development regulated by?
cascade of TFs that turn on genes
5 genes involved in drosophila development
- egg-polarity genes (maternal-effect genes)
- gap genes
- pair-rule genes
- segment-polarity genes
- homeotic genes
what is the role of the maternal-effect genes?
establish anterior-posterior axis
2 maternal-effect genes
- Bicoid
- Nanos
what is the role of bicoid?
form anterior axis
bicoid mutants
lack anterior structures (head, thoracic segments)
how does bicoid work?
concentration gradient: high [bicoid] at anterior end, low [bicoid] at posterior end
tells nucleus how far cell is from the front of organism
how is the bicoid concentration gradient formed? (3 steps)
- bicoid mRNA from mother localizes at anterior of embryo
- bicoid protein translated at anterior
- bicoid protein diffuses toward posterior
bicoid is _______ and ________ to form anterior structures
bicoid is REQUIRED and SUFFICIENT to form anterior structures
how do we know that bicoid is sufficient to form anterior structures?
inject bicoid mRNA –> anterior structures form
what is the role of nanos?
form posterior structures
what happens to nanos mutants?
lack posterior abdominal structures
how is the nanos concentration gradient formed?
- nanos mRNA from mother localizes at posterior of embryo
- nanos protein translated at posterior
- nanos protein diffuses towards anterior
role of nanos
inhibits translation of maternal HUNCHBACK mRNA
where is hunchback mRNA localized?
uniformly-spread thru embryo
describe the inhibition of hunchback by nanos
nanos is found in high [ ] at posterior and low [ ] at anterior –> since hunchback is uniformly-distributed, it gets inhibited by nanos so there is low [hunchback] at posterior and high [ ] at anterior
what is the role of gap genes?
translate maternal anterior-posterior gradients into subdomains
what is an example of a gap gene?
hunchback
how many gap genes are there? what type of proteins do they encode?
9
all encode TFs
what regulates gap genes? how?
regulated by maternal-effect genes, TRANSCRIPTIONALLY
what happens to gap gene mutants?
large gaps in the body due to loss of several consecutive segments
how does bicoid affect hunchback transcription?
hunchback promoter has 3 bicoid binding sites to activate hunchback expression at anterior (where there’s high [bicoid])
role of pair-rule genes?
make 7 pairs of segments –> make 14 segments
what happens to pair-rule gene mutants?
absence of every other segment
how many pair-rule genes are there? what type of proteins are they?
8
all TFs
how are pair-rule genes expressed?
each gene is expressed in 7 stripes (stripes at diff positions depending on gene)
2 examples of pair-rule genes?
Eve and Ftz
what happens if you mutate Eve
7 stripes will be lost, but not the other stripes in each pair bc those are determined by Ftz
what does Eve mean?
Even-skipped
what regulates pair-rule gene expression?
maternal effect and gap genes
in Eve stripe 2, what is the relative expression of hunchback, bicoid, and two gap genes (giant and kruppel)?
high expression of Eve
since at anterior, high [hunchback] and medium [bicoid]
low [giant and kruppel]
describe the activation of eve in specific stripes
eve gene has different enhancers to regulate eve expression in diff stripes (ex. stripe 2 has its own enhancer, stripe 3,7 share an enhancer, stripe 4,6 share an enhancer, etc.)
each enhancer has diff arrangements of binding sites for maternal-effect and gap genes to regulate expression
which genes are eve activators? how many binding sites do they each have?
bicoid - 5 binding sites
hunchback - 1 binding site
which genes are eve repressors? how many binding sites do they each have?
kruppel - 3 binding sites
giant - 3 binding sites
when are segment-polarity genes expressed?
once cellularization has occurred
what is the purpose of segment-polarity genes? (2)
to encode components of cell-cell signaling pathways (Hedgehog and Wingless) and define anterior/posterior within each segment
3 examples of proteins encoded by segment-polarity genes?
- secreted proteins
- membrane receptors
- TFs
what genes control segment polarity genes?
pair-rule genes
what happens to segment polarity gene mutants?
half of each segment is mirrored (lose defined anterior vs posterior)
what is the purpose of homeotic genes?
determine the function of each stripe
what happens to homeotic gene mutants?
drosophila lack a certain structure which is replaced by another structure normally found on other body segments –> HOMEOTIC TRANSFORMATION
2 examples of homeotic mutants
- Ultrabithorax - second thorax and set of wings
- Antennapedia - leg instead of antenna
what are the homeotic genes called?
hox genes
how many hox genes are there?
8
what do hox genes encode?
homeo-domain family TFs
where are hox genes expressed?
expressed in specific segments and there is some overlap btwn segments
what control hox gene expression?
gap gene and pair-rule gene products
