Lecture 34

Question 1

Brief drosophila life cycle

Answer 1

Egg is laid, it goes through embriogenesis in a day, then it reaches teh larval stage where it’ll go through different phases, from days 1-5 it’ll go from larva to pupa, will basically eat and grow, from day 5-9 the fly attains full size and form and reaches sexual maturity

Question 2

Briefly go through the embriogenesis of drosophila, those first 2 hrs

Answer 2

Fertilization occurs, diploid cell, and instead of starting to divide right away we only get nuclear divisions, 9 of them specifically, but its still all one cell, this is a multinucleate syncytium, these nuclei migrate to the periphery of the mega cell, divides 4 more times to get a bunch of nuclei, at this point we starts developing individual cells where these nuclei have landed, forming the embryo, some cells on one end become pole cells and by 2 hours in we have an embryo with positional info, axis established
Note that everything that has happened up until this point has been mediated by the maternal info

Question 3

Maternal effect genes in drosophila

Answer 3

In drosophila its the mother and her genome that set up the embryo throughout a good portion of embryonic development, so the mother needs to have functional maternal effect genes because her DNA is crafting that environment, not the embryos
So even if the mother is heterozygous for a maternal effect genes and she actually passed on a mutant allele such that the progeny is homozygous mutant, it will develop normally (at least in the beginning) because the mother had at least 1 good/functional maternal effect genes
Contrarily, if the mother is homozygous mutant, she doesnt have any good allele to make the maternal effect genes protein or mRNA or wtv tf it is, so even if the progeny is heterozygous (from its father), it’ll still show a mutant phenotype, because mother didnt have the proper allele to support its development

Question 4

When screening for genes that when mutated resulted in mutant embryos, what did they have in common

Answer 4

Many, the majority, were TFs, so when mutant they led to bad embryos
There were also some common classes (like affecting certain aspects), so it wasn’t as scattered as they thought it would be its really mutations to certain aspects that are bad

Question 5

What maternal effect genes affect the anterior/posterior axis

Answer 5

Bicoid: involved with establishing the anterior, without it flies lack anterior structures like head and thoracic segments
Nanos: involved with establishing the posterior, without it they lack posterior structures, some abdominal ones too
Note: because maternal effect genes this is something mom needs to have a good copy of, even if progeny does not

Question 6

How does bicoid establish anterior positional info

Answer 6

It works on a morphogenic gradient kind of thing, so produced on one side of the cell and it diffuses down to teh middle, under normal conditions it should stop around middle and meet the nanos gradient for posterior info, but if theres no nanos present then the gradient diffuses to the end and theres no posterior info to give
The presence of this gradient reaches different nuclei to different extents, meaning itll elicit a graded response as it gets further down

Question 7

How is the bicoid gradient in embryos established

Answer 7

The mRNA itself is clustered on one end of the cell, and if its translated there its already in anterior
MRNA seems to be anchored to the anterior end of the cell but the protein can diffuse down its gradient down to the middle (possible at all because in early embryogenesis its all just one giant cell)
Note: bicoid is a TF

Question 8

How does nanos maintain the morphogenic gradient and of what

Answer 8

Nanos is a translational inhibitor of the uniformly spread maternal TF hunchback, its tethered to the posterior end of the embryo so it can inhibit the translation of hunchback in the posterior, so hunchback PROTEIN will be higher in anterior, mRNA will be the same everywhere,
Note: hunchback is maternally loaded, so provided by mother

Question 9

GAP genes (brief)

Answer 9

A class of genes found in the screen for genes affecting drosophila embryonic development
There are 9 type of Gap genes, all are TFs and mutants in these lose several consecutive segments
They take A-P gradients established by mother and translate them into the segments
Note: drosophila embryos have 14 segments, so a few consecutive segments is a solid chunk

Question 10

Zygotic hunchback

Answer 10

A type of Gap gene, its transcription is regulated by bicoid (cuz its a TF), which kind of makes sense since its anterior
This particular gene has 3 enhancer/promoter sequences that can interact with bicoid, and its only when all 3 are present that the gradient of hunchback reaches the middle of the embryo (as it should)

Question 11

Why does the zygotic hunchback gene have 3 binding sites in its promoter

Answer 11

Bicoid is pretty well diffused but there is a gradient down to the embryos middle, so different levels of bicoid will reach the gene depending where its at meaning it’ll elicit more or less transcriptional response, which is consistent with the idea of having a gradient of hunchback
Most anteriorly there will be a lot of bicoid and therefore a lot of zygotic hunchback transcription, and this will decrease as it goes down to the middle

Question 12

GAP gene regulation (generally speaking)

Answer 12

Expression is initially controlled by previously established gradients by the maternal-effect genes, but once things start to even out and we start getting some transcription of GAP genes, they can feedback to modulate each others expression

Question 13

Pair-rule genes

Answer 13

Another classes they discovered when looking for genes affecting drosophila embryonic development
These ones are all (8) TFs with mutants missing every other segment (since there expressed in alternating segments), thats why its called “pair”
Note these are zygotic genes and are expressed after Gap genes

Question 14

Eve pair-rule gene

Answer 14

Eve as in even-skipped, affects stripe 2, and this particular stripe has different levels of different maternal-effect gene proteins (providing A-P info) and different gradients of activators and repressors to bind to this genes enhancer
Basically it makes a strip in which the overlap is just right such that eve can be active and make its TF that will go on to activate other proteins specific to that strip, to the extent required by that strip

Question 15

Segment polarity genes

Answer 15

These genes encode components of 2 cell-cell signalling pathways, components of the pathway (including porteins, receptors, TFs, etc)
Their expression is contingent on the previously established gradient of maternal-effect genes
These are activated after segments are defined, not contributing to the number of segments but rather the polarity of segments

Question 16

Homeotic genes

Answer 16

Something we worry about after segment polarity genes, they specify the specific body strictures associated with the established segment, so if segment 4 needs front legs that’s something that homeotic genes control
Characteristics of these mutations isnt only the missing of a structure but rather its replacement with another that shouldn’t be there

Question 17

Go through the classes of genes established for drosophila embryonic development

Answer 17

First we have the maternal effect genes that lay the gradient and ground work for all to come, then come Gap genes, then pair-rule genes, then segment polarity genes and finally homeotic genes