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Cell and Developmental > Early Embryo Development > Flashcards

Early Embryo Development Flashcards

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1
Q

Why is Drosophila the animal geneticist’s model organism?

A
  • Easy to breed
  • High fecundity
  • Tolerant of diverse conditions
  • Fast lifecycle - approx 33 days
  • Sequenced genome
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2
Q

Life cycle of fruit fly

A

Female 3mm long
12 day life cycle
After egg is fertilised, embryo emerges in approx 24 hours
Embryo undergoes successive moults to become the first, second and third instar larva
Larval stages characterised by consumption of food and resulting growth
Then pupal stage, where there is a dramatic reorganisation of body plan (metamorphosis) followed by emergence of adult fly

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3
Q

What happens after fertilisation of the zygote in Drosophila?

A

Zygotic nuclei undergo successive mitosis divisions without cell division

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4
Q

What is the syncitjal stage following fertilisation and nuclei mitotic divisions?

A

6000 nuclei in one cell

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5
Q

What is laid down as the oocyte forms in the Drosophila ovary?

A

Maternal gene mRNAs in a polar arrangement

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6
Q

What happens to these maternal gene mRNAs after fertilisation?

A

Maternal gene products are expressed
Some act as translation inhibitors for other maternal mRNAs, producing gradients of proteins across the developing embryo

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7
Q

What genes influence establishment of embryo polarity?

A

Maternal effect genes

When mutated in the mother cause defects in offspring

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8
Q

What is a morphogen?

A

Substance which controls development across a concentration gradient

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9
Q

What do maternal effect genes do when translated?

A

Produce maternal morphogen gradients

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10
Q

Examples of maternal effect gene products

A

Nanos

Bicoid

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11
Q

How many maternal genes are involved in setting up positional information in the egg?

A

about 50

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12
Q

What do the proteins produced from translation of maternal genes regulate?

A

They regulate each other and the zygotic genes in the nuclei of the embryo

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13
Q

What happens when proteins are synthesised from the maternal effect mRNAs?

A

Diffusion produces protein gradients in the developing embryo

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14
Q

What are the two functions of bicoid (bifunctional protein)?

A
  1. Acting as a transcription factor, Bicoid influences fate (gene expression) in a concentration-dependent manner
  2. Acting as an RNA binding protein, Bicoid inhibits the translation of caudal mRNA in the anterior part of the embryo, resulting in a posterior to anterior gradient of Caudal protein
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15
Q

How does Nanos block Hunchback translation?

A

nanos mRNA is localised to the posterior pole
After fertilisation Nanos protein diffuses throughout the posterior part of the embryo
Nanos blocks the translation of maternal hunchback mRNA in the posterior part of the embryo resulting in an anterior to posterior gradient of Hunchback protein

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16
Q

What is the function of Hunchback protein?

A

Represses abdominal-specific genes, allowing the region of hunchback expression to form the head and thorax

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17
Q

Types of genes required for segmentation

A

Gap gene
Pair-rule genes
Segment polarity genes

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18
Q

Examples of gap genes

A 
hunchback
giant
Krüppel
knirps
tailless
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19
Q

What are gap genes?

A
  • Expressed early in development in broad regions
  • Define the head, thorax and abdomen
  • Loss-of-function mutants have large sections of their body plan missing
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20
Q

What leads to the expression of zygotic gap genes?

A

Bicoid expression

Gap genes are the first zygotic genes to be expressed along the anterior-posterior axis

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21
Q

What do all gap genes encode?

A

Transcription factors

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22
Q

What happens once gap proteins are synthesised?

A

They diffuse away from their site of synthesis, but the proteins have very short half lives, so they don’t get far before being degraded. This produces bell-shaped protein concentration profiles

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23
Q

What is the exception to gap genes producing a bell-shaped concentration profile?

A

Zygotic hunchback - expresses over a broad region producing a steep anterior-posterior gradient

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24
Q

What regulates gradients of gap gene expression?

A

Gradients of maternal effect gene proteins

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25
What must happen for zygotic hunchback (not maternal hunchback mRNA) to be switched on?
Bicoid (transcription factor) switches on zygotic hunchback This only happens when the concentration of Bicoid exceeds a threshold. Bicoid binds to regulatory sites in the hunchback promoter. Increasing the amount of maternal bicoid increases the size of the domain of hunchback expression
26
What do gradients of Bicoid and Hunchback proteins do?
Activate and repress the expression of giant, Krüppel, knirps and tailless Both Bicoid and Hunchback are transcription factors that bind to the promoters of gap genes and regulate their expression They are both morphogens
27
How do Bicoid and Hunchback regulate the expression of Krüppel?
Krüppel expression is activated by a combination of Bicoid and low levels of Hunchback Krüppel expression is repressed at high levels of Hunchback Similar threshold effects of Hunchback define the anterior borders of expression of knirps and giant
28
Describe an example of cross-interactions between gap genes affecting gap gene expression
There are depressive interactions between gap gene products E.g. the posterior boundary of anterior knirps expression is specified by interaction with the product of the tailless gap genes
29
What kind of system can the model of delimiting regions of proteins be possible in
An acellular system, where transcription factors can diffuse into concentration gradients across the embryo
30
What do pair-rule genes do?
Establish parasegments
31
Examples of pair rule genes
even skipped | fushi tarazu
32
What do pair rule gene mutants lack?
Portions of every other segment
33
How do gap gene products affect pair rule gene expression?
Specific combinations of gap genes produce stripes of pair rule gene expression
34
How many stripes/parasegments are there in a syncitium?
14
35
Which parasegments is even skipped expressed in?
Odd-numbered parasegments
36
Which parasegments is fushi tarazu expressed in?
Even-numbered parasegments
37
What activates even in a broad domain?
Bicoid and Hunchback
38
How are the anterior and posterior borders of parasegment 3 formed?
Through repression by Giant and Krüppel proteins respectively
39
Why do pair-rule genes have complex promoters?
They respond to specific combinations of activators and repressors
40
What occurs after pair-rule gene expression?
Cellularisation | Cell membranes form around nuclei
41
What genes are expressed after cellularisation?
Segment polarity genes
42
Examples of segment polarity genes
hedgehog wingless engrailed
43
What are segment polarity genes?
Establish final segment boundaries Activated in response to pair-rule gene expression Segment polarity genes do not encode transcription factors, instead they encode molecules that are involved in cell-cell signalling
44
What genes determine segment identity?
Homeotic (Hox) genes
45
Examples of Hox genes
antennepedia | bithorax
46
What are homeotic genes?
They specify the identify of body segments Share a conserved DNA sequence (approx 180 bp) called the homeobox The homeobox encodes a 60 amino acid homeodomain which recognises specific DNA sequences Mutations in homeotic genes cause one structure to be replaced by another
47
What do the three thoracic segments of Drosophila produce?
1. 1 pair of legs 2. 1 pair of legs and wings 3. 1 pair of legs and halteres (balancers)
48
Hox gene expression order
The order of Hox gene expression along the embryo corresponds to the order of genes along the chromosome Initial domains of homeotic gene expression are influenced by gap genes and pair-rule genes
49
bithorax mutant
Mutations in ultrabithorax gene transforms third thoracic segment into another second thoracic segment - two pairs of wings This is a loss-of-function mutant
50
antennepedia mutant
Antennepedia protein expressed in the head and thorax Legs, rather than antennae, grow out of head sockets This is a gain-of-function mutant
51
Which parasegments does the antennapedia complex control?
Parasegments 1-5 | these eventually form the head and thoracic segment
52
Which parasegments does the bithorax complex control?
Controls the development of parasegments 5-14 | These eventually form the rest of the thorax and the abdomen
53
Who won the Nobel Prize for Physiology and Medicine for work on Drososphila embryo development and when?
Christiane Nüsslein-Volhard Edward B. Lewis Eric Wieschaus 1995
54
Example of Hox gene conservation: eyeless / Pax6
- Mutatjons in the human Hox gene PAX6 are responsible for the disease aniridia (a suite of conditions affecting eye development) - Genes with similar sequences have been identified in mice (Pax6) and Drosophila (eyeless) - Pax6 and eyeless genes have 94% amino acid sequence identity in the paired domain and 90% amino acid sequence identity in the homeodomain - Misexpression of Drosophila eyeless gene induces ectopic eyes on the legs, wings and antennae of flies
55
Are ectopic eyes functional?
Ectopic eyes on antennae and legs extend axonal projections into the central nervous system Some response to light but ectopic axons do not connect to correct optical lobe targets
56
What happens if Pax6 gene from mouse expressed in Drosophila
Induces ectopic Drosophila eyes on fly (cannot produce mouse eyes) eyeless/Pax6 is a homeobox master control gene for eye morphogenesis
57
Why is Arabidopsis thaliana the plant geneticist’s model organism?
- Simple growth requirements - Rapid development - Small genome - Complete genome sequence
58
Early embryo development steps in A. thaliana
1. Egg and sperm cells fuse to form a zygote 2. Asymmetric transverse division occurs to form apical cell and basal cell 3. Basal cell divides to form suspensor 4. Apical cell divides to form globular stage embryo 5. Rapid cell division either side of future shoot apex to form triangular stage embryo 6. Outgrowths will later become cotyledons. Cell elongation produces torpedo stage embryo 7. Mature stage: embryo and seed lose water and become metabolically inactive to enter dormancy. Storage compounds accumulate in cells
59
How is early embryo development regulated in Arabidopsis?
- Wuschel-related homeobox (WOX) transcription factors | - Auxin signalling
60
How are wuschel-related homeobox (WOX) mRNAs distributed in early development?
They are asymmetrically localised, although we are unsure how this is achieved
61
What is IAA?
Indole acetic acid | An auxin
62
What are auxins similar in structure to?
The amino acid tryptophan
63
What is auxin transport mediated by?
Auxin pumps
64
What does auxin specify at the start of development?
The apical cell
65
What does auxin specify later in development?
The hypocotyl, root meristem and embryonic root
66
What does the flow of auxin through root tips and leaves do?
Conveys polarity and drives growth
67
What controls auxin distribution in the early Arabidopsis embryo?
PIN proteins
68
What do mutations that affect auxin transport disrupt?
Aspects of plant development including embryo polarity, meristem function and organ development
69
What auxin receptor regulates the degradation of a specific class of transcriptional repressors in response to auxin?
TIR1 | Transcriptional repressors called AUX/IAA proteins
70
What do AUX/IASs do?
Bind auxin-response factors (ARFs) and prevent them from activating the transcription of auxin-response genes
71
What does degradation of AUX/IAA proteins in response to auxin do?
Allows auxin-response factors to activate auxin-response genes
72
Example of an AUX/IAA transcriptional repressor
Bodenlos
73
Example of an auxin-response factor (ARF)
Monopteros
74
What do bodenlos and monopteros form?
Heterodimers
75
What causes the degradation of bodenlos?
PIN7-directed auxin flow into the embryo
76
What does monopteros activate and what does this allow?
Activated auxin-responsive gene expression | Allows hypocotyl and roots to develop

Cell and Developmental (4 decks)

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