Building Brains 4 - Invertebrates Flashcards
(169 cards)
1
Q
Identify two nervous system trends that have been identified throughout evolution. (2)
A
- Centralisation
- Cephalisation
2
Q
Throughout evolution, describe how the symmetry of the nervous system in an organism has changed. (1)
A
Gone from having radial symmetry to bilateral symmetry.
3
Q
Name four aspects of the nervous system which appear to have been conserved across species throughout evolution. (4)
A
- Cell types
- Overall architecture
- NTs/receptors
- Genes underlying development
4
Q
In general, why are more primitive organisms good to study in terms of development? (2)
A
- Easier to understand
- More ways to manipulate/test them (especially genetic)
5
Q
In what way can mice best be used to study nervous system development? (1)
A
Genetic manipulations
6
Q
In what way can the sea slug (aplysia) best be used to study nervous system development? (1)
A
Learning and memory (conditioning)
7
Q
In what way can C.elegans best be used to study nervous system development? (1)
A
Defined cell lineage (only 302 neurones)
8
Q
Give four reasons why fruit flies (drosophila) make good models to study nervous system development. (4)
A
- Complex genome with lots of genes
- Human genes have functional orthologues in flies
- Nervous system smaller (easier to study) but bisymmetrical
- Able to learn and display complex behaviours
9
Q
Describe the organisation of the nervous system in invertebrate organisms. (4)
A
Brain
Central cord
Peripheral nerves
which are defined/split into segments
10
Q
Compare the location of the nerve cord in vertebrates and invertebrates. (1)
A
In vertebrates the nerve cord lies dorsally
however in invertebrates it lies ventrally.
11
Q
When does the nervous system develop in fruit flies? (2)
A
Established during embryogenesis
but refined and added to during larval stages.
12
Q
All cells in the embryo contain exactly the same genes.
How are cells able to differentiate and perform different functions? (1)
A
Altering gene expression
13
Q
Give two ways that a cell is able to alter its gene expression in the embryo. (2)
A
- Transcription factors
- microRNAs
14
Q
Which layer of the embryo does the nervous system develop from? (1)
A
Ectoderm
15
Q
Give the four stages of invertebrate nervous system development. (4)
A
- Neural induction
- Neural patterning
- Segregation of neural progenitor cells
- Division and differentiation
15
Q
Briefly describe what is meant by ‘neural induction’ in the development of invertebrate nervous systems. (1)
A
Regions of the ectoderm are endowed with neurogenic capabilities, and embryonic ectodermal cells make the decision to acquire a neural fate.
15
Q
Briefly describe what is meant by ‘neural patterning’ in the development of invertebrate nervous systems. (1)
A
Cells are subdivided along the dorsal-ventral and anterior-posterior axis
16
Q
Briefly describe what is meant by ‘segregation of neural progenitor cells’ in the development of invertebrate nervous systems. (1)
A
A few neural progenitor cells are ‘selected’ to display full commitment to a neural fate.
17
Q
In the early invertebrate embryo, how are transcription factors able to diffuse between nuclei to form gradients? (1)
A
The early embryo is a syncytium (nuclear divisions within one big shared pool of cytoplasm)
18
Q
Describe how the invertebrate syncytial blastoderm transforms into a cellular blastoderm. (2)
A
- Nuclei migrate to periphery of cytoplasm
- Membranes then form around nuclei to create a cellular blastoderm
19
Q
The early invertebrate blastoderm splits into two layers.
What are these layers called and along which axis is this carried out? (2)
A
Ectoderm and Mesoderm
Carried out along the dorsal-ventral axis
20
Q
At the stage where the early invertebrate blastoderm splits to form the ectoderm and mesoderm, is the blastoderm a syncytium or a cellular structure.
Why is this important? (2)
A
Syncytium
This is important to allow concentrations of transcription factors to be set up along the dorsal-ventral axis.
21
Q
In the early invertebrate blastoderm (before it has split to form different layers) the cells in the embryonic tube are split into three rough sections.
Name these sections and describe which is the most dorsal and which is the most ventral. (3)
A
- Lateral ectoderm (epidermis; most dorsal)
- Neuroectoderm (middle)
- Mesoderm (most ventral)
22
Q
Name the molecule which produces a gradient in the single cell, multi-nucleate early invertebrate embryo and begins to induce splitting into the ectoderm and mesoderm. (1)
A
Dorsal
23
Describe the concentration gradient of dorsal protein in the early invertebrate embryo. (1)
Highest ventrally
Lowest dorsally
24
What kind of molecule is 'dorsal'? (1)
Transcription factor
25
Complete the sentence. (1)
Dorsal protein promotes ..................... fates in the early invertebrate embryo.
Ventral
26
Dorsal is a transcription factor in the early invertebrate embryo which forms a gradient in the dorsal-ventral axis.
Describe the effect on a tissue of being exposed to high levels of dorsal protein. (2)
- Production of snail
- Induces mesoderm
27
What type of molecule is snail protein? (1)
Transcription factor
28
Dorsal is a transcription factor in the early invertebrate embryo which forms a gradient in the dorsal-ventral axis.
Describe the effect on a tissue of being exposed to low levels of dorsal protein. (2)
Production of decapentaplegic (DPP)
which promotes epidermal ectoderm
29
What type of molecule is decapentaplegic (DPP)? (1)
Extracellular signalling molecule
30
Dorsal is a transcription factor in the early invertebrate embryo which forms a gradient in the dorsal-ventral axis.
Describe the effect on a tissue of being exposed to intermediate levels of dorsal protein. (3)
Production of SOG
which inhibits DPP
so induces neuroectoderm.
31
What type of molecule is SOG? (1)
Extracellular signalling molecule
32
Describe the DPP pathway which promotes epidermal fates. (3)
- DPP binds to serine-threonine kinase receptor
- p-Mad (transcription factor) activated
- Upregulates epidermal genes and inhibits neural genes
33
Describe how SOG acts as a negative regulator of DPP. (1)
Sog binds to DPP in extracellular space to stop it binding to its receptor.
34
What type of molecule is Mad? (1)
Transcription factor
35
What is a morphogen? (1)
A molecule which is distributed non-uniformly, and can therefore induce different cellular responses depending on concentration thresholds.
36
Describe how a concentration gradient of a morphogen is produced. (2)
Local synthesis at one particular site
diffusion throughout a tissue.
37
Give three transcription factors which help to pattern the neuroectoderm in the dorsal-ventral axis. (3)
msh (muscle segment homeobox)
ind (intermediate neuroblasts defective)
vnd (ventral nervous system defective)
38
What types of molecules are msh, ind, and vnd? (1)
Homeodomain transcription factors
39
What two factors determine whether areas of neuroectoderm in the early invertebrate embryo express msh, ind, or vnd? (2)
- Concentration of dorsal
- Concentration of DPP
40
Describe the levels of dorsal and DPP associated with expression of msh in the invertebrate neuroectoderm. (2)
- Low dorsal
- High DPP
41
Describe the levels of dorsal and DPP associated with expression of ind in the invertebrate neuroectoderm. (2)
- Medium dorsal
- Medium DPP
42
Describe the levels of dorsal and DPP associated with expression of vnd in the invertebrate neuroectoderm. (2)
- High dorsal
- Low DPP
43
DPP inhibits vnd and ind.
Describe the relative sensitivities of vnd and ind to inhibition by DPP. (2)
Vnd more sensitive
Ind less sensitive
44
Describe how clear boundaries are produced between regions of neuroectoderm expressing msh, ind, and vnd. (1)
The molecules inhibit each other
45
In general, does dorsal protein activate or inhibit production of msh, ind, and vnd? (1)
Activate
46
Which molecule is responsible for initial patterning of the early invertebrate embryo in the anterior-posterior axis? (1)
Bicoid
47
What type of molecule is bicoid? (1)
Transcription factor
48
Is bicoid a morphogen? (1)
Yes
49
Describe the gradient of bicoid in the early invertebrate embryo. (1)
Higher anteriorly
Lower posteriorly
50
What is the effect of bicoid on other transcription factor/s in the early invertebrate embryo? (1)
Activates Hunchback expression
51
What type of molecule is hunchback? (1)
Transcription factor
52
Is hunchback a morphogen? (1)
No - however it still forms a gradient due to the bicoid gradient
53
What is the effect of bicoid and hunchback gradients in the early invertebrate embryo? (1)
Switch on gap genes
54
What kind of molecules do gap genes produce? (1)
Transcription factors
55
Give an example of a gap gene. (1)
Kruppel
56
Describe the distribution of gap genes in the early invertebrate embryo. (1)
Each gap gene corresponds to multiple future segments - gap genes separate the embryo into broad AP domains.
57
What type of molecule is Kruppel, and what type of patterning gene codes for it? (1)
Transcription factor
Coded for by a gap gene
58
Describe how borders between gap gene expression in the early invertebrate embryo are made sharp and distinct. (1)
Adjacent gap genes inhibit each other
59
What is the role of gap rule genes in the early invertebrate embryo, in terms of activating/inhibiting other genes? (1)
Activation of pair-rule genes
60
Give two examples of pair rule genes. (2)
eve
ftz
61
Describe how segments in the early invertebrate embryo are determined by pair rule genes. (1)
Each pair rule gene corresponds to one segment.
62
Which segments in the early invertebrate embryo does the eve gene mark? (1)
Even segments
63
Which segments in the early invertebrate embryo does the ftz gene mark? (1)
Odd segments
64
What type of molecules do pair rule genes encode in the early invertebrate embryo? (1)
Transcription factors
65
Give two factors that determine Hox gene expression in the early invertebrate embryo. (2)
- Gap genes
- Pair rule genes
66
What types of molecules do Hox genes encode? (1)
Transcription factors
67
What is the role of Hox genes in the early invertebrate embryo? (1)
Pattern the embryo as a whole (not just nervous system) by activating programs of gene expression.
68
What is the difference between a segment and a Hox gene in the early invertebrate embryo? (2)
Segments are repeated along the length of the embryo.
Hox genes 'overlay' several segments to slightly alter gene expression in relation to AP axis.
69
Describe the concept of 'segmental repeats' when patterning the AP axis of an early invertebrate embryo. (4)
Embryo divided into obvious segments along AP axis
Specific neurone/neuroblast patterning WITHIN each segment
This pattern is repeated along embryo
With small alterations based on location of segment relative to AP axis (determined by Hox genes)
70
What is the general name for genes which are expressed WITHIN each segment of the early invertebrate embryo? (1)
Segmental polarity genes
71
Name a factor which helps to determine which genes are expressed in each segment (segmental polarity genes) of an early invertebrate embryo. (1)
Pair-rule genes
72
Name two segmental polarity genes expressed in the early invertebrate embryo. (2)
- Wingless (WG)
- Hedgehog (HH)
73
Describe the distribution of wingless and hedgehog gene expression WITHIN a segment of the early invertebrate embryo. (2)
Activated in opposite ends of each segment
Form 'opposing' concentration gradients (eg. where WG is high, HH is low)
74
What type of molecule is wingless? (1)
Extracellular signalling molecule
75
What type of molecule is hedgehog? (1)
Extracellular signalling molecule
76
True or false? (1)
Wingless acts as a morphogen in the early invertebrate embryo, but hedgehog does not.
False - they both act as local morphogens
77
What is the role of wingless and hedgehog proteins in the early invertebrate embryo? (1)
Activate expression of other segmental genes in stripes WITHIN each segment of embryo
78
Name the receptor for the hedgehog protein in early invertebrate embryos.
Where in/on the cell is this receptor located? (1)
Patched
Located on cell membrane
79
Briefly describe the hedgehog signalling pathway in the early invertebrate embryo. (4)
- HH binds to patched
- Activates smoothened
- Stops GLI from being cleaved
- GLI acts as transcription activator
80
Describe how DV patterning and AP patterning are combined in the early invertebrate embryo to confer identities to each individual neuroblast. (2)
AP stripes and DV stripes form a grid, so each neuroblast in a segment sits at a specific coordinate.
Each coordinate contains a unique combination of gene expressions.
81
From which part of the early invertebrate embryo do sense organ precursor cells (SOPs) develop, and what do they turn into in the adult? (2)
- Arise from lateral ectoderm
- Form sensilla and peripheral nervous system
82
From which part of the early invertebrate embryo do neuroblasts develop, and what do they turn into in the adult? (2)
- Arise from neuroectoderm
- Form central nervous system (neurones and glia)
83
Some neural precursor cells in the early invertebrate embryo (SOPs or neuroblasts) fully delaminate, and some only partially delaminate from the rest of the ectoderm.
Which one is which? (2)
- SOPs partially delaminate
- Neuroblasts fully delaminate
84
Describe what is meant by the sentence:
'In the early invertebrate embryo, all ectodermal cells have neural competence, but they will not all turn into neural precursor cells.'
(1)
Ectodermal cells have the potential to differentiate into neuroblasts or sense organ precursors, but only a fraction become committed.
85
Name the mechanism by which just one cell in a cluster from the ectoderm in the early invertebrate embryo turns into a neuroblast/SOP and the rest revert to an epidermal state. (1)
Lateral inhibition
86
Describe the process of lateral inhibition, the mechanism by which just one cell in an ectodermal cluster in the early invertebrate embryo becomes committed to a neural fate. (6)
- All cells in the cluster express delta
- Delta binds to notch receptor on neighbouring cells
- Notch activation = expression of HES
- HES molecules block expression of proneural genes
- One cell escapes inhibition to become committed to a neural fate but continues to inhibit neighbours
- Neighbours revert to epidermal state
87
What type of molecule is delta? (1)
Transmembrane ligand
88
What type of molecule is notch? (1)
Transmembrane receptor
89
What type of molecule is HES, and what does it do? (2)
Transcriptional repressor
Block expression of proneural genes
90
Describe how notch, a cell membrane receptor, is able to affect HES in the cell nucleus. (2)
When delta binds, the intracellular Notch domain is cleaved
and can enter the nucleus.
91
Name three proneural genes which are inhibited by HES during lateral inhibition, therefore becoming gradually restricted to a select number of committed neural cells. (3)
- Achaete
- Scute
- Atonal
92
What types of proteins do the genes achaete, scute, and atonal produce? (2)
bHLH (basic helix-loop-helix)
transcription factors.
93
In experiments, neuroblasts which have delaminated from the ectoderm in the early invertebrate embryo can be killed.
What would happen if this neuroblast is destroyed? (3)
One of the neighbouring ectodermal cells (with neural capabilities) will take over and become the neuroblast.
Because inhibition from previous neuroblast isn't present,
but the new cell can continue inhibiting neighbours.
94
Describe how cell division is able to create diversity in the developing embryo. (1)
Asyemmetric division
95
Describe the asymmetric cellular divisions of SOPs in the early invertebrate embryo. (2)
Each SOP undergoes 2 asymmetric cell divisions (divides once asymmetrically, then each daughter cell divides again asymmetrically)
to produce three support cells and a neurone.
96
Name the four specialised cells which are produced from a single sense organ precursor in the early invertebrate embryo. (4)
- Neurone
- Glial cell
- Socket
- Bristle cell
97
Describe the asymmetric cellular divisions of neuroblasts in the early invertebrate embryo. (2)
Each neuroblast undergoes repeated asymmetric divisions to self renew and generate a ganglion mother cell (GMC).
Each GMC undergoes one cell division to generate 2 neurones.
98
How are cells able to divide asymmetrically in the early invertebrate embryo? (2)
Asymmetric distribution
of numb.
99
Describe two functions of the numb protein in the early invertebrate embryo. (2)
- Regulates notch signalling
- Asymmetric cell division
100
Describe how the asymmetric distribution of numb in SOPs facilitates asymmetric division in the PNS of the early invertebrate embryo. (2)
Numb distributed unevenly throughout cell
so daughter cells inherit different amounts of numb.
101
Describe the effect of mutating the numb protein on the division and differentiation of SOPs in the early invertebrate embryo. (2)
All cell divisions would be symmetrical
so all daughter cells would be the same (no diversity).
102
Describe how the polarisation of ectodermal cells facilitates asymmetric division of neuroblasts in the early invertebrate embryo. (2)
Polarisation results in proteins (eg. numb) being distributed differently in the plane of cell division.
GMCs and regenerating neuroblasts contain different distributions of protein.
103
Describe the three sets of genes which overall pattern the early invertebrate embryo and ultimately determine the position of a neuroblast within the embryo. (3)
- Segmental polarity genes
- Hox genes
- Columnar genes in DV axis
104
Describe the temporal control of neuroblast identity in the early invertebrate embryo. (4)
During development, dividing neuroblasts express different transcription factors at different points in time
The GMC will inherit the factor which was expressed in the neuroblast at the time it was produced
As GMCs are produced, the new cell displaces the existing cell and pushes it toward the centre of the embryo
First born neurones end up on the inner side of the nerve cord, later neurones end up on outside of nerve cord.
105
True or false? (1)
Cell division and neurogenesis only occurs in the drosophila embryo.
False - a second wave of adult neurogenesis occurs in drosophila
106
Describe what happens to neuroblasts in the abdominal region of the drosophila embryo after completing their cell lineages. (1)
They undergo programmed cell death
107
Describe what happens to neuroblasts in the cephalic and thoracic regions of the drosophila embryo after completing their cell lineages. (4)
- Neuroblasts arrest their cycle and enter G0-like quiescent state
- Neuroblasts re-enter mitosis during 1st instar larval stage
- Neurogenesis continues in larval and pupal stages
- Neuroblasts then exit cell cycle and disappear
108
Describe one difference between embryonic neuroblast cell divisions and larval neuroblast cell divisions in drosophila. (2)
- Embryonic neuroblasts shrink as they self-renew
- Larval neuroblasts are able to regrow to their original size as they self-renew
109
What is meant by 'forward genetics' when studying development and disease in invertebrates and applying this to humans? (2)
Screening for gene mutations associated with a particular phenotype in invertebrate model,
identify gene and human homologue.
110
What is meant by 'reverse genetics' when studying development and disease in invertebrates and applying this to humans? (4)
Identify a human disease-causing gene
Identify fly homologue
Manipulate gene in fly
Phenotype analysis to gain insight into function of known disease gene
111
Describe what is meant by a 'connectome', and its limitations in studying neurodevelopment. (2)
A connectome is a comprehensive map of neural connections within an organism's nervous system.
LIMITATION - does not show functional relationships between neurones
112
Name a technique which can be used to investigate functional neuronal circuits in drosophila. (1)
GAL4-UAS
113
What is the function of the GAL4-UAS system when investigating functional neuronal circuits in drosophila? (1)
Upregulate or downregulate specific genes in specific cells/tissues.
114
What is GAL4? (1)
A transcription factor found in yeast
115
What is UAS? (1)
Upstream activating sequence which is activated by GAL4
116
Describe how the GAL4-UAS system can be used to induce gene expression changes in drosophila. (3)
- Modify one fly for tissue-specific expression of GAL4
- Modify another fly to express UAS for a specific gene of interest (upregulation) or RNA inhibitor (downregulation)
- Offspring from these 2 flies produce GAL4 in specific tissues, leading to activation of UAS and gene of interest in specific tissue
117
What is the goal when GAL4-UAS is combined with optogenetics? (1)
Controllable activation of selective and definable neurones.
118
Describe how the GAL4-UAS technique and the optogenetics technique can be combined to control activation of select neurones in drosophila. (5)
- Fly modified to express GAL-4
- Fly modified to express UAS for channel-rhodopsin gene
- Cross breed these flies
- Offspring expresses channelrhodopsin only in specific neurones
- Specific neurones can be activated using light and results analysed
119
Give another technique other than optogenetics that GAL4-UAS can be combined with and describe briefly how this works. (2)
Thermogenetics
- Neurones express TRP channels and become active by temperature change
120
Name the gene which is the master regulator of male courtship behaviour in drosophila. (1)
fruitless
121
Describe how the fruitless gene produces different protein products in the male and female fruit fly. (1)
Different splicing
122
Describe 2 possible outcomes of modifying the fruitless gene in male drosophila. (2)
- Males court other males
- Defective in courtship behaviour
123
Name the cluster of neurones in drosophila which express fruM and appear to be essential for courtship behaviours. (1)
P1
124
What would be the effect be of activating artificially-implanted P1 neurones expressing fru in the female fruitfly? (1)
Female would show male courtship behaviour
125
Give an advantage of studying axonal navigation in the early invertebrate embryo rather than trying to study connections in the adult brain. (1)
Possible to see early axons navigating in a simple environment.
126
Name the part of the early axon which directs axon growth. (1)
Growth cone
127
Describe the cytoskeletal structure of the axonal growth cone. (2)
Core of microtubules extending longitudinally down axon
filopodia formed of actin.
128
True or false? (1)
When an axon grows in the early invertebrate embryo, new material is added at the growth cone and not at the cell body.
True
129
What is the role of the filopodia on the early axonal growth cone? (1)
Explore the environment
130
Name four methods/techniques which axons in the embryo use to migrate to their target. (4)
- Follow existing tracts (fasciculation)
- Guidepost cells (intermediate targets)
- Contact guidance
- Diffusible cues (chemotropism)
131
Briefly describe how axons may find their target via fasciculation in the early embryo. (2)
Pioneer axon finds target.
Follower axons bundle together to form fascicle.
132
Briefly describe how axons may find their target via guidepost cells in the early embryo. (1)
Axons use other neurones in the environment as guides to find their target.
133
Describe what is meant by 'contact guidance' when referring to axonal path finding. (2)
Short range signals
in the extracellular matrix or attached to cells.
134
Describe what is meant by 'chemotropism' when referring to axonal path finding. (2)
Long range signals
which diffuse to axons.
135
Name the part/structure of the early migrating axon which allows the growth cone to change direction to respond to cues in the external environment. (1)
Cytoskeleton
136
Describe the location where neurones with branching dendrites tend to grow in the early invertebrate embryo. (2)
Single layer
beneath epidermis.
137
True or false? (1)
When dendrites branch, they also use growth cones and extrinsic cues to know where they are going.
True
138
Describe why dendrites of neurones in the same class do not like to overlap each other. (1)
Waste of dendrites - dendritic redundancy
139
True or false? (1)
While sensory neurones of the same class avoid their dendrites overlapping, sensory neurones of different classes have overlapping dendrites.
True - this is essential to develop a full somatosensory map of the body
140
Describe what is meant by the term 'neurones of the same class' when referring to sensory neurones in the early invertebrate embryo. (1)
Sensory neurones which respond to the same modality.
141
Give two mechanisms by which dendrites in the early invertebrate embryo avoid overlapping as they are branching. (2)
- Self-avoidance
- Tiling
142
Describe 'self-avoidance', when talking about the spatial patterning of dendritic branches in the early invertebrate embryo. (1)
Dendrites from the same cell avoid crossing, resulting in evenly-spaced dendrites and minimal redundancy.
143
Describe 'tiling', when talking about the spatial patterning of dendritic branches in the early invertebrate embryo. (1)
Dendrites of neighbouring sensory neurones of the same class avoid crossing, resulting in contiguous, but non-overlapping fields.
144
Name the molecule that allows dendrites to carry out self-avoidance when branching in the embryo. (1)
Dscam (Down's Syndrome cell adhesion molecule)
145
What family of molecules is Dscam part of? (1)
Immunoglobulin superfamily
146
Describe how Dscam molecules allow branching dendrites in the embryo to avoid crossing over each other. (3)
Many possible isoforms of dscam protein made possible by extensive alternative splicing
Different neurones express multiple different dscam isoforms
dscams bind homophilically in an isoform specific manner
147
Describe what would happen if an embryo expressed a mutant form of dscam in neurones. (1)
Dendrites on developing neurones would cross and be disorganised.
148
Describe what would happen if an embryo mis-expressed the same form of dscam in 2 different neurones. (1)
Dendrites of the neurones would not cross
149
Name four axon guidance ligands. (4)
- Ephrins
- Slits
- Netrins
- Semaphorins
150
Name the receptor for ephrins.
Are ephrins cell surface or secreted ligands? (2)
Eph
Cell surface
151
Name the receptor for slits.
Are slits cell surface or secreted ligands? (2)
Robos
Secreted
152
Name the receptor for Netrins.
Are netrins cell surface or secreted ligands? (2)
Unc40/DCC and frazzled
Secreted
153
Name the receptor for semaphorins.
Are semaphorins cell surface or secreted ligands? (2)
Plexins and Neuropilins
Can be secreted or cell surface
154
Which axon guidance ligands/receptors are attractive and which are repulsive? (4)
All can be either depending on specific receptor
155
True or false? (1)
All interactions between axon guidance ligands/receptors are unidirectional.
False - most are unidirectional, but there are some exceptions.
156
Apart from axon guidance, name two other developmental processes which use axon guidance molecules. (2)
- Heart formation
- Neural crest cells
157
Describe the 'normal' path of commissural axons in the early invertebrate embryo. (2)
Axons extend across midline of ventral nerve cord
then join the longitudinal fascicle on the opposite side to where they originated.
158
Name the molecule expressed in the ventral nerve cord midline in the early invertebrate embryo which acts as a chemoattractant. (1)
Netrin
159
Describe what would happen in the early invertebrate embryo if the organism expressed a mutant version of netrin. (1)
Reduction in number of axons which cross midline.
160
Name the molecule expressed in the ventral nerve cord midline in the early invertebrate embryo which acts as a chemorepellent. (1)
Slit
161
Describe what would happen in the early invertebrate embryo if the organism expressed a mutant version of slit. (1)
All axons travel in the midline of the ventral nerve cord.
162
What would be the effect on commissural neurones of expressing a mutant form of robo in the early invertebrate embryo?
Explain why this happens. (3)
Axons would continually cross midline, exit midline, then recross.
Because another form of robo is present which allows enough repulsion from slit to exit midline
however is not strong enough to stop it crossing back over.
163
Axons must be attracted to the midline to enable them to cross over, however must then be repelled from the midline to stop them recrossing.
How is this achieved? (3)
Low robo receptor so axon attracted to midline.
As axon crosses midline robo receptor increased
so axon repelled from midline.
164
Name the molecule which changes the level of robo receptor in the commissural axons of the early invertebrate embryo. (1)
Commissureless (comm)
165
How does comm alter the levels of robo in the early invertebrate embryo? (1)
Prevents robo from reaching the growth cone surface.
166
Describe the levels of comm in an embryonic commissural axon as it crosses the midline. (2)
Comm levels high as axon crosses midline
Comm levels decrease once axon has crossed midline
167
Describe the effect of a mutant version of comm on commissural fibres in the early invertebrate embryo. (1)
No commissural fibres in ventral nerve cord.
