Lecture 19. Cellular Response to Positional Information: The Hox Code Flashcards
How do cells respond to positional information ?
- Cell division
- Cell differentiation
- Changes in cell shape
- Changes in cell movement
- Apoptosis
What is a tell - tale sign for when a gene is switched on in a eukaryotic cell ?
The presence of mRNA specific for the gene within the cell
What are the methods for detecting gene expression ?
- Transcriptomics
- RT-PCR amplification from mRNA
- In situ hybridisation
What is in situ hybridisation ?
The approach that allows examination of gene activity in both time and space in the embryo
What is in situ hybridisation based on ?
Detecting mRNA within cells within the whole embryo.
How does in situ hybridisation work ?
A labelled probe is generated that has a sequence complimentary to the mRNA and can bind tightly to it. The probe can be visualised by a stain or can be made fluorescent
How might a cell respond to positional information ?
- Proliferate
- Change gene expression - differentiate
- Change cell shape
- Move - migrate
- Apoptosis
What can stimulate proliferation ?
Mitotic factors
What is proliferation ?
The division of cells to form new cells
Is cell proliferation highly controlled ?
Yes highly controlled at all stages
What do most cancers derive from ?
Mutations that give rise to a single abnormal somatic cell where cell division and growth go unchecked
What is cell differentiation brought about by ?
Change in gene expression
What sort of process is cell differentiation ?
Its a progressive, multilayered - receipt and response to multiple signals
What do specialised cells produce in cell differentiation ?
Proteins tailored to their specialised function
What are the specialised proteins in red blood cells ?
Haemoglobin
What are the specialised proteins in epidermal cells ?
Keratin
What are the specialised proteins in neurons ?
Voltage gated ion channel
What are the specialised proteins in muscle ?
Specific actin and myosin proteins
What causes localised contraction of particular cell ?
Contraction of cytoskeleton elements
What does cell motility do ?
Trigger the cell to move
What is cell motility brought about by ?
Changes in cytoskeleton
How do cell crawl in cell motility ?
Production of filopodia or lamellipodia
What do cells need to allow cell motility ?
Dismantle cell adhesion proteins
What can cell only move through ?
A receptive environment
As a result of cells only being able to move through a receptive environment what happens ?
They will be attracted or repelled depending on the extra cellular matrix of neighboring cells
What is blastocoel inside the gastrulating embryo rich in ?
Fibronectin
What is fibronectin ?
An attractant that encourages internal migration
What is apoptosis ?
Cell elimination by programmed cell death
How does apoptosis work ?
The cell is induced by an external or internal signal to fragment its DNA and to shrink, without release of the cell contents
Where is apoptosis common ?
Widespread in development where cells need to be removed to sculpt the right shape and structure
Where was apoptosis discovered ?
C. elegans
What carries out apoptosis ?
Caspases
What carries out apoptosis in nematodes ?
CED-3
What carries out apoptosis in mammals ?
Caspase - 9
What does the lack of caspase 9 result in ?
Loss of apoptosis, disrupting in normal brain development due to too many neurons
What is apoptosis present in ?
Homeostasis
What are the three categories if genes that build the body plan ?
- Maternal effect gene
- Segmentation genes
- Homeotic genes
What does maternal effect gene do ?
Set up the major axes of the embryo (anterior posterior and dorsal ventral) early in development
What does the segmentation gene do ?
Divide the embryo into segments along the anterior posterior axis
What does the homeotic gene do ?
Determine segment identity
What do mutations in homeotic genes do ?
Structure characteristic of a particular part arising in the wrong place
What does a molecular analysis of homeotic genes show ?
That they encode cell specific transcription factors
What does the combination of homeotic genes express in any segment confer ?
Identity
What were homeotic genes found to be ?
Similar to each other and are clustered in the genome
Where are homeotic genes expressed ?
In a sub set of segments in the embryo where they contribute to segment identity
What is colinearity ?
Position of the gene in the cluster reflects the position along the anterior posterior axis where the gene is expressed
What are the known as the hox genes ?
Clustered genes, that determine the identity of structures in the embryo and clustered in the genome
What is the function of cell specific transcription factors ?
Bind to specific sequences in the control regions of a gene, and switch on or off gene expressions
What are homeotic genes ?
When mutated show phenotypes where structures appropriate to one part of the body plan appear elsewhere
Who originally studied drosphilia body plans ?
Ed Lewis
What is a homeobox ?
A 180bp conserved DNA sequence found in a family of cell specific transcription factors
What is the homeodomain ?
60 amino acid within the resulting protein encoded by the 180bp conserved sequence.
It forms a helix-turn-helix structure in the protein that binds to DNA in a sequence specific manner - the protein is a cell specific transcription factor
What are the hox genes ?
Homeotic genes that contain a homeobox sequence that encodes a homeodomain transcription factor
What are hox genes a subset of ?
All homeobox genes particularly involved in positional identity
What must animals have in order to have genes similar to hox genes ?
Organised body plan
How many clusters are there in mammals ?
4
What is the advantage of having four clusters of hox genes ?
- DNA sequence is conserved
2. Chromosomal arrangement of the gene is conserved
What does removal of multiple hox genes in vertebrates result in ?
A much more dramatic mutant phenotype
What happens when hoxc8 is mutated in mice ?
An extra thoracic vertebra with ribs is formed, vertebrae being produced in one position that are appropriate to another position
How do hox genes function ?
By conferring a complicated positional code called the hox code
What is the structure of hox genes in humans and mouse ?
There are 39 hox genes arranged in four clusters
How are the hox genes expressed in the mouse and human ?
Arranged in overlapping domains along the anterior posterior axis of the embryo
What do hox transcription factors regulate ?
Development by coordinating the transcription of batteries of other developmental genes
What are the features of the hox genes ?
- They contain a sub class of highly conserved homeobox, so they encode transcription factors
- They are involved in organising the body plan of an animal
- They exist in clusters of similar genes in the genome
Where are the hox gene expression turned on in the vertebrate limb ?
Overlapping domains along the developing limb bud
Where are the genes of the hox a cluster expressed ?
In overlapping domains along the posterior anterior axis
Where are the genes of hox d cluster expressed ?
In the overlapping domains along an axis between p/d and a/p
What do hox genes pattern ?
Both the p/d and a/p axes
What does the human mutation HOXD13 cause ?
Synpolydactyly
What are the A, B and C genes that encode transcription factors ?
MADS family transcription factors
How many genes and clusters are there in drosphilia ?
1 cluster and 11 genes
