lecture 23 Flashcards
1
Q
What is pattern formation?
A
- the process by which a spatial and temporal pattern of cellular activities is organised within the embryo so that a well ordered structure develops
- development of a body plan, axes
2
Q
What happens if there are errors in pattern formation?
A
- can have severe consequences for the development of an organism
- e.g. duplicated thorax in fly
- fusions of digits, knuckles
3
Q
What are simple stages in development of an organism?
A
- zygote
- cleavage
- blastula
- gastrulation
- neurulation
- neurula
- segmentation (somitogenesis)
- determination of segment identity
- Hox genes
4
Q
What is the definition of determination?
A
Formal definition
- determination implies a stable change in the internal state of a cell such that its fate is now fixed, or determined
- a determined cell will follow that fate when grafted into other regions of the embryo
- cells often “know” where they are positioned in a developing embryo before morphological differentiation occurs
- they are determined
5
Q
What is an experiment that shows the pre-somitic mesoderm has a positional identity prior to somite formation?
A
- with chick embryos
- the segmental plate mesoderm is determined as to its position along the anterior-posterior axis before somitogenesis
- when segmental plate mesoderm that would ordinarily form thoracic somites is transplanted into a region in a younger embryo (caudal to the first somite) that would ordinarily give rise to cervical (neck) somites
- the grafted mesoderm differentiates according to its original position and forms ribs in the neck
6
Q
By what is the positional identity of somites along the antero-posterior axis specified?
A
- Hox genes
- e.g. head to tail separated to four regions: W, X, Y, Z
- gene I expressed in W, X, Y, Z
- gene II expressed in X, Y, Z
- gene III expressed in Y, Z
- gene IV expressed in Z
- therefore each region has/is defined by the expression of certain numbers of these genes
7
Q
What are Hox genes?
A
- are transcription factors
- contain a DNA-binding region of approx. 60 amino acids called the homeodomain (contain a helix-turn-helix DNA-binding motif)
- the DNA sequence of 180 base pairs that encodes the homeodomain is called the homeobox (hence the name)
- first identified in the fruitfly, Drosophila
- required for anterior-posterior positional identity in flies and mammals (and all other segmented animals)
8
Q
Why are they called homeobox genes?
A
- homeobox received their name because mutations in these genes can result in homeotic transformations
- i.e. one body segment is transformed into another with different positional identity
9
Q
Are clusters of homeotic genes only found in the fly?
A
- no
- while clusters of homeotic genes were first identified in the fly
- similar clusters were subsequently identified in vertebrates
- known as Hox complexes
- in the mouse there are four Hox clusters that have arisen during evolution of vertebrates by duplication of an ancestral cluster
- therefore, corresponding genes within the four clusters resemble each other closely (known as paralogs)
- the similarity is most pronounced within the homeobox
a point of confusion:
- most genes that contain a homeobox are not part of a Hox complex and are not associated with homeotic transformations
10
Q
What are orthologs cf paralogs?
A
- ortholog = similar gene between species
- paralog = similar gene with in an organism’s/species’ genome
11
Q
How are Hox genes aligned in the drosophila?
A
- along chromosome from 3’ to 5’
- labial - expressed in the head
- expression of genes aligns with their position along the chromosome
- matches anterior to posterior structures of the fly
- collinearity
- theoretically there’s no reason why this should be the case in genes
12
Q
What are features of hox gene expression?
A
- Hox genes typically have sharp anterior boundaries of expression and less well defined posterior boundaries
- “anterior” genes are expressed first
- genes found more 3’ in the cluster are expressed more anteriorly
- collinearity
13
Q
How do we know that Hox genes influence segment identity?
A
- appear to be expressed in the right regions
- particular Hox genes specify particular regions in mouse and chick, even if the number of segments in each region is different
- e.g. boundary between expression of hox 5 and hox 6 is found at the boundary between cervical and thoracic vertebrae in both the chick and the mouse despite the chick having 14 and the mouse having 7 cervical vertebrae (or sometimes that give rise to these vertebrae)
14
Q
What is posterior dominance?
A
- aka posterior prevalence
- the more posteriorly expressed Hox genes tend to inhibit the action of Hox genes expressed more anteriorly to them
- hence back to our 4 region example
- in region Z, gene I, II, III and IV are expressed but if gene IV inhibits all the three genes anterior to it, the only functioning gene in region Z is therefore gene IV
15
Q
What do you have to consider when doing KO mice for hox genes?
A
- they have paralogs, so if you want to see knockout function you have to ko all paralogs
