2.6 - 2.7 Development and Model Organisms Flashcards
What are the three key early stages of development?
- From fertilisation to 4 cell stage
- From morula to blastocyst
- From blastocyst to week 4 embryo
What happens during the fertilisation to 4 cell stage?
- The egg is in the final stage of meiosis as the sperm enters and delivers the pronucleus
- Polar bodies develop
- There are cleavage divisions
What happens in the stage going from morula to blastocyst?
- Those cells undero compaction and express cell adhesion molecules
- A watertight epithelium seal is made
- Water pumped in for fluid filled cavity called the blastula
What happens in the blastocyst to week 4 embryo stage?
- The embryo implants into the wall of the uterus - embryogenesis
- A furrow forms called the primitive streak
- Gastrulation means there are 3 tissue types created
How do we go from zygote to gastrula?
- Early development involves the cleavage of the ZYGOTE into smaller and smaller cells eventually forming the BLASTULA, a layer of cells enclosing a fluid filled cavity.
- Cell movements then take place during GASTRULATION to form the three germ layers: ECTODERM (outer skin), ENDODERM (inner skin) and MESODERM (middle skin).
In a frog how do the three germ layers give rise to distinct tissue?
- The Ectoderm gives rise to the epidermis and nervous system.
- The Endoderm gives rise to the lining of the gut, liver, pancreas, lung, etc.
- The Mesoderm gives rise to connective tissue, muscles, blood, cartilage, bone.
What are the four key processes for cells in development?
What is pattern formation in development?
What is morphogenesis in development?
How does ‘Dorsal’ demonstrate pattern formation?
A dorsoventral (top to bottom) gradient of a the Dorsal morphogen results in Twist being expressed in the ventral cells of the embryo.
How is Twist and example of morphogenesis?
Twist turns on expression of hundreds of genes which affect the behaviour of the cells. Expression of some genes cause cells to constrict on their apical side resulting in the formation of a furrow
What is the difference between mesenchymal cells and epithelial cells?
Mesenchymal cells are able to move around and they can also coalesce into epithelial tissue
What development features is pattern formation responsible for?
How can differences in gene expression between sibling cells be caused by internal factors?
- During an asymmetric division, internal molecules may be segregated to one cell after division which can change the fate of those cells. These are called “cell fate determinants”
- Its asymmetric division because the two cells are different from the beginning
How can differences in gene expression between sibling cells be caused by external factors?
Alternatively a symmetric division may produce two identical cells, but due to their position they come under the influence of different external factors/signals which can alter their cell fate
What is the concept of cell differentiation?
- The earliest cells in the embryo have the potential to give rise to many different cell types.
- As development proceeds, cells become more specialised and restricted in their possible fates, due to differential expression of genes
- Eventually they achieve their final mature form and are referred to as terminally differentiated
What is morphogen?
It is an important mechanism for establishing different cell fates across a field of cells
What are the two key features of morphogen?
What is induction in the context of development?
- It is where one cell type produces extracellular molecules that spread out and activate signalling pathways in neighbouring tissues.
- Those cells take on a particular fate
What is this phenomenon known as?
Progressive refinement
What are the four classes of mutant phenotypes in drisophila?
Where is the egg polarity gene Bicoid expressed?
Anterior half
Where is the gap gene Hunchback expressed?
In broad regions
Where is the pair rule gene Even skipped expressed?
In alternate segments
Where is the segment polarity gene Engrailed expressed?
In the posterior half of every segment
What is the hierarchy of genes that give rise to segmentation?
At a particular level genes only get regulated by things above them or at the same level
What do egg polarity genes do?
- Egg polarity genes like Bicoid establish the overall anterior-posterior pattern of the embryo.
- Their expression pattern is established during the formation of the egg.
- Bicoid is an egg polarity gene which acts as a morphogen
How is Bicoid protein expressed in a gradient form?
How does bicoid dose affect the position of the cephalic furrow?
- The position of the cephalic furrow depends on the levels of bicoid.
- Increasing bicoid dose moves the furrow posteriorly.
- The cephalic furrow starts to invaginate at key position 65%
What do Gap genes do?
They have expression patterns that cover large regions of the embryo. Their expression patter is controlled by egg polarity genes and by other gap genes
How does bicoid regulate Hunchback transcription?
How do Gap genes regulate each other?
- Gap genes control the expression domains of other gap genes
- Kruppel is repressed in anterior regions by Giant and in posterior regions by Knirps
- Also activated by intermediate levels of Hunchback
How do intermediate levels of Hunchback activate Kr expression?
How is eve stripe expression modular?
- Eve is a pair rule gene
- Each of these stripes it independently managed with its own enhancer
- The stripes are modular
How is the eve stripe expression regulated?
- Eve stripe 2 enhancer element has binding sites for Bicoid, Hunchback, Giant and Kruppel
- Egg polarity gene and pair rule gene, Bicoid and Hunchback can activate expression in a broad domain as they are above in the heirarchy
- Giant and Kruppel repress Eve, defining the anterior and posterior limits of expression
What are segment polarity genes?
- They are expressed in a subregion within each segment
- Their expression is initiated by pair rule proteins and maintained by other segment polarity proteins
- They establish patterns of cell fate within the segment
What are two important segment polarity genes, and what is their expression activated by?
- Wingless (wg) and engrailed (en)
- en expression is activated by high concentrations of Ftz or Eve and repressed by other pair rule genes resulting in 14 stripes of cells which are initially one cell wide
- wg expression is activated where Ftz and Eve are absent, in the gap between concentrations
How is segment polarity gene expression maintained?
- The initial expression of engrailed and wingless is maintained by a positive feedback signalling loop
- Uses two signalling molecules wg and hedgehog causing activation of the wingless pathway
- Wing turning on engrailed
- hedgehog binds to its receptor and turns on wingless which was put their initially by pair rule genes
How is segment identity determined by homeotic genes?
- Each of the segment gives rise to different parts of the embryo
- HOX gene controlled by all 4 level of hierarchy
How do HOX genes exhibit colinearity?
- Colinearity refers to the fact that the order of expression along the anterior-posterior axis in the embryo is also the order in which the genes are found on the chromosomes
How has the pattern of Hox genes been conserved in humans?
- They are orthologues
- Colinearity also conserved
What role do HOX genes play in humans?
THey are involved in the patterning of the limbs.
What does HOXD13 control?
What are some popular developmental model organisms?
Which organisms provide the best tradeoff between utility and relevancy?
What are some approaches to determining where the gene is transcribed?
- For in situ make an antisense mRNA moleucle that binds to it via complementary base pairing and tag with chemical stain
What are some methods for determining where the protein is expressed?
How is eve-stripe2-lacZ used as an example of a gene reporter?
Gene reporters are a powerful approach to analysing how the expression of a gene is controlled. Regulatory DNA elements such as the eve-stripe 2 enhancer, are plaed upstream of a reporter gene such as lacZ. The lacZ can then be detected by immunostaining for Beta Glactosidase
What is the difference between loss of function and gain of function analysis?
What is GAL4 and where does it bind?
It is a transcriptional activator from yeast
How does the GAL4-UAS system work?
Left shows where GAL4 is expressed and right WHAT is expressed
How can ectopic expression of Eyeless be achieved in the leg?
