Development Flashcards
Why is gene expression important in development?
Why Drosophilia used
- single cell differentiates into many different organs, tissues, limbs etc.
- causes cells to perform different functions
- Drosophilia used to study development because of;
- its v. short life cycle
- Egg develops into complete adult in 9 days
How molecular biologist understand differentiation
- compare mutants and normal cells
- Mutants whose embryos have problem in development & identify mutated gene or missing products in mutants
3 groups of genes that are important in embryo development
- Maternal genes/ egg polarity genes
- Segmentation genes
- Homeotic genes
Maternal/ Egg polarity genes
- what it does
- Drosophila follicle
- Fate of cell largely determined by its position in the egg
- cells at anterior end develop into head structures
- cells at posterior end develop into legs or arms
- there is a specific way things develop
- Drosophilia follicle contains outer surface of follice cells that surround nurse cells that are in close contact w/ oocyte
- nurse cells contected by cytoplasmic bridges to each other and to the anterior end of the oocyte. Follicle cells somatic; nurse cells and oocyte are germline in origin.
Maternal/ Egg polarity genes
-3 genes that determine anterior, posterior, dorsal and ventral ends
- Dorsal protein in nuclei helps determine dorsal - bentral axis of drosophilia embryo
- dorsal protein conc. in nuclei on ventral surface (does diffuse across cell) - Bicoid and Nanos proteins determine the anterior-posterior axis in Drosophila embryo
- Bicoid: localised at anterior end of egg
- Nanos products located at posterior end of drosophilia embryo
Segmentation genes
- embryo will cosist of 3 thoracic and 8 abdominal segments in addition to head and tail structures
- Mutations in segmentation genes lethal -> causes improper segmentation and embryo dies before maturity
- total of 30 segmental genes - divided into 3 subgroups
1. Gap genes
2. Pair Rule Genes
3. Segment polarity genes
Segmentation genes - 3 subgroups and effect of mutation
- Gap genes: mutation causes elimination of anterior segments
- Pair-Rule genes: mutation causes deletion of even-numbered segments
- Segment-polarity genes: mutations causes posterior half of each segment to be replaced by mirror image of anterior half an adjacent segment
Homeotic Genes (hox genes)
-responsible for the development of body parts
Homoios - something has changed to something else
-mutations in any one of these genes causes one body part to develop as another
-Each gene has a region of homology, called Homeobox (is 180bp in length)
-codes for a DNA binding domain (Homeodomain) that is similar in all hox gene products
-homeodomain present near C-terminus and 80% conserved in all Hox proteins
Hox genes in Drosophila - present in 2 complexes
- antennapedia complex (has 5 genes)
- homeotic genes confer identity on the most anterior segments of the fly
- genes vary in size and are interspersed w/ other genes
- Bithorax complex (contains 3 hox genes)
- proceeding from left to right each homeotic gene in complex acts upon a more posterior region of the fly
- formation of a compartment that requires the gene product(s) expressed in previous compartment plus a new function coded by next gene along cluster
How do Hox Proteins work?
- Homeodomain is a helix turn helix motif, a DNA binding region
- provides specificity in DNA binding
- can act as an activator or a repressor (depends on a.a. seq. and N terminus & other proteins interacting with it)
- Hox proteins frequently found complexed with other proteins
- hetrodimer formation extends range of regulatory activities of Hox proteins
Regulation of hox gene expression 2 groups
- Different hox proteins required at various stages of development
- tight control is important for normal development
- genes controlling hox genes placed in 2 groups;
1. Polycomb group (pc-G) - Pc-G proteins act by recognizing a DNA region called polycomb response element - their binding to target DNA causes REPRESSION of nearby genes
2. Trithorax group (Trx-G): are activators of hox genes - their binding to DNA maintains chromatin in transcriptionally active state