Development

Question 1

Why is gene expression important in development?

Why Drosophilia used

Answer 1

• 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

Question 2

How molecular biologist understand differentiation

Answer 2

• compare mutants and normal cells

- Mutants whose embryos have problem in development & identify mutated gene or missing products in mutants

Question 3

3 groups of genes that are important in embryo development

Answer 3

1. Maternal genes/ egg polarity genes
2. Segmentation genes
3. Homeotic genes

Question 4

Maternal/ Egg polarity genes

• what it does
• Drosophila follicle

Answer 4

• 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.

Question 5

Maternal/ Egg polarity genes

-3 genes that determine anterior, posterior, dorsal and ventral ends

Answer 5

• 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

Question 6

Segmentation genes

Answer 6

• 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

Question 7

Segmentation genes - 3 subgroups and effect of mutation

Answer 7

1. Gap genes: mutation causes elimination of anterior segments
2. Pair-Rule genes: mutation causes deletion of even-numbered segments
3. Segment-polarity genes: mutations causes posterior half of each segment to be replaced by mirror image of anterior half an adjacent segment

Question 8

Homeotic Genes (hox genes)

Answer 8

-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

Question 9

Hox genes in Drosophila - present in 2 complexes

Answer 9

1. 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
2. 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

Question 10

How do Hox Proteins work?

Answer 10

• 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

Question 11

Regulation of hox gene expression 2 groups

Answer 11

• 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