Developmemtal Genetics Flashcards
Who won the Nobel prize of 1995 for developmental genetics?
- Edward Lewis
- Eric Wieschaus
- Christianne Nusslein-Volhard
For the results of their large scale mutagenesis screen
What were the efforts of Eric Weischaus, Edward Lewis and Christianne Nusselein-Volhard?
The efforts of these 3 scientists
- 30,000 independent mutants in genes required for life
- 8,000 mutants required for embryonic development
- 750 mutants affecting anterior/posterior or dorsal/ventral patterning
- 150 genes identified that affect anterior/posterior or dorsal/ventral
(I.e. they found approximately 5 mutant alleles per gene 750/150= 5)
What two different gene sets control embryonic development in fruit fly?
- Maternal effect genes
2. Zygotic genes
Explain maternal effect genes
- mRNA and proteins deposited in the egg cytoplasm during oogenesis (source maternal genome)
- Products distributed in gradients or concentrated in specific regions of the cell
- Encode for transcription factors and proteins that regulate gene expression and activate or repress expression of genes in the embryo
- mutation which when present in the mother do not damage her but have effects on the development of her progeny
- maternal-effect mutations can not be rescued by the paternal genetic contribution in the sperm
Explain zygotic genes
- transcribed in nuclei of developing embryo
- typically not transcribed immediately following fertilization
- transcribed in specific regions in response to distribution of maternal-effect proteins
- deleterious recessive mutations in homozygotes lead to embryonic lethality
How to build a fly?
Specify position by a graded response to a morphogen molecule.
- First - subdivide then embryo into anterior and posterior regions (ie head and tail)
- Second- define smaller segments later on (ie. The number of segments in the thorax)
- Assign identity to each segment
Experiments have shown that destruction of maternal mRNA or transplantation of maternal mRNAs…
…to different locations in the embryo can result in dramatic abnormalities in embryonic development and patterning of the body plan
What is drosophila?
Cytoplasmic determinants of anterior-posterior axis
What is the Bicoid maternal effect gene?
Bicoid mRNA is normally synthesized by nurse cells (oocyte support cells) and deposited in the anterior of the oocyte
What is the Bicoid mutation?
bcd-bcd- mother fly produces defective oocyte lacking Bicoid mRNA —> fertilization leads to embryo lacking head and thorax
Embryos can be rescued:
Purified normal Bicoid mRNA—> embryo of bcd-bcd mother develops injection site —> head
What is the Nanos maternal effect gene ?
nanos mRNA is deposited at posterior of the egg after fertilization, nanos protein is involved in specifying the abdominal segments
Explain the nanos mutation and what embryo can be rescued by
Offspring of nanos-/nanos- mother —> embryo lacking abdomen and posterior
Embryo can be rescued by:
- Cytoplasm from posterior of normal egg (with nanos mRNA) is injected
- Posterior abdomen region of embryo from nanos-/nanos- mother
- Embryo develops normal abdominal structures
What can mutation in Drosophilia?
Mutations in drosophilia maternal effect genes disrupts normal development of their offspring
Bicoid mutants lack anterior structures
Nanos mutants lack abdominal and posterior
What kind of morphogen is Bicoid?
An anterior morphogen
What are morphogen?
Morphogens are gene products that directly govern the process of tissue development during pattern formation
What does superficial cleavage create?
Superficial cleavage creates a Syncytial blast Oder mood
- 30 minutes - fusion of spermatogonia and egg nuclei
- 70 minutes- nuclear division creating syncytium
- 90 minutes- nuclei migrate to periphery of cytoplasm
- 2 hours - synctial blastoderm
- 3 hours- cellular blastoderm
What can happen before cellular blastoderm formation?
Prior to formation of the cellular blastoderm proteins can diffuse freely accr8ss the embryo
Explain determination of the A/P axis of the fruit fly
Bcd protein forms a gradient with highest amounts in anterior
Bcd is a transcription factor that activates genes at different concentrations as well as preventing the translation of specific mRNA
What is a caudal protein?
Caudal protein assists in defining the posterior end of the embryo
How is the caudal mRNA distribution converted to a posterior to anterior Caudalprotein gradient ?
Bicoid protein represses caudal mRNA translation
How does BCD affect translation?
Interaction of translation initiation complex with 3’ UTR bound proteins
Proteins like Bicoid (Bcd) to elements in the 3’ UTR (of Caudal mRNA that recruit proteins that bind to the m7G cap and prevent binding of the translation initiation complex
Explain the concentration gradient of Nanos
Nanos mRNA is translated to produce a concentration gradient highest at the posterior
Does Nanos act as a morphogen ?
Nanos doesn’t act as a morphogen
It represses maternal hunchback which gets distributed throughout the embryo
-nanos specifically binds Hb mRNA
What causes the opposing concentration gradients of Bicoid and nanos proteins?
Localized deposition and tethering of nanos and Bicoid mRNA results in a protein gradient following translation
a. Gradient formation by localization and diffusion