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
What does Nanos inhibit?
Nanos inhibits the translation of hunchback mRNA (producing an anterior to posterior hunchback protein gradient )
What does Bicoid inhibit?
Bicoid inhibits the translation of caudal mRNA (producing a posterior to anterior caudal protein gradient)
Protein gradients establish…
The anterior and posterior axis
Gradients of maternal gene products (and hunchback) in the syncytial blastoderm
How do protein concentration gradients lead to a defined fruit fly body plan?
Protein gradients established in the egg perpetuate a cascade of positional information that ultimately defined the fruit fly body plan
What do maternal effect genes assist in?
Maternal-effect genes assist in the sequential activation of gap genes, pair rule genes, segment polarity genes and homeotuc genes
Outline the process by which maternal effect genes
- Anterior-posterior gradients formed by maternal-effect genes
- Zygotic gap genes divide embryo into broad regions
- Zygotic pair-rule genes divide embryo into stripes about two segments wide. The combined action of all pair-rule genes defines segment borders.
- Zygotic segment polarity genes divide segments into anterior and posterior halves
- Homeotic selector genes specify the identify of each segment
What are the patterns of gap gene expression?
- Hunchback expression is controlled by Bicoid
- Hunchback is the master gap gene
- Hunchback and Bicoid together control other gap gene
What is understood from the positional information and French flag model (Wolpert, 1996) ?
The expression of genes is either activated or repressed based on the concentration of another protein
Kruppel is specified by…
Kruppel is specified by hunchback protein
- above a threshold krupple is repressed
- at lower concentration krupples is activated
Transcription of GAP gene Kruppel is inhibited by…
Nanos and high concentrations of Bicoid and hunchback. Regulation is complex and involve multiple gene product interactions
Why do gap genes have multiple binding sites in promoter regions?
Gap genes have to respond to many genes and therefore have multiple binding sites for these genes in their promoters
-some binding sites mediate repression and some activate and this is how gap genes regulate pair-rule genes
Binding of gap gene proteins to one of the regulatory regions in the promoter of even-skipped
What do protein gradients established in the egg cause?
Protein gradients established in the egg perpetuate a cascade of positional information that ultimately defines the fruit fly body plan
At the end of this gene cascade are the homeotic genes (HOX genes) that confer identity to each body segment
What are Hox genes?
Hox genes are transcription factors that regulate the expression of other genes
What is needed for transcription factor function?
DNA:Protein and Protein:protein interactions are important for transcription factor function
How does the modular structure of transcription factors help with function?
Note modular structure of transcription factors: One part of the protein is responsible for DNA binding, another for dimer formation, another for transcriptional activation (I.e. interaction with basal transcription machinery)
Dimer formation adds an extra element of complexity and versatility
What is the homeodomain?
The homeodomain is a conserved 60 amino acid protein motif, found all HOX genes, that has DNA binding capacity
The homeodomain and Hox genes themselves are conserved between different Hox genes and between species
How Genes:
- Expressed in a colinear fashion in the embryo
- Occur in the genome a clusters
- Cluster duplication has occurred in a number of evolutionary lineages
- Define segment identity
- Genes are homologous across groups
What activates the overlapping domains of homeobox gene expression?
Patterning of the gap and rule genes activates overlapping domains of homeobox gene expression
What are homeotic mutants ?
Structure formed by one segment is transformed into that formed by another segment
Describe Homeotic genes(HOX genes) as selector genes
Homeotic genes (selector genes)
- regulate other zygotic genes
- Selector genes: determine which adult structures will be formed by each body segment - antennae, mouth parts, legs, wings, thorax, and abdomen
Hox genes expression domains are…
Similar in flies and vertebrates
How do Hox genes pattern vertebral identity?
- complete knockout of Hox 10 genes converts lumbar into more ribbed thoracic vertebrae
- complete knockout of Hox 11 genes transforms the sacral vertebrate into copies of lumbar vertebrae
Homeotic transformation in mammals requires mutation of all paralogs of a specific Hox gene
Why do all vertebrates have 7 cervical vertebrae?
The pattern of Hox gene expression specifies the identity of anterior-posterior axis elements.
In vertebrates these elements are the vertebrae
Do Hox gene expression pattern limb elements?
Yes
Which Hox paralog group patterns the scapula?
Hox paralog group 9
Which Hox paralog group patterns the humerus?
Hox paralog group 10
Which Hox paralog group patterns the ulna and radius?
Hox paralog group 11
Which Hox paralog group patterns the metacarpals ?
Hox paralog group 12
Which Hox paralog group patterns digits?
Hox paralog group 13
Explain how Hox gene expression patterns limb elements
Proximal-distal patterning of the limb bud appears to be controlled by a linear arrangement of Hox gene expression
-The pattern of Hox gene expression changes as limb outgrowth occurs, and genetic ‘knockout’ of paralogous groups results in loss of specific structures along the proximal-distal axis
What patterns the elements of the hands ?
Hox gene expression across the anterior-posterior axis of the limb patterns elements in the hand
Contrast paralogs and homologs
Homolog genes- are inherited in two species by a common ancestor
Paralogs- homologs that are evolved by duplication and code for protein with similar, but not identical functions
How are Hox genes expressed?
- Expressed in a colinear fashion in the embryo
- Occur in the genome as clusters
- Clusters duplication has occurred in a number of evolutionary lineages
They define segment identity
Contrast Homeotic genes in mammals and in vertebrates
In vertebrates:
- Hox genes in the cluster act specifically in the development of vertebrate limbs
- complete knockout of hox10 genes c9nverts lumbar into moreribbed thoracic vertebrate
- complete knockout of hox 11 genes transforms the sacral vertebrate into copies of lumbar vertebrae
In mammals- Genetic ‘knockout’ of paralogs groups results in loss of specific structures along the proximal-distal axis
-Mutations in HoxD13 in humans can cause a genetic condition called sympolydactyly
Describe Homeotic mutations in mammals
Hox genes have been duplicated over evolutionary history and now exist similar gene clusters
The genes of the different clusters work together to estsblish the identity of body segments along the head-tail axis.
Proximal-distal patterning of the limb bud appears to be controlled by a linear arrangement of Hox gene expression
Describe Homeotic mutations in fruit flies
Homeotic mutations are responsible for determining the identity of particular segments or structures of the body
-when homeotic genes are inactivated or expressed in unusual locations due to mutations, they may cause body segments to take new identities
