Transvection
Zygotic Gene Activity in
Development
The differentiation of cell types and the
formation of organs depend on genes
being activated in particular spatial and
temporal patterns.
Zygotic Gene Expression
 The earliest events in animal development are
controlled by maternally synthesized factors.
 At some point after fertilization, zygotic gene
expression, or expression of genes from the
embryo’s genome, begins.
 The initial wave of zygotic gene expression is a
response to maternally synthesized factors. As
development proceeds, activation of zygotic genes
leads to complex cascades of gene expression.
Body Segmentation in Drosophila
Cuticle patterns
-Denticle belts
Homeotic Genes
 Homeotic mutations
transform one segment
into another; this is
called homeosis.
 Mutation in bithorax (bx)
affect two thoracic
segments.
 Mutations in
Antennapedia transform
the antennae into legs.
The Homeotic Genes of Drosophila
 The Drosophila homeotic genes form two large
clusters on one of the autosomes.
 All of the homeotic genes encode helix-turn-helix
transcription factors with a conserved homeodomain
region involved in DNA binding. These genes control
a regulatory cascade of target genes that control
segment identity.
 The expression pattern of these genes along the
anterior-posterior axis corresponds to the order of the
genes along the chromosome.
The Bithorax and Antennapedia
Complexes of Drosophila
Organ Formation
Organ formation requires the
organization of many different types
of cells.
Organ formation is under genetic
control.
The eyeless Gene
 Mutant flies lack eyes.
 The wild-type eyeless
gene encodes a
homeodomain
transcription factor that
activates a pathway
involving thousands of
genes.
Expression of eyeless in Other Tissues
Causes Formation of Extra Eyes
Mammalian Homologues of eyeless
The mouse homologue of eyeless,
Pax6, produces extra eyes when it is
inserted into Drosophila.
In mice, mutations in the eyeless
homologue reduce the size of the eyes.
Mutations in the human homologue
cause aniridia.
Two Hybrid
Genetic Analysis of
Development in Vertebrates
Geneticists can study development in vertebrates by
applying knowledge gained from the study of model
invertebrates, by analyzing mutations and
phenocopies of mutant genes in model vertebrates
such as mice and zebrafish, and by examining the
differentiation of stem cells.
The Mammalian Hox Genes
Evolution of the Hox Genes
The four mammalian Hox gene clusters were
created by quadruplication of a primordial
cluster early in vertebrate evolution.
The organization of the HOM/Hox genes has
been preserved during evolution.
Within each cluster, the physical order of the
Hox genes corresponds to their anteriorposterior expression pattern.
T2
A1
61 amino
Acid conserved
DNA
Binding
Sequence
A-DNA, B-DNA and Z-DNA
The Z-DNA helix is left-handed and has a structure that repeats every 2 base pairs.
The major and minor grooves, unlike A- and B-DNA, show little difference in width
B-DNA and Z-DNA
A-DNA, B-DNA and Z-DNA
Lambda repressor
EcoRV