Transcript Pair rule genes also encode TFs

Gap genes also encode TFs
They regulate each other’s txn:
Ex: Hb represses txn of Kr, helping to refine anterior
boundary of Kr expression.
They also regulate txn of other genes:
Ex: Pair Rule genes:
Each is expressed in 7 stripes.
Pair rule genes also encode TFs
They regulate each others’ txn.
Also regulate txn of Segment Polarity genes:
Each is expressed in 14 stripes along A-P axis.
Some encode TFs.
Others encode proteins involved in cell-cell communication.
(Growth Factor)
Drosophila embryogenesis
3h
Cellular blastoderm
Anterior
pole cells (germ line)
Posterior
10 h
Segmented embryo
22 h
1st instar larva
TFs
Divide the embryo
into segments
TFs
TFs
Cell-cell
communication
Regulate txn of
homeotic genes
Homeotic genes = Selector genes
Selector Gene Model
Selector Genes
(master control genes)
Effector Genes
Antp
Scr
T2
T1
T3
A8
Ubx
AbdB
Homeotic Phenotypes
Development of an inappropriate
body part in place of the correct
body part.
Ubx
WT
T1
T2
T3
T3 Effector Genes
Ubx
Ubx-
Antp off in T3 cells
Antp on in T3 cells
Ubx
mutant
WT
Antp incorrectly expressed in head
antenna
T2 legs
Two clusters of Drosophila
homeotic genes:
Antennapedia-complex (ANT-C)
Bithorax-complex (BX-C)
Hox genes
Mouse Hox gene expression
Hoxc-8 mutant
L1 = 1st lumbar vertebra
WT: L1 does not have ribs
Hoxc-8- mutant: L1 develops
like a more anterior vertebra
(homeotic phenotype)
Hoxc-8- mutants are viable
and have additional
phenotypes
Maintenance of Homeotic Genes
Expression Patterns
Trithorax-group:
Maintain homeotic genes in ON state.
Maintain active chromatin structure.
Polycomb-group:
Maintain transcriptional SILENCE of homeotic genes.
Maintain repressive chromatin structure.
Both involve covalent modifications of histones.
(Acetylation/Deacetylation & Methylation)
Collectively, help to maintain determined states of cells.
Clones = Genetically identical individuals
Two Kinds of Animal Cloning
Reproductive Cloning:
Remove nucleus from egg cell.
Replace it with nucleus from somatic cell (e.g., skin cell).
Stimulate cell to divide (like a fertilized egg).
Implant embryo into surrogate mother.
Therapeutic Cloning (Nuclear Transplantation):
Start the same, but stop development at blastocyst stage.
Remove inner cell mass cells and use to create
embryonic stem cell line.