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 embryogenesis3 h

10 h

22 h

Cellular blastoderm

Segmented embryo

1st instar larva

pole cells (germ line)

Anterior Posterior

TFs

TFs

TFsCell-cell communication

Divide the embryo into segments Regulate txn of

homeotic genes

Homeotic genes = Selector genes

Selector Gene Model

Selector Genes (master control genes)

Effector Genes

Scr

T1

Antp

Ubx AbdB

T2

T3A8

Homeotic Phenotypes

Development of an inappropriate body part in place of the correct body part.

Ubx

T3 Effector Genes

Ubx Antp off in T3 cells

Ubx- Antp on in T3 cells

WT

Ubx mutant

T3

T2

T1

WT Antp incorrectly expressed in head

antenna

T2 legs

Antennapedia-complex (ANT-C)Bithorax-complex (BX-C)

Two clusters of Drosophila homeotic genes:

Hox genes

Mouse Hox gene expression

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

Hoxc-8 mutant

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.

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.

Clones = Genetically identical individuals