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 3 h 10 h 22 h Cellular blastoderm Segmented embryo 1st instar larva pole cells (germ line) AnteriorPosterior

TFs Cell-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 UbxAbdB T2 T3 A8

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