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Anterior – Posterior axis formation in embryo Sompid Samipak Part I Models organisms Embryonic development Involves cell division, cell differentiation, and morphogenesis A single-celled zygote gives rise to cells of many different types, each with a different structure and...

Anterior – Posterior axis formation in embryo Sompid Samipak Part I Models organisms Embryonic development Involves cell division, cell differentiation, and morphogenesis A single-celled zygote gives rise to cells of many different types, each with a different structure and corresponding function Mitotic cell division : give rise to a large number of cells Cell differentiation : cells become specialized in structure and function Morphogenesis : processes that give shape to the organism and its parts https://www.ck12.org/book/ck-12-biology/section/8.1/  Totipotency in plants - Testing genomic equivalence - A differentiated cell can Generate a whole organism Totipotent : capable of developing into a complete organism or differentiating into any of its cells or tissues. Nuclear transplantation in animals In nuclear transplantation - the nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell (cloning = using one or more somatic cells from a multicellular organism to make another genetically identical individual)  Frog egg cells are exposed to UV light, destroying the nucleus. Nuclei from cells of embryos are transplanted into the enucleated egg cells. Most of the recipient eggs developed into tadpoles when the transplanted nuclei came from relatively undifferentiated cells of an early embryo. However, the success rate decreases as the donor cell becomes more differentiated.  fasciated wildtype Used tomato mutation fasciated, which causes an increased number of floral organs per whorl, and tomato wild type for fasciated. Interspecific chimeras were generated between tomato and L. peruvianum, which differ in number of carpels per flower Stem cells can be isolated from early embryos at the blastocyst stage Totipotent stem cells can divide into all cell types in an organism. A totipotent cell has the potential to divide until it creates an entire, complete organism. Pluripotent stem cells can divide into most, or all, cell types in an organism, but cannot develop into an entire organism on their own.  In induction process Cell- Cell signals) Signal molecules from embryonic cells cause transcriptional changes in nearby target cells The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression Pattern formation – has been well studied in Drosophila Key developmental events in the life cycle of Drosophila Gene regulation Regulation of transcription Alternative splicing Regulation by microRNAs ALSO….. Don’t forget protein mediated regulation of translation! Best studies is found in the study of embryo development in fruit fly Drosophila melanogaster.  Fruit fly egg has distinct polarity Give rises to polarity of the body plan in embryo, larva and adult fly Anterior to posterior polarity in the eggs depends on anchoring mRNAs and proteins to each end of the egg and the regulation of translation  Anterior determination relies on bicoid gene All of offsprings will show the same phenotype regardless of its own genotype Mirror image duplication of posterior structures bicoid gene must provide something in the mother that is necessary for the development of the anterior end in the offspring embryo Nearly all cytoplasm components in embryo are provided by the mother If mother fails to provide any components, offspring will exhibit the consequences and show changes in phenotypes Genotype of the mother affect the phenotype of the offspring : maternal effect  Nurse cells Oocyte Oocyte (egg cell) connects to special cells called nurse cells providing nourishment to cytoplasmic components to the egg cell - Mother transcribe bicoid gene in nurse cells, and distribute in the cytoplasm of oocyte. It will not be translated until after fertilization. - The end of the oocyte with tethered bicoid mRNA will become anterior of embryo and the head of larvae - microtubules are used for anchoring and localization - Cell doubling in 9 min - Nucleus divides without cell division Fertilized egg and move to the nucleus periphery of embryo - After the 13th round of nuclear division, cell Dividing nuclei membrane form. - Nuclei reside in the same cytoplasm is known as “syncytium”, More nuclei division here macromolecules Movement to periphery can freely diffuse and affect all nuclei Cell divisions to form cellular blastoderm - But bicoid mRNA is tethered to anterior end, so cannot diffuse freely - Upon fertilization, nuclear division begin, bicoid gene is translated into protein (brown) that diffuse freely creating concentration gradient - In nucleus, many genes Require Require Require low are regulated by Bicoid high levels medium levels Bicoid protein of Bicoid levels Bicoid protein protein protein Morphogens = Substances that are found in a gradient distribution and can direct different repones at different concentration levels Targeted genes that bind weakly requiring high level of Bicoid protein Targeted genes that bind strongly require low level of Bicoid protein Hunchback gene is a gene that needs Bicoid protein to be activated  Hunchback gene Promoter region Strong Bicoid binding Weak Bicoid binding Cooperative binding at the promoter, once Bicoid binding at one site, it is more likely that the other site will bind Cooperative binding Bicoid driven Hunchback’s expression and feedback mechanism Hunchback gene’s expression throughout anterior end of embryo Hunchback’s expression cutoff where Bicoid protein level drops below the threshold needed for activation Mother deposits hunchback mRNA throughout the egg… …but translation of hunchback mRNA is inhibited in the posterior by posteriorly restricted nanos protein. - Posterior repression of hunchback translation further restricts hunchback protein to the anterior end of the embryo Until now, regulating transcription of target genes along the Bicoid gradient is one mode of Bicoid protein  Caudal mRNA is also deposited (uniformly in blue color) into the egg by the mother Caudal mRNA Caudal protein is produced only in the posterior end and plays a role in the development of the posterior end of Caudal protein the embryo  Posterior production of Caudal protein is achieved by Bicoid Caudal mRNA protein inhibiting the Bicoid protein translation of the Caudal Caudal caudal mRNA mRNA protein Caudal protein  Initiation factors unable to bind cap Unable to recruit small ribosomal subunit complex caudal mRNA caudal mRNA Onset of translation BLOCKED Bicoid protein bind to the 3’ UTR of the caudal mRNA Bound Bicoid protein interacts with other protein that prevents the 5’ cap structure of the caudal mRNA from interacting with the initiation factors that is needed to recruit the small ribosomal subunit complex  Later studies showed that Bicoid mediated repression of Caudal translation also involve microRNAs that bind alongside Bicoid protein at 3’ UTR of caudal mRNA Recap : control of anterior – posterior axis of embryo  Regulatory control of embryo development establishing anterior-posterior axis Maternal effects by deposition of morphogens Localization and anchoring of mRNA Control of transcription by maternal supplied morphogens Translation regulation of maternally contributed mRNA