Wormbook.org

1. Development of a complex multicellular organism is more than just mitosis- we certainly do not look like gigantic fertilized eggs. • Zygote-----many specialized cell types (differ in size, shape, longevity, biochemistry) • Plus they are arranged spatially in tissues (cells that share a certain function) • Wow! • What does it take to go from a single celled zygote to a multicellular individual?? • 1. Mitosis…..2. Differentiation

2. How do cells grow to look a certain way and occur in a particular place? C. elegans Wormbook.org

3. Figure 14.9 1 2 3 Promoter • How do cells grow to look a certain way and occur in a particular place? Nontemplate strand DNA 5 3 T A T A A A A 3 5 A T A T T T T A eukaryotic promoter TATA box Start point Template strand Transcription factors 5 3 Several transcription factors bind to DNA. 3 5 • …. each cell is “told” by transcription factors to transcribe different genes to make mRNA and then to make different proteins. • Differential transcription and translation of genes drives differentiation! RNA polymerase II Transcription factors Transcription initiation complex forms. 5 3 3 5 3 5 RNA transcript Transcription initiation complex

4. So lets go back to our zygote that is waiting to go through mitosis to grow into an adult with many different cell types…. Transcription factors initiate differentiation.

5. So genes are being transcribed, mRNA is being made and in turn proteins are made. NOW…… “The expression of one gene can influence the expression of several other genes.” What does that mean??? The protein being made may be a transcription factor for another gene!

6. Figure 16.4-3 Nucleus • A specific example!!!! Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF MyoD protein (transcription factor) Myoblast (determined) mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell cycle– blocking proteins MyoD Another transcription factor Part of a muscle fiber (fully differentiated cell)

7. In turn, the altered expression patterns of these genes can then influence the expression of an even larger number of genes. By this process, called a cascade, a change in one or a few genes can alter the expression patterns of numerous genes.

8. Transcription factors bind to DNA sequenceslocated near the coding region of the gene in question. When they bind they alter the “transcriptional machinery” and, thus, the level of transcription can change. In some cases the binding of transcription factors causes transcription to increase (up-regulation); in other cases it causes transcription to decrease (down-regulation).

9. Back to the ZYGOTE There are transcription factors already INSIDE zygotes! (b) Induction by nearby cells Unfertilized egg Early embryo (32 cells) Sperm Who puts those little green triangles and brown circles in the cell? Cytoplasmic determinants serve as transcription factors and regulate expression of the cell’s genes as cells differentiate. Nucleus Figure 16.3 Fertilization Zygote (fertilized egg) NUCLEUS Signal transduction pathway Mitotic cell division Signal receptor Two-celled embryo Signaling molecule (inducer)

10. Induction by nearby cells (a) Cytoplasmic determinants in the egg Unfertilized egg Early embryo (32 cells) Sperm Sometimes transcription is also initiated from OUTSIDE cell during development. Nucleus Figure 16.3 Fertilization Molecules of two different cytoplasmic determinants Zygote (fertilized egg) NUCLEUS Signal transduction pathway Mitotic cell division Signal receptor Two-celled embryo Signaling molecule (inducer)

11. Revisit one way this process starts… Egg establishes a molecular coordinate system-provides a way of telling "which end is up” A specific cytoplasmic determinant is called bicoid-establishes the anterior-posterior polarity in Drosophila. How does bicoid do this?

12. Researchers looked for mutants!! Bicoid-two tails-lack front half of body! They hypothesized that gradients of substances were important. In mutants discovered bicoid mRNA is NOT concentrated in anterior end (head end). (Nurse cells surround the anterior region of the egg in Drosophila-is transported from the nurse cells into the egg).

13. Figure 16.7b-5 1 2 3 4 5 Follicle cell Egg developing within ovarian follicle Nucleus Egg Nurse cell Egg shell Unfertilized egg Depleted nurse cells Fertilization Laying of egg Fertilized egg Embryonic development Segmented embryo 0.1 mm Body segments Hatching Larval stage (b) Development from egg to larva

14. Figure 16.7a Body plans! Ultimately positional information comes from cytoplasmic determinants but inductive signals also come into play… Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Posterior Anterior Left Ventral (a) Adult These cues determine how the cell and the cell’s progeny will react to future molecular signals

15. Apoptosis During development but also when cells are damaged or infected.. What happens? cells parts are packaged up and scavenged by other cells (saves neighbors from reactive chemicals that might leak out) Details were worked out in C. elegans(as adults they have 1,000 cells but on the way cell death happens regularly) Are triggered to die thru signal transduction pathway by external signals with relay molecules passing message to nucleus-then proteins are produced that start process!

16. Figure 16.6 1 mm Interdigital tissue Cells undergoing apoptosis Space between digits