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) What does it take to go from a single celled zygote to a multicellular individual?? 1. Mitosis…..2. Differentiation

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

How do cells grow to look a certain way and occur in a particular place? Different genes turned on and off due to Transcription factors Epigenetics Differential transcription and translation of genes drives differentiation!

Figure 14.9 Transcription factors TATA box Promoter Nontemplate strand Start point Transcription initiation complex forms. Transcription initiation complex DNA RNA transcript A eukaryotic promoter Several transcription factors bind to DNA Template strand Transcription factors RNA polymerase II TATAAAA ATATTTT …. each cell is “told” by transcription factors to transcribe different genes to make mRNA and then to make different proteins When different proteins are made the cell differentiates

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 cell differentiation.

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!

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

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.

Transcription factors bind to DNA sequences located 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. Can cause transcription to increase (up- regulation) or decrease (down-regulation).

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

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

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 (an mRNA) establishes the anterior- posterior polarity in Drosophila. This mRNA is put into the egg by “nurse cells” This one mRNA tends to aggregate in anterior end because that is where nurse cells are.

If there is a problem with this mRNA then it does not aggregate in the anterior region and you end up with a mutant fly. If it is put there by mom…who is mutated? If mom homozygous for “Bicoid” (remember fly mutants named by mutation) will have offspring with two tails-lack front half of body!

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

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

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!

Figure mm Interdigital tissueCells undergoing apoptosis Space between digits

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Questions for-Cells in the early embryo battle each other…. What did the researchers study and what did they find?? If all cells in a body come from a fertilized zygote and are thus genotypically identical why would there be differences between cells?? Where would these differences come from?? Why might this process be an important force in longer lived animals? Is this just like the process of apoptosis??