Question

1. What is one big difference between how frogs and fish gastrulate? (1-2 sentences) (Hint -explain how each of these organisms gastrulates and the difference will be obvious!)

2. Number the following steps of germ cell development in the appropriate order.

A. The primordial germ cells undergo meiosis.

B. The primordial germ cells are specified

C. The primordial germ cells migrate to where the gonads are developing

3.

What are the signals that are required for the induction of the organizer?

		TGF-beta signaling is the mesoderm inducing signal, Wnt signaling comes from the Niewcoop center
		Wnt signaling is the mesoderm inducing signal, TGF-beta signaling comes from the Niewcoop center
		Signals come from the animal hemisphere to induce the organizer
		the organizer is autonomously specified

4. How did C. Easley make sperm-like cells from human foreskin fibroblasts? (2-3 sentences)

5. What is special about how the genes (whose mutations result in the flies above) are arranged in the fly genome and how they are expressed in the developing embryo? (1-2 sentences)

Answer 1

ans 1.the end product of gastrulation is brastula -disk shaped of undifferentiated cells inside embryo consisting of three different layers .

gastrullation depends on yolk distribution .

FISH

The fish embryo is telolecithal and contains lots of yolk. Cleavage results in a flat mass of cells on one side of the embryo. Gastrulation follows, consisting of two major processes: epiboly and involution. In epiboly, cells at the animal pole (termed the blastoderm or blastodisc in fish) spread out and expand to cover the yolk at the vegetal pole (in amphibians(frog), yolk-filled cells are covered instead).1ST DIFFERENCE

During epiboly, cells at the edge of the blastoderm began to involute (fold in) and later form the endoderm and mesoderm, with non-involuted cells forming the ectoderm. During epiboly and involution, blastodermal cells converge on one side of the embryo . and interviate via convergent extension to form axis

FROG

Like fish eggs, amphibian eggs are telolecithal and have bulky yolk at the vegetal pole which prohibit the simple invagination seen in sea urchins. In fish and amphibians, the blastopore is located just above the largest of the yolk. As animal pole cells increase in area and spread over the embryo (via epiboly), the lip of the blastopore widens. Beginning at the dorsal side, cells at the edge of the blastopore lip begin moving inward via involution.

The archenteron -- which gives rise to the endoderm -- is now formed from the vegetal yolky cells that were surrounded via epiboly, and by the cells involuted from the dorsal lip. Other cells that involute over the dorsal lip form the intervening mesoderm. Convergent extension is responsible for the capacity for a small ring of cells just above the dorsal lip to lengthen along the anterior-posterior axis and form the internal endodermal and mesodermal sheets.

Gastrulation in amphibians requires fibronectin, a glycoprotein in the extracellular matrix that provides a substrate for cell migration. Fibronectin binds to integrins (receptors for extracellular matrix proteins). By binding extracellular fibronectin and intracellular actin, integrins link the external environment to the cytoskeleton. In amphibians, the surfaces of cells in the blastocoel roof become covered with oriented fibronectin fibrils. Blastocoels injected with fibronectin antibodies do not undergo gastrulation; the cell layer near the dorsal lip folds repeatedly without going inward.

2.the order are as follows

C<A<B