Chapter 18 (4)

True or False:
In embryonic development, cells give rise to cells of many different types, with different structure and function.
Any developmental program must produce cells of different types that form higher-level structures arranged in a particular way in 3 dimensions
True False

The stages between a zygote and the organism it becomes: __, __, + __.
Through a succession of mitotic cell divisions, the zygote gives rise to a large number of cells. __alone would produce only a great ball of identical cells.

During embryonic development, cells not only increase in number, but also undergo __, the process by which cells become specialized in structure and function.
Moreover, the different kinds of cells are not randomly distributed but are organized into __ in a particular 3D arrangement.

The physical processes that give an organism its shape is __, meaning “creation of form.” It can be traced back to changes in the shape, motility, and other characteristics of the cell that make up various regions of the embryo.

Activity of a cell depends on the gene it expresses and the proteins it produces. Almost all cells in an organism have the same genome; therefore, __results from the genes being regulated differently in each cell type.
Each fully __cell has a particular mix of specific activators that turn on the collection of genes whose products are required in the cell.

How do different sets of activators come to be present in two liver cells?
· It turns out that materials placed into the __by the mother set up a sequential program of gene regulation that is carried out as cells divide, and this program makes the cells become different from each other in a coordinated fashion.

The specific genes expressed in any particular cell of a developing organism determines its path.

Two sources of information, used to varying extents in different species, “tell” a cell which genes to express at any given time during embryonic development.
What are they?

mRNA, proteins, other substances, and organelles are distributed unevenly in the unfertilized __, and this unevenness has a profound impact on the development of the future embryo in many species. Maternal substances in the egg that influence the course f early development are __. After fertilization, early mitotic divisions distribute the zygote’s cytoplasm into separate cells. The nuclei of these cells may thus be exposed to different __, depending on which portions of the zygotic cytoplasm a cell received. The combination of __ in a cell helps determine its developmental fate by regulating expression of the cell’s genes during the course of __.

In an environment, most influential are the signals impinging on an embryonic cell from other embryonic cells in the vicinity, including contact with __ molecules on neighboring cells and the binding of __secreted by neighboring cells. Such signals cause changes in the target cells, a process called __. The molecules conveying these signals within the target cell are __ and other proteins expressed by the embryo’s own genes. In general, the signaling molecules send a cell down a specific developmental path by causing changes in its gene expression that eventually result in observable cellular
changes. Thus, interactions between embryonic cells help induce __ of many specialized cell types making up a new organism.

True or False:
As the tissues and organs of an embryo develop and their cells differentiate, the cells become noticeably different in structure and function. These observable changes are actually the outcome of a cell’s developmental history beginning at the first mitotic division of the zygote.
The earliest changes that set a cell on a path to specialization are subtle ones, showing up at the molecular level.
True False

Biologists use __to refer to the events that lead to the observable differentiation of a cell. Once it has undergone __, an embryonic cell is irreversibly committed to its final fate. If a committed cell is experimentally placed in another location of the embryo, it will still differentiate into the cell type that is its normal fate.

The outcome of __, observable cell differentiation, is marked by the expression of genes for tissue-specific proteins. These proteins are found only in a specific cell type and give the cell its characteristic structure and function.
The first evidence of __is the appearance of mRNAs for these proteins. Eventually, __ is observable with a microscope as changes in cellular structure.

On the molecular level, different set of genes are sequentially expressed in a regulated manner as new cells arise from division of their __.
A number of these steps in gene expression may be regulated during __, with transcription among the most important. In the fully differentiated cell, __remains the principle regulatory point for maintaining appropriate gene expression.

__ cells are specialists at making tissue-specific proteins.

__cells develop from embryonic precursor cells that have the potential to develop into a number of cell types, including cartilage cells and fat cells, but particular conditions commit them to becoming muscle cells.
· Although the committed cells appear unchanged under the microscope, __ has occurred, and they are now __, which eventually start to churn out large amounts of muscle-specific proteins and fuse to form mature, elongated, __skeletal muscle cells.

In an experiment of muscle cell determination, different genes were expressed in a separate embryonic precursor cell, and then scientists looked for differentiation into myoblasts and muscle cells.
· In this way, they identified several so-called “__” whose proteins products commit the cells to becoming skeletal muscle. Thus in the case of muscle cells, the molecular basis of __ is the expression of one or more of these __.

__encodes the __protein, a transcription factor that binds to specific control elements in the enhancers of various target genes and stimulates their expression. Some target genes for __ encode still other muscle-specific transcription factors. __ also stimulates expression of the __ gene itself, thus perpetuating its effect in maintaining the cell’s differentiated state. Presumably, all the genes activated by __ have enhancer __ recognized by __ and are thus coordinately controlled. Finally, the secondary transcription factors activate the genes for proteins such as myosin and actin that confer the unique skeletal muscle cell properties.

The MyoD protein deserves its designation as a __. It is even capable of changing some kinds of fully differentiated nonmuscle cells, like fat and lover cells, into muscle cells.

o Why doesn’t it work on all kinds of cells?
§ One likely explanation is that activation of the muscle-specific genes is not solely dependent on __but requires a particular combination of regulatory proteins, some of which are lacking in cells that do not respond to __.

For tissues to function effectively in the organism as a whole, the organism’s body plan- its overall 3D arrangement- must be established and superimposed on the __process.
__ and __ both contribute to the development of a spatial organization in which the tissues and organs of an organism are all in their characteristic places, called __.

Pattern formation:
· In animals: begins in the early __, when the major __of an animal are established. Before the tissues and organs of a bilaterally symmetrical animal appear, the relative positions of the animal’s __ and __, __ and __, and __ and __ are set up, thus establishing the 3 major body axes. THe molecular cues that control __, collectively called__, are provided by __ and __.
o These cues tell a cell its location relative to the body axes and to neighboring cells and determine how the cell and its progeny will respond to future molecular signals.

True or False
Genes control development and have led to an understanding of the key roles that specific molecules play in defining position and directing differentiation.
True False

The life cycle of Drosophila
· Fruit flies and arthropods have three segments: __, __, and __
o __
o __
o __
· __are localized in the unfertilized egg and provide __ for the placement of anterior-posterior and dorsal-ventral axes even before fertilization.

The life cycle of Drosophila
· Egg develops in the female’s ovary, surrounded by ovarian cells called __ and __.
These support cells supply the egg with nutrients, mRNAs and other substances needed for development and make the egg shell.
· After fertilization and laying of the egg, embryonic development results from formation of a segmented larva, which goes through __larval stages. The fly then forms a __in which it __ into the adult fly.

__: control pattern formation in the late embryo, larva, and adult

Drosophilic genes affect __and might just be a few needles in a haystack or might be so numerous and varied that the scientists would be unable to make sense of them.

Mutations affecting a process as fundamental as __would surely be __, mutations with phenotypes causing death at the embryonic or larval stage. Because organisms with __ mutations never reproduce, they cannot be bred for study.
__ in the egg were known to play a role in __. These substances are encoded by genes of the mother, fittingly called __.

__are genes that, when mutant in the mother, results in a mutant phenotype in the offspring, regardless of the offspring’s own genotype.
Because they control the orientation (__) of the egg and consequently of the fly, maternal effect genes are also called __.
· One group sets up the __ axis of the embryo
· Second establishes the __ axis.

Like mutations in __genes, mutations in __ are generally embryonic details.
__ is a term meaning “two-tailed.” It is essential for setting up the anterior end of the fly and might be concentrated at the future anterior end of the embryo.

__: gradients of substances called morphogenes establish an embryo’s axes and other features of its form.

__mRNA is highly concentrated at the extreme anterior end of the mature egg, as predicted by the hypothesis.

mRNA is produced in __, transferred to the egg via __, and anchored to the cytoskeleton at the anterior end of the egg.
After the egg is fertilized, mRNA is translated into protein.
The __protein then diffuses from the anterior end toward the posterior, resulting in a gradient of protein within the early embryo, with the highest concentration at the anterior end.