Important words and concepts from Chapter 21, Campbell et al., 1999 (2/17/01):

by Stephen T. Abedon ( for Biology 113 at the Ohio State University


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(1) Chapter title: The Genetic Basis of Development

(a)                    "This chapter applies much of what you've learned about molecules, cells, and genes to one of biology's most important questions--how a complex multicellular organisms develops from a single cell… The scientific study of development got under way about a century ago, at roughly the same time as genetics. But for decades the two disciplines proceeded along mostly separate paths. We have seen how genetics advanced from Mendel's laws to an understanding of the molecular basis of inheritance. Meanwhile, developmental research focused on embryology, the study of the stages of development leading from fertilized egg to fully formed organism. Only recently have the concepts and tools of molecular genetics reached the point where a real synthesis has been possible. The synthesis is a challenge, for it means relating the linear information in genes to a process of development that takes place in four dimensions, three of space and one of time." p. 388, Campbell et al., 1999

(b)                    Understanding the genetic basis of development is difficult for a number or reasons: (i) understanding the process requires an understanding of numerous underlying biological concepts and phenomena, (ii) the processes required to produce a morphologically complex organism starting only with a single cell are themselves complex, and (iii) the concepts involved are only beginning to be understood by scientists.

(c)                    Development links: [index]

(i)                     [genetic regulation of development (Dynamic Development) (overall site contains tons of individual articles on many aspects of animal development)]

(ii)                   [the genetic basis of development (Google Search)]

(2) Zygote (see also zygote)

(a)                    A zygote is a fertilized egg

(b)                    Development of many multicellular organisms traditionally begins at this fertilization step

(c)                    However, note that processes necessary for the successful development of an multicellular organisms in fact typically begin within the egg even before fertilization has occurred

(d)                   [zygote (Google Search)] [index]

(3) Development (see also development)

(a)                    Development from zygote to multicellular organism involves cell division, the organization of cells into tissues, the organization of tissues into organs, the organization of organs into organ systems, and the organization of organism into whole organisms

(b)                    Thus, development involves not only cell division, but the differentiation of cells into ones that do different jobs, and then the three-dimensional organization of cells that do different jobs into groups of cells that together do specific jobs within the multicellular organism

(c)                    We traditionally differentiate these many steps of development into categories of development called

(i)                     Cell division

(ii)                   Cell Differentiation

(iii)                 Morphogenesis

(d)                   See Figure Some key stages of development in animals and plants

(e)                    [organism development (Google Search)] [index]

(4) Cell division (see also cell division)

(a)                    We have already considered cell division in this course in the chapter titled, "The cell cycle"

(b)                    Note that the majority of the cell division involved in development is mitotic division

(c)                    "Cell division alone… would produce a great ball of identical cells, which is nothing like an animal or plant. During embryonic development, cells not only increase in number, they also undergo differentiation, becoming specialized in structure and function. Moreover, the different kinds of cells aren't just mixed up randomly but are organized into tissues and organs."

(d)                   [cell division (Google Search)] [index]

(5) Cell differentiation (differentiation) (stem cells) (see also cellular differentiation)

(a)                    Cell differentiation involves the functional and structural specialization of cells within a multicellular organism as that organism develops

(b)                    Specifically, differentiation occurs via differential gene activity in cells such that the proteins produced by one cell differ from those produced by another cell

(c)                    Differentiation occurs in stages such that a cell can be less or more differentiated, with increased differentiation occurring as a cell lineage becomes more specialized

(d)                   Typically the replacement of a well-differentiated cell occurs via the division and subsequent differentiation of a less-well-differentiated cell

(e)                    For example, the so-called stem cells that are all the rage these days in medical transplant studies are examples of less-differentiated cells that can divide and then give rise to highly differentiated cell lines such as replacing an immune system or neurons in the central nervous system

(f)                     [cell differentiation, differentiation (Google Search)] [index]

(6) Morphogenesis (see also morphogenesis)

(a)                    Morphogenesis is the process that gives a developing organism a specific three dimensional shape

(b)                    This is more than just the external shape of the organism, but also includes the organization of cells into tissues, tissues into organs, etc.

(c)                    See Figure Some key stages of development in animals and plants

(d)                   Note in the figure that morphogenesis proceeds cell differentiation; what this temporal order means is that cells tend to find their positions in developing organisms before they specialize, rather than specializing before they find their specific positions

(e)                    "Cell division and differentiation play important roles in morphogenesis in all organisms, as does the death of certain cells." p. 388, Campbell et al., 1999

(f)                     [morphogenesis (Google Search)] [index]

(7) Genomic equivalence (see also genomic equivalence)

(a)                    Genomic equivalence means that the genomes of all or most of the somatic cells within a multicellular organism are essentially the same, i.e., have the same amount of DNA and the same nucleotide sequence

(b)                    Thus, the differentiation of cells does not (usually) involve changes in the nucleotide sequence of DNA

(c)                    Genomic equivalence can be demonstrated via various cloning experiments including the growing of a whole plant from an isolated cell (other than a zygote) and the cloning of an animal via the injection of a somatic-cell genome into a zygote (or unfertilized egg) whose own genome has been destroyed

(d)                   Note that just because the cloning of a multicelled organism from a differentiated cell may be possible, it is not necessarily technically easy

(e)                    See Figure Test-tube cloning of carrots

(f)                     See Figure Nuclear transplantation

(g)                    See Figure Cloning a mammal

(h)                   [genomic equivalence (Google Search)] [index]

(8) Determination (see also cell fate determination)

(a)                    Determination refers to the processes that lead to cellular differentiation

(b)                    "Cellular differentiation is actually the outcome of a cell's developmental history extending back to the first mitotic divisions of the zygote." p. 395, Campbell et al., 1999

(c)                    That is, the process of cellular differentiation is a step-wise one where not only increasing differentiation occurs, but the fate of the cells (their determination) is constrained (narrowed) as increasing differentiation occurs

(d)                   Thus, we can observe cells as they pass through developmental stages of increasing differentiation that serve to determine the ultimate structure and function of the resulting cells

(e)                    Note that what is going on within a cell lineage during determination are changes in gene expression including constraints on gene expression such that the ultimately "determined" product of determination sports a specialized pattern of gene expression and therefore of cellular proteins

(f)                     Furthermore, specific regulatory genes are expressed within different differentiating cells that control gene expression thereby assuring that one array of genes is expressed by one kind of cell (e.g., muscle cell) whereas a different array of genes is expressed by a different kind of cell (e.g., adipose cell)

(g)                    Once can then envisage the differentiation process of one in which one kind of regulatory gene constrains the kinds of subsequent regulatory genes that may be expressed, thereby resulting in certain types of only partially differentiated cell lineages can give rise to only certain types additionally differentiated cell lineages (e.g., connective tissue precursor cells give rise only to connective tissue cells)

(h)                    [determination differentiation (Google Search)] [index]

(9) Cytoplasmic determinants (see also cytoplasmic determinants)

(a)                    Given that the history of a cell lineage can impact substantially on specialization fates, what then determines early on within an embryo that one lineage will give rise to cells contributing to one kind of tissue whereas another lineage will give rise to cells contributing to a different kind of tissue (when, in fact, all cells ultimately can trace their lineages back to the original zygote)

(b)                    The answer is positional information, such that cells found in one part of the embryo contain or receive different information from cells that are found in different parts of the embryo

(c)                    Positional information can be traced back to the zygote itself, which contains a heterogeneous cytoplasm such that one part of the cytoplasm does not resemble the cytoplasm found in another part of that cell

(d)                   We call these cytoplasmic differences, that give rise to differently differentiated cell lineages, cytoplasmic determinants

(e)                    Cytoplasmic determinants include proteins as well as mRNAs found in the cytoplasm of the zygote

(f)                     See Figure Nuclei in the early embryo are exposed to different concentrations of cytoplasmic determinants

(g)                    [cytoplasmic determinants (Google Search)] [genetic regulation of development (various maternal and zygotic control of development articles) (Dynamic Development)] [index]

(10) Induction (see also signal induction)

(a)                    Another means by which positional information is transduced into cellular differentiation occurs with the interaction of cells within the multicellular embryo

(b)                    Here cell-to-cell signaling occurs, and via a process called induction cells in different positions respond to these signals by altering their gene expression

(c)                    Induction, since it can occur following the interaction of many different types of cells in spatially complex structures, is a general term for a vast collection of cell-to-cell signaling mechanisms that give rise to the complex morphologies and arrays of differentiation that one typically observes within the bodies of multicellular organisms

(d)                   [genetic regulation of development (various induction articles) (Dynamic Development)] [index]

(11) Model organism

(a)                    Often as particular field of biological study may be performed more easily using one kind of organism versus another

(b)                    Organisms studied for the sake of gaining a general understanding of common biological processes are called model organisms

(c)                    For example, Mendel used pea plants as model organisms for genetics while phage T4 was used an early model organism for development of the field of molecular genetics

(12) Model organisms for the study of development

(a)                    Though there exists general principles for animal (or plant) development, still the processes of development are sufficiently complex and organism specific (i.e., occur within the unique contexts of individual species), the study of development cannot occur completely divorced from the study of specific species of organisms; these specific species represent the specific model organisms used for the study of development

(b)                    Common model organisms for the study of development include:

(i)                     Frogs

(ii)                   Caenorhabditis elegans (a nematode/round worm)

(iii)                 Drosophila melanogaster (a fruit fly)

(iv)                 The mouse

(v)                   The zebra fish

(vi)                 Arabidopsis (a plant)

(c)                    See Figure Model organisms

(13) Note: this lecture is incomplete, and stops here only abruptly; my apologies for any inconvenience--someday I'll finish it

(14) Vocabulary [index]

(a)                    Cell differentiation

(b)                    Cell division

(c)                    Cytoplasmic determinants

(d)                   Determination

(e)                    Development

(f)                     Differentiation

(g)                    Genomic equivalence

(h)                    Induction

(i)                      Model organism

(j)                      Model organisms for the study of development

(k)                    Morphogenesis

(l)                      Zygote

(15) Practice questions [index]

(a)                    No entry

(16) Practice question answers [index]

(a)                    No entry