The Questions of Developmental Biology

determine the anterior-posterior axis of the Drosophila embryo and adult. (The ability of cells to
be specified by gradients of proteins is a critical phenomenon and is discussed in more detail in
Chapters 3 and 9.)
Epilogue: Posttranslational Gene Regulation
When a protein is synthesized, the story is still not over. Once a protein is made, it
becomes part of a larger level of organization. For instance, it may become part of the structural
framework of the cell, or it may become involved in one of the myriad enzymatic pathways for
the synthesis or breakdown of cellular metabolites. In any case, the individual protein is now part
of a complex "ecosystem" that integrates it into a relationship with numerous other proteins.
Thus, several changes can still take place that determine whether or not the protein will be active.
Some newly synthesized proteins are inactive without the cleaving away of certain inhibitory
sections. This is what happens when insulin is made from its larger protein precursor. Some
proteins must be "addressed" to their specific intracellular destinations in order to function.
Proteins are often sequestered in certain regions, such as membranes, lysosomes, nuclei, or
mitochondria. Some proteins need to assemble with other proteins to form a functional unit. The
hemoglobin protein, the microtubule, and the ribosome are all examples of numerous proteins
joining together to form a functional unit. And some proteins are not active unless they bind an
ion such as calcium, or are modified by the covalent addition of a phosphate or acetate group.
This last type of protein modification will become very important in the next chapter, since many
important proteins in embryonic cells are just sitting there until some signal activates them. We
turn next to how the embryo develops by activating certain proteins in specific cells.
Principles of Development: Developmental Genetics
1. Differential gene expression from genetically identical nuclei creates different cell types.
Differential gene expression can occur at the levels of gene transcription, nuclear RNA
processing, mRNA translation, and protein modification.
2. Genes are usually repressed. Activation of a gene often means inhibiting its repressor. This
leads to thinking in double and triple negatives: Activation is often the inhibition of the inhibitor;
repression is the inhibition of the inhibitor of the inhibitor.
3. Eukaryotic genes contain promoter sequences to which RNA polymerase can bind to initiate
transcription. The eukaryotic RNA polymerases are bound by a series of proteins called basal
transcription factors.
4. Eukaryotic genes expressed in specific cell types contain enhancer sequences that regulate their
transcription in time and space.
5. Specific transcription factors can recognize specific sequences of DNA in the promoter and
enhancer regions. They activate or repress transcription from the genes to which they have bound.
6. Enhancers work in a combinatorial fashion. The binding of several transcription factors can act
to promote or inhibit transcription from a certain promoter. In some cases transcription is
activated only if both factor A and factor B are present, while in other cases, transcription is
activated if either factor A or factor B is present.