Chapter 21

Differential gene expression affects the developmental process in animals. Positional information controls pattern formation. Homeotic genes code for transcription factors that control the development of segment-specific body parts. Cells receive molecular signals that communicate their position in relation to other cells. Pattern formation is the development of a multicellular organism's spatial organization, including the arrangement of tissues and organs. Cells receive molecular signals, collectively called positional information, that inform the cells about their location in relation to other cells and to the body axes. Positional information regulates pattern formation. Homeotic genes are regulatory genes that work after the body axes have been established. Homeotic genes identify particular body segments in an individual, eventually leading to the formation of segment-specific body structures in the correct locations on the body.

Antenna

Offspring will show a mutant phenotype if the mother has a mutant genotype. This statement is true; the gene is transcribed in the mother, and the mRNA is delivered to the zygote.

True Pattern formation is the process by which the location of an embryo's body segments is determined.

Fertilized egg bicoid mRNA is translated at the anterior pole of the fertilized egg.

Abdomen The regions with low concentrations of Bicoid protein develop into posterior structures such as the abdomen.

The Bicoid protein is a regulatory transcription factor. The Bicoid protein regulates expression of the embryo's early developmental genes.

The larva would be normal with one head at the anterior pole. The injected mRNA would rescue the mutant phenotype of the egg.

two-headed fly The head develops where there is a high concentration of bicoid protein.

the establishment of the anterior-posterior axis The concentration gradient of bicoid protein determines the anterior-posterior axis of a developing Drosophila.

Head The head develops where there is a high concentration of bicoid protein.

fertilization of the egg Fertilization of the egg triggers translation of bicoid mRNA.

egg-polarity Egg-polarity genes are responsible for establishing the polarity of the egg.

abdomen The abdomen develops where there is a low concentration of bicoid protein.

diffusion Bicoid protein is produced at the anterior end and diffuses toward theposterior, resulting in a gradient.

the fertilized egg BicoidmRNA is translated in the fertilized egg.

nurse cells

there is an anterior to posterior gradient in BICOID protein concentration in the embryo Correct. After fertilization, BICOID mRNA is translated and a concentration gradient of BICOID protein forms along the anterior-posterior axis of the embryo.

Homeotic genes Correct. Homeotic genes specify segment identity with respect to the body part that will develop at metamorphosis.

False Correct. Maternal effect genes are transcribed during oogenesis (in the mother). The mRNA is translated in the embryo after fertilization.

mRNA localization Correct Three strategies have evolved for instructing cells to express specific genes during development: cell-to-cell contact, signaling through the diffusion of a secreted signaling molecule (morphogen), and localization of maternal mRNAs in an egg (either before or after fertilization). Only the third listed process—mRNA localization—does not represent a pathway of cell-to-cell signal transduction or establishment of an extracellular gradient for regulating gene expression. Localization of mRNAs allows cells to distribute critical regulatory molecules asymmetrically to specific parts of a cell, typically via transport along elements of the cytoskeleton.

The daughter cell cannot change its mating type. A mother cell and its daughter cell can show different mating types. Although both mother and daughter cells might initially, for example, exhibit the "a" mating type, only the mother is capable of switching to the "α" mating type. The mother is able to switch mating types because the HO gene, which controls switching, can be activated in the mother because of the absence of Ash1 repressor protein, whereas HO in the daughter is repressed by Ash1. Ash1 mRNAs transcribed in the nucleus of the mother cell before budding become localized in the daughter cell during the budding process because of cytoskeletal-mediated transport of Ash1 mRNAs from mother cell to daughter cell.

SpoIIR and Delta In the bacterium Bacillus subtilis, spore formation begins when a forespore cell produces a signaling protein called SpoIIR. SpoIIR then travels to the septum separating the forespore cell from the mother cell where it triggers the proteolytic activation of an inactive form of σE called "pro-σE" found in the mother cell. The activated σE subunits then help initiate transcription of specific genes in the mother cell. In the early development of insects, a neuroblast (precursor cell to a neuron) secretes signaling molecules called Delta. These secreted molecules bind to Notch receptors on the surfaces of neighboring cells in the neurogenic ectoderm. Upon activation, the intracellular domain of Notch is cleaved by intracellular proteases. Cleavage produces a molecular subunit called NotchIC, which then travels to the nucleus where it associates with a DNA-binding protein called Su(H). The NotchIC-Su(H) complex activates genes that encode transcriptional repressors that prevent the cell from becoming a neuron. This developmental mechanism prevents cells in physical contact with a neuroblast from developing into neurons themselves and instead they become skin cells.

Dorsal protein activates Twist expression in the ventral neurogenic ectoderm regions of the embryo. In the early developing Drosophila embryo, a concentration gradient of the morphogen Dorsal protein is established with the highest concentration in the ventral region and lowest in the dorsal region of the embryo. One of the gene targets of Dorsal is twist, which has two low-affinity Dorsal-binding sites that can be occupied only by peak levels of the Dorsal gradient. This means that twist is only expressed in the ventralmost 18 cells, which will form the mesoderm of the embryo. twist cannot be expressed in what will become neurogenic ectoderm regions because the concentration of Dorsal protein is too low.

Expression of Ubx represses expression of DII genes in the abdominal segments of crustaceans and the formation of thoracic limbs. Expression of Ubx in the abdominal segments of insects does repress the DII genes and prevents formation of thoracic limbs. In contrast, expression of Ubx does not repress DII expression in the abdominal segments of crustaceans. The explanation for this is that the Ubx proteins produced by insects are functionally different than the Ubx proteins found in crustaceans. The crustacean Ubx protein has a short motif consisting of 29 amino acid residues. This "anti-repression" motif prevents Ubx from repressing DII; if this motif is deleted from the crustacean Ubx, then these crustacean proteins can repress DII activity as effectively as the fly Ubx proteins. Similarly, if this motif is attached to a fly Ubx protein, then fly Ubx no longer represses DII.

a block of genes linked together on the same chromosome that is conserved between distantly related animals Comparative genome analysis has revealed that distantly related animals share conserved blocks of linked genes (show a high degree of synteny). Even organisms that have not had a common ancestor for hundreds of millions of years can exhibit some synteny.

Differential gene expression is the synthesis of a protein or non-coding RNA in a subset of embryonic cells.

1) A cell or cells synthesize and release the morphogen or signaling molecule. 2) The distribution of morphogen establishes an extracellular concentration gradient. 3) The morphogen binds receptors on the surface of other cells. The percent occupancy of morphogen decreases as the distance increases between the source cell and the receptor cell. 4) Through a signaling pathway, activated receptor leads to an increase in expression of a transcriptional regulator that controls expression of many genes.

the localization of ash1 mRNAs to the daughter cell

cell-to-cell contact

All of them

a. Researchers engineered an mRNA that had the 3' UTR of OSKAR mRNA replaced with the 3' UTR from BICOID mRNA. b. They observed OSKAR mRNA localization in the anterior pole as normally observed for BICOID mRNA normally. This is enough to induce pole cell formation at the wrong locations.

Knirps protein creates the posterior border of the eve stripe 2. Even-skipped or eve for short is a "pair-rule" gene that is expressed in a series of seven alternating stripes that extend along the length of the embryo. The second eve stripe is created by the interplay among two transcriptional activators (Bicoid and Hunchback) and two transcriptional repressors (Giant and Krüppel). Binding sites for all four of these proteins are found on one of the two 5′ enhancers upstream of the eve gene. The high concentration of Bicoid and Hunchback at the anterior end of the embryo activates eve, whereas repression by Giant and Krüppel in specific regions of the embryo create the anterior and posterior borders of stripe-2, respectively. The repressors Giant and Krüppel are known as "gap proteins." Knirps, another gap protein, is involved in creating other eve stripes.

D) all of them

A