180 public questions tagged with this topic.
An **inducible operon (such as the lac operon) is normally off but can be activated by an inducer** like lactose. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
L-Maf, member of large Maf family containing bZIP domain, executes terminal lens fiber differentiation after early Pax6/Sox2 specification. It accumulates in equatorial lens epithelium where FGF gradient is high, binding MARE sequences in delta-crystallin enhancer and alphaA-crystallin promoter, strongly transactivating them. L-Maf knockout chick lacks crystallin accumulation and shows defective fiber elongation. It works synergistically with Sox2 and Prox1 to drive cytoskeletal changes and gap junction formation for transparency. Thus L-Maf links extracellular FGF signal to structural gene output essential for refractive properties of lens, not vesicle induction itself.
Ref: Reza and Yasuda 2004, Intl J Dev Biol Chapter: Lens differentiation and crystallin regulation - L-Maf regulates delta-crystallin.
Competence of head ectoderm to form lens is established by transcription factor Pax6, master regulator of eye development expressed early in anterior neural plate and surface ectoderm. Pax6 directly binds enhancers of lens genes Six3, Sox2, and crystallins, rendering ectoderm responsive to BMP and FGF from optic vesicle. Sox2 cooperates with Pax6 but not alone, L-Maf acts later downstream, Rx1 specifies retinal field. Pax6 knockout ectoderm fails to form lens even with wild-type optic vesicle, while Pax6 overexpression confers competence, confirming essential priming function for lens.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Pax6 competence and head ectoderm specification for lens.
Segmental identity along proximodistal axis maps onto nested HoxA and HoxD cluster expression. Hoxa11 paralog specifically marks middle segment zeugopod encompassing radius and ulna in forelimb and tibia and fibula in hindlimb. Hoxa9-Hox10 dictate stylopod humerus-femur, Hoxa13 defines autopod wrist and digits. Compound Hoxa11 and Hoxd11 double mutants develop severely truncated dysmorphic zeugopod with fusions, while stylopod remains less affected. This precise mapping demonstrates temporal collinearity driving segmental limb architecture through Hox-controlled proliferation and differentiation programs and growth timing. Conserved across chick mouse human models, this mechanism illustrates classic embryological principles integrated with modern molecular genetics.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Hoxa11 and Hoxd11 – zeugopod specification and knockout phenotype.
FGF8 from AER maintains distal progenitors and progressive proximal-distal patterning through temporal exposure. Gradual loss reduces time distal cells spend in progress zone under FGF influence, so late-forming structures fail to be specified due to premature Meis activation. Conditional genetic deletion of Fgf8 in mouse AER results in limbs with relatively normal stylopod humerus femur but severely truncated autopod lacking digits, because proximal specification occurred before depletion while distal specification needs prolonged FGF exposure combined with Cyp26b1 mediated RA clearance. Complete loss requires earlier FGF10 elimination, extra digits relate to SHH augmentation.
Ref: Moon and Capecchi 2000, Gilbert Chapter 20: Loss of FGF8 leads to distal truncation.
Nested Hox activity subdivides proximal-distal axis through overlapping temporal expression domains controlled by global enhancers. In mouse and chick, Hoxa11 and Hoxd11 expressed in region fated as forearm and shank, between Hox9 proximal domain and Hox13 distal domain, specifying zeugopod intermediate segment under balanced RA and FGF inputs and Cyp26b1. Fate maps combined with Cre-mediated knockouts demonstrate Hox11 inactivation deletes radius ulna and tibia fibula. Stylopod marked by Hox9, autopod by Hox13, digits overlapping Hox13 plus Hox12. Therefore Hoxa11 expression identifies zeugopod segment specifically and coordinates growth and patterning.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Hoxa11 expression specifies zeugopod region.
Hydra head formation relies on conserved Wnt pathway defining oral organizer. Wnt3 is transcribed at extreme tip of hypostome, most apical region, where it forms autostimulatory loop with beta-catenin and Brachyury maintaining organizer identity and driving expression of head-specific genes including BMP inhibitor and tentacle genes. Transgenic Hydra overexpressing Wnt3 generate ectopic tentacle whorls and secondary heads; knockdown prevents head regeneration causing transient foot-like transformation. HoxD13 regulates vertebrate distal limb, BMP4 ventral fate, SHH vertebrate limb posterior patterning. Therefore Wnt3 is crucial genetic trigger for Hydra head organizer, budding initiation, and axis establishment throughout life and regeneration.
Ref: Nature, Hobmayer et al., Wnt3 defines head organizer in Hydra regeneration and axis formation.
Notum functions as localized brake on Wnt signaling at anterior-facing wounds producing microenvironment where Wnt ligands are deactivated. When Notum expression is knocked down by RNAi, inhibition absent, so wound retains high residual Wnt/beta-catenin activity normally reserved for posterior identity. High Wnt then imposes tail transcriptional program onto anterior blastema, leading to ectopic tail formation where head should regenerate, yielding two-tailed animals. Complete regeneration failure would require loss of stem cells, extra heads arise from beta-catenin depletion not elevation, normal regeneration depends on precise Notum mediated Wnt dampening to protect anterior fate from posteriorizing cues.
Ref: PNAS, Notum RNAi in planaria - excessive Wnt causes tail formation at anterior wound.
Zeugopod segment comprising radius-ulna in forelimb and tibia-fibula in hindlimb depends on group 11 Hox paralogs, especially HOXD11, HOXA11, HOXC11. These genes expressed in intermediate limb bud mesenchyme during middle phase of Hox colinear activation. Double knockout Hoxa11/Hoxd11 mice exhibit severely shortened, fused zeugopod elements while stylopod and autopod partially preserved, indicating specific requirement. They regulate proliferation of chondroprogenitors, Fgf10 maintenance period, and integration of SHH timing. Distinct enhancers sensitive to FGF duration control intermediate expression window, establishing zeugopod identity.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: HOX11 paralogs and zeugopod development, knockouts.
Dorsal-ventral limb polarity originates from non-AER ectodermal signals rather than ridge. Wnt7a transcript localized specifically to dorsal ectoderm induces LIM homeodomain transcription factor Lmx1b in dorsal mesoderm, conferring dorsal characteristics including nails, dorsal tendon pattern, hair orientation and pigmentation. Ventral ectoderm expresses Engrailed1 repressing Wnt7a and activating BMP signaling promoting ventral fates like footpads. Ectoderm rotation experiments inverting DV orientation produce inverted limb polarity, confirming ectodermal instruction. Wnt7a also contributes to maintaining SHH posteriorly, integrating axes, but primary function remains dorsalizing mesenchyme through Lmx1b induction pathway.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Wnt7a dorsal ectoderm and Lmx1b dorsalization of mesoderm.
Vegetal T-box factor VegT induces Nodal-related genes in vegetal endoderm via Smad-activated enhancers, while nuclear beta-catenin on dorsal side binds TCF to activate Siamois. Siamois and VegT cooperatively bind Xnr enhancers, synergistically upregulating Xnr1,2,5,6 on dorsal side highest, ventral side moderate, establishing gradient. BMP and Activin also modulate but transcriptional activation fundamentally requires both beta-catenin dorsal cue and VegT mesendoderm competence factor. Double knockdown eliminates mesoderm entirely. Thus intersection explains dorsal-high Nodal gradient establishing organizer position and patterning mesoderm along dorsoventral axis.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Xnr activation by beta-catenin and VegT co-regulation.
Maternally encoded Wnt11 ligand normally activates canonical Wnt pathway vegetally, stabilizing beta-catenin dorsally via Dishevelled and GBP inhibiting GSK-3. Injecting antisense Wnt11 morpholino oligonucleotides depletes ligand, preventing Dishevelled activation, leaving GSK-3 active throughout embryo, causing beta-catenin degradation ubiquitously. Consequently Siamois and Xnr genes not transcribed in Nieuwkoop center, dorsal mesoderm absent, organizer fails, embryo becomes ventralized with excess blood and epidermis lacking notochord and neural plate. Rescue with stabilized beta-catenin mRNA restores organizer and secondary axis, proving Wnt11 upstream required for organizer formation and dorsal axis specification.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Antisense Wnt11 leads to failure of organizer formation.
