#signaling

Paracrine Factor

Which of the following is an example of a receptor tyrosine kinase (RTK) ligand?

Receptor Tyrosine Kinase ligands are typically soluble growth factors like EGF, FGF, PDGF, VEGF, Insulin, HGF forming dimers bridging receptors. Fibroblast Growth Factor binds FGFR1-4 extracellular IgII-III domains causing dimerization, trans-autophosphorylation of activation loop tyrosines Y653/654 activating Ras-MAPK, PI3K-AKT, PLCγ signaling driving proliferation, survival, chemotaxis. This mitogenic RTK signaling contrasts Hedgehog which signals via Patched-Smoothened GPCR-like seven-pass mechanism not involving tyrosine kinase, Wnt via Frizzled-LRP beta-catenin, and Delta via Notch proteolytic S2 S3 cleavage producing NICD. Therefore FGF exemplifies classic RTK ligand paradigm used in developmental induction including limb AER maintenance.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: FGF as prototypic receptor tyrosine kinase ligand for induction.

View Details

C-Elegans Development-l

Which of the following describes the function of LIN-3 in C. elegans?

LIN-3 is epidermal growth factor family ligand synthesized in anchor cell under control of EGL-38 Pax2 and lag-2 gonadal inputs. The gene encodes transmembrane protein cleaved by rhomboid protease SUP-17 and ADAM10 metalloprotease, releasing EGF domain that diffuses short distance as morphogen. Binding receptor LET-23 on underlying vulval precursors activates tyrosine kinase signaling, Ras activation, MAPK phosphorylation. Dosage matters critically: low expression fails to induce, high dose causes multivulva phenotype beyond P6.p. Thus LIN-3 functions as primary inductive cue coordinating uterine-hypodermal development.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: LIN-3 as EGF-like ligand from anchor cell inducing vulval precursors.

View Details

Eye Lens Induction -ll

What is the main function of retinoic acid in eye development?

Retinoic acid signaling synthesized by retinal pigment epithelium via Raldh1-3 enzymes plays pivotal role in retinal differentiation patterning. Retinoic acid gradient along dorsal-ventral axis regulates expression of Tbx5, Vax2, and Cyp26 to specify dorsal-ventral retinal identity, promote photoreceptor differentiation, and stimulate neurite outgrowth. RA also induces Pax2 in optic stalk and supports vascular development. It does not primarily induce lens which depends on BMP-FGF, nor optic vesicle formation which precedes RA, nor cornea formation dependent on surface ectoderm, but fine-tunes retinal progenitor competence and patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Retinoic acid role in retinal dorsal-ventral patterning and differentiation.

View Details

Sex determination-l

β-catenin accumulation favors:

Nuclear β-catenin accumulation in supporting precursors drives ovarian fate downstream of Wnt4/Rspo1. Ligand binding inhibits destruction complex composed of Axin, Apc and Gsk3β, preventing phosphorylation and proteasomal degradation of β-catenin. Stabilized protein enters nucleus associating with Lef1/Tcf to transcribe Foxl2, Fst and repress Sox9 enhancer. Genetic stabilization of β-catenin in XY gonads induces male-to-female reversal with granulosa differentiation, whereas conditional deletion in XX causes masculinization and ectopic Sox9. Therefore β-catenin favors ovary formation, maintaining follicular organization and germ cell meiosis entry.

Ref: NCBI, β-catenin favors ovary formation; Gilbert Chapter 6 Wnt/β-catenin ovarian pathway.

View Details

Dictyostelium

cAMP production regulated by:

Biosynthesis of cAMP from ATP is catalyzed by adenylyl cyclases encoded by acaA, acrA, and acgA genes expressed at distinct stages. ACA, a twelve-transmembrane protein, dominates early aggregation, activated downstream of cAR1, heterotrimeric G proteins, and cytosolic regulator CRAC. It generates both intracellular second messenger for protein kinase A and extracellular signal for relay. Phosphatases remove phosphate groups, while lyases and esterases perform unrelated cleavages. Later ACA activity declines, replaced by ACB during culmination and ACG during spore dormancy, each regulated by developmental promoters and environmental cues like osmolarity and ligand binding.

Ref: Kessin, Dictyostelium: Evolution, Cell Biology, Chapter 4: Adenylyl cyclases ACA, ACB, ACG regulation.

View Details

Dictyostelium

Signaling molecule guiding Dictyostelium aggregation:

Aggregation centers broadcast pulsatile 3',5'-cyclic AMP, the universal chemoattractant for Dictyostelium. Starving cells relay cAMP outward as spiral or concentric waves detectable by dark-field optics. Neighbors sense increasing concentration via high-affinity G protein-coupled cAR1 receptors that activate adenylyl cyclase ACA, producing further cAMP. This excitable relay generates directional movement up gradient. Calcium ions, ATP, or ADP do not elicit chemotaxis in this species. Periodic cAMP secretion every six minutes arises from adaptation of receptor and degradation by extracellular phosphodiesterase PdsA, ensuring wave propagation over centimeter-scale territories.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 22: cAMP oscillations, cAR1 GPCR and PdsA phosphodiesterase relay.

View Details

Regulation of chromatin structure

Which modification is directly linked to DNA repair signaling?

Among histone modifications, phosphorylation uniquely creates rapid, reversible signaling platforms for DNA damage response. Upon double-strand breaks, ATM, ATR and DNA-PK rapidly phosphorylate H2A.X at serine 139, generating γ-H2A.X that spreads megabases around lesions. This phospho-epitope is recognized by BRCT and FHA domains of MDC1, 53BP1 and repair factors, concentrating ubiquitylation and remodeling activities. Unlike acetylation or methylation that modulate transcription, phosphorylation provides high-turnover signal integration, coupling checkpoint activation, cell-cycle arrest and homologous recombination or non-homologous end joining pathway choice.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: DNA Repair, Histone Phosphorylation Signaling, γ-H2AX

View Details

Cancer

Cancer cells communicate with stroma primarily by

Secreting signaling proteins and proteases, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

View Details

Cell Signaling GPCR Pathways - I

Adaptor proteins differ from scaffold proteins because adaptors

Adaptor proteins contain modular interaction domains but lack intrinsic catalytic activity; they function solely to bring other proteins into proximity. Scaffold proteins organize multiple components of a pathway into a discrete complex, thereby enhancing specificity, speed and spatial restriction of signaling. Both classes of proteins are essential for the fidelity of intracellular information transfer.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

View Details

Cell Signaling GPCR Pathways - I

PH domains mainly bind

Pleckstrin-homology (PH) domains bind specific phosphoinositides such as PIP3 or PIP2 with high affinity. Recruitment of PH-domain-containing proteins to membrane regions enriched in these lipids is a key mechanism by which PI3K signaling localizes effectors such as AKT and PDK1. The PH domain therefore functions as a lipid-regulated membrane-targeting module.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

View Details

Cell Signaling GPCR Pathways - I

SH3 domains recognize

Src-homology 3 (SH3) domains bind short proline-rich motifs conforming to the consensus PxxP. These interactions mediate assembly of signaling complexes independently of phosphorylation and are common in pathways that regulate cytoskeletal dynamics, endocytosis and kinase localization. The modular nature of SH3 domains allows combinatorial assembly of diverse multiprotein complexes. Careful

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

View Details

Cell Signaling GPCR Pathways - I

SH2 domains bind to

Src-homology 2 (SH2) domains are modular protein modules that recognize phosphotyrosine residues in a sequence-specific context. Binding of an SH2 domain to a phosphorylated tyrosine creates a high-affinity docking site that recruits downstream signaling proteins to activated receptor tyrosine kinases or adaptor complexes. This interaction is a central mechanism of phosphotyrosine-dependent signal propagation.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

View Details