#protein

ZOOLOGY

Which protein is most abundant in the human body?

Collagen is the most abundant protein in the human body, providing structural support to connective tissues. This follows from latest NCERT 2026-27 principle explaining the concept clearly for NEET students in simple steps as per rationalized syllabus.

Ref: NCERT Biology Textbook - Latest Edition for Academic Session 2026-27 (Zoology section, Rationalized Textbook for Class XI and XII), Chapter: Biology - Zoology portion (Latest NCERT Textbooks for Academic Session 2026-27 - Rationalized Edition for Class XI and XII), Topic: Structural organization, physiology, human health and related concepts as per latest syllabus.

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ZOOLOGY

Which of the following is an example of a fibrous protein?

Collagen is a fibrous protein that provides structural support in connective tissues. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.

Ref: NCERT Biology Textbook for Class XI and XII (Zoology section), Chapter: Structural Organisation in Animals and Frog, Topic: Tissues, skeletal and organ systems.

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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.

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Paracrine Factor

Which protein acts as an intracellular transducer in the BMP signaling pathway?

BMP receptors are serine/threonine kinase family phosphorylating receptor-regulated Smads Smad1,5,8 upon BMP2/4 binding to ALK2/3/6 type I and BMPR2 type II. Phosphorylated Smad1/5/8 complexes with Co-Smad Smad4, translocates to nucleus via importin beta binding MH2 domain to activate Id1-3, Msx1/2 transcription factors patterning ventral mesoderm and inhibiting neurogenesis by repressing Sox2. STAT proteins transduce JAK cytokine signaling via tyrosine phosphorylation and dimerization, JAK is cytoplasmic tyrosine kinase, beta-catenin transduces Wnt via TCF/LEF. Thus Smad proteins canonical intracellular transducers distinguishing TGF-beta/BMP pathway from RTK and GPCR signaling logic for dorsoventral patterning.

Ref: Heldin et al., Nature 1997: Smad proteins transduce TGF-beta and BMP signaling to nucleus.

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C-Elegans Development-l

Which protein is involved in lateral inhibition during anchor cell specification?

Anchor cell versus ventral uterine precursor decision employs canonical lateral inhibition via Notch family receptors. Two equipotent cells Z1.ppp and Z4.aaa coexpress receptor LIN-12 Notch and ligand LAG-2 Delta-like. Initial stochastic difference amplifies via transcriptional feedback loop: cell with slightly higher LIN-12 signaling downregulates LAG-2 transcription via LAG-1 CSL repression, adopting ventral uterine fate, while neighbor with high LAG-2 low LIN-12 becomes anchor cell secreting LIN-3 and lag-2 for pi specification. LIN-12 intracellular domain released by presenilin SEL-12 cleavage translocates to nucleus ensuring single anchor cell.

Ref: Greenwald, WormBook Notch signaling: LIN-12 mediates lateral inhibition in anchor cell versus uterine precursor decision.

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C-Elegans Development-l

Which protein is responsible for the migration of P-granules during early development?

P-granule segregation depends on microtubule cytoskeleton immediately after fertilization. Sperm centrosome generates radial microtubule array, and cytoplasmic dynein heavy chain DHC-1 with accessory LIS-1, DNC-1 dynactin and DLC-1 light chain transports RNP granules containing PIE-1, GLH-1, PGL-1 toward posterior cortex using minus-end directed movement. Actomyosin cortical flows assist initiation, but dynein inhibition via RNAi randomizes granules throughout AB and P1, disrupting germline. Proper posterior localization ensures partitioning solely into P1-P4 lineage, protecting germline RNAs, preventing somatic differentiation, preserving totipotency until Z2/Z3 formation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: P-granule segregation and dynein-dependent transport mechanisms.

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Elegans -ll

Which protein is involved in vulval precursor cell lateral inhibition?

Vulval development provides model for lateral inhibition via Notch. Anchor cell secretes LIN-3 EGF inducing P5.p-P7.p to become vulval precursor cells. Among these, P6.p receiving highest LIN-3 dose adopts primary fate and expresses DSL ligands LAG-2, APX-1, DSL-1. These activate LIN-12 Notch receptor in neighboring P5.p and P7.p, triggering transcription of secondary fate genes lip-1, lst genes and repressing primary fate gene lin-39 hyperactivation. LIN-12 signaling thus ensures only one central cell becomes primary, flanking cells become secondary, preventing excessive primary lineage formation, central mechanism of lateral signaling patterning.

Ref: Sternberg, WormBook 2005; Gilbert Chapter 15: LIN-12 Notch lateral inhibition in vulval precursor cells.

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Sea urchin -lll

Which protein prevents β-catenin degradation?

Beta-catenin stability is controlled by destruction complex containing Axin, APC, and GSK-three kinase that phosphorylates conserved serine residues marking beta-catenin for proteasomal degradation via beta-TrCP. Wnt pathway activation recruits Dishevelled to plasma membrane via Frizzled receptor, where Dishevelled inhibits GSK-three kinase activity through protein interactions, preventing phosphorylation. Inhibition allows beta-catenin accumulation and nuclear translocation to activate TCF targets like siamois and Pmar1. In sea urchin, maternal Dishevelled localized vegetally performs this inhibition constitutively, protecting beta-catenin without exogenous Wnt ligand, establishing vegetal gradient required for endomesoderm and micromere identity establishment after cleavage.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Dishevelled prevents beta-catenin degradation sea urchin vegetal.

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Fertilization in mammals-ll

Protein promoting trophoblast differentiation:

Trophectoderm identity is driven by transcription factor Cdx2 acting as master regulator opposing pluripotency program. In outer blastomeres Hippo pathway inactive, YAP translocates to nucleus complexing with TEAD4 to activate Cdx2 expression. Cdx2 then drives genes for epithelial polarization, water transport and invasion while directly repressing Oct4 and Nanog transcription. Forced Cdx2 expression converts embryonic stem cells into trophoblast stem cells. Hence Cdx2 promotes differentiation toward placental lineage rather than embryonic lineages, establishing first cell fate decision, implantation competence, extraembryonic commitment essential for pregnancy and placental patterning processes.

Ref: Strumpf et al., Development 2005: Cdx2 determines trophectoderm differentiation opposing Oct4 pluripotency program.

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Fertilization in mammals-iv

Fertilin protein mediates:

Fertilin, also known as ADAM1-ADAM2 heterodimer, is sperm surface metalloprotease-disintegrin expressed in testis and processed during epididymal maturation, localizing to posterior head and equatorial segment. Its disintegrin domain interacts with integrins on egg plasma membrane microvillar region, facilitating adhesion and promoting membrane apposition preceding fusion pore formation coordinated by Izumo1-Juno and CD9. Fertilin knockout sperm show reduced fusion efficiency despite normal zona penetration. It does not function in acrosome biogenesis, zona digestion or capacitation but specifically bridges gamete membranes, making it critical component of mammalian sperm-oolemma binding and fusion machinery evolutionarily conserved across rodents and primates.

Ref: Evans JP, Human Reproduction Update 2002: ADAM fertilin roles in sperm-egg adhesion and membrane fusion.

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Cytoskeleton

Which protein links intermediate filaments to actin filaments and microtubules?

Plectin is a giant cytolinker of the plakin family that simultaneously recognizes actin, microtubules and intermediate filaments, integrating cytoskeletal networks into a cohesive mechanical continuum. Its structure includes an N-terminal actin-binding domain composed of two calponin-homology motifs, a central 200-nm coiled-coil rod dimerization domain, and a C-terminal repeat domain that binds vimentin, keratin, desmin and glial fibrillary acidic protein with distinct isoform specificity. Alternative splicing of first exons targets isoforms to hemidesmosomes via integrin beta4, to focal adhesions, desmosomes, mitochondria and nuclear envelope. At these sites plectin recruits microtubule-associated proteins and directly contacts tubulin, coordinating plus-end dynamics with intermediate filament anchorage. Deletion produces skin blistering with muscular dystrophy due to failure of stress transfer. In contrast spectrin forms tetramers capping actin at membranes, fimbrin bundles actin in microvilli, filamin crosslinks actin orthogonally, none containing high-affinity intermediate filament repeats. Thus plectin alone provides universal bridging activity essential for epithelial and muscle integrity.

Ref: Alberts Ch 16; Fuchs & Cleveland, Science 1998 plakin plectin crosslinks IF, actin, microtubules.

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Cytoskeleton

Which protein is responsible for anchoring actin filaments to the Z-disc in muscle cells?

Organization of sarcomere depends on anchoring thin filaments at Z disc providing mechanical integrity during repetitive contraction. Z disc 100 nanometer thick electron dense structure contains alpha actinin antiparallel dimer crosslinking antiparallel barbed ends from adjacent sarcomeres into orthogonal lattice spaced 20 nanometer, plus titin Z repeats, telethonin, myotilin, FATZ. Nebulin huge filamentous protein 600 to 900 kilodalton with 185 repeats of 35 residue nebulin motif SDXXYK each binding one actin monomer, plus N terminus interacting with tropomodulin capping pointed end and C terminal SH3 domain binding myopalladin CapZ inside Z disc embedding. By acting as molecular ruler nebulin dictates thin filament length and stabilizes filament against cofilin severing and depolymerization, loss causing nemaline myopathy with shorter thin filaments. Spectrin provides membrane skeleton in erythrocytes, dystrophin links actin via cysteine rich domain to dystroglycan complex at costameres for lateral force transmission, not Z disc anchoring. Titin provides elasticity and scaffold but nebulin specialized for actin Z disc anchorage and length specification.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Nebulin Anchoring Actin at Z-Disc.

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