32 public questions tagged with this topic.
Mutation introduces new genetic variations by altering DNA sequences. 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: Principles of Inheritance, Molecular Basis of Inheritance and Biotechnology, Topic: Genetics, DNA techniques and applications.
Not all mutations are inherited; only those in germ lls can be passed to the next generation. 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 (Botany section), Chapter: Morphology and Anatomy of Flowering Plants, Topic: Plant structure and tissue systems.
SmlA functions as negative regulator of counting factor production. In smlA null cells, countin and associated cf proteins accumulate to supranormal levels in conditioned medium, causing premature fragmentation of aggregation streams. Cells interpret elevated CF as indicator that aggregate is already too large, triggering increased random motility and reduced adhesion that breaks territories into numerous small mounds. Resulting fruiting bodies are diminutive with fewer spores each, though total spore number remains similar. This phenotype demonstrated counting factor as size-regulating quorum substance and revealed smlA role analogous to brake preventing overproduction, thereby fine-tuning optimal multicellular group dimensions for developmental success.
Ref: PNAS, Counting factor complex and smlA- mutants - overproduction leading to smaller aggregates.
GNOM encodes ARF-GEF controlling formation of COPI-independent transport vesicles that recycle PIN auxin carriers from endosomes to basal and apical plasma membrane domains. Loss-of-function gnom alleles accumulate PINs in intracellular agglomerations, abolish differential auxin maxima at embryo apex and root pole, and disrupt apical-basal axis specification. Resultant embryos display abnormal zygote division, fused collar-like cotyledons, complete loss of axial distinctions, root pole absence, and dwarf stature phenocopying chemical inhibition with NPA. Hence vesicle trafficking machinery translates biochemical hormone gradients into morphological polarity and stable tissue identities during plant embryogenesis.
Ref: Mayer et al., Development 1993; Gilbert, Developmental Biology, Chap 20: GNOM mutation causes loss of apical-basal polarity.
Somaclonal variation denotes phenotypic and genetic variability observed among plants regenerated from somatic cells in tissue culture, term introduced by Larkin and Scowcroft 1981 to describe variation in clones derived from same explant. Unlike seed derived variation, somaclonal variation originates from stress imposed by in vitro environment high concentrations of synthetic auxins especially 2,4-D, rapid cell cycles, oxidative burst from autoclaved medium components, and epigenetic reprogramming during dedifferentiation redifferentiation. Underlying mechanisms include point mutations due to error prone DNA repair, transposable element mobilization such as activation of Tos17 retrotransposon in rice callus, chromosome aberrations aneuploidy polyploidy translocations due to spindle failures, and altered DNA methylation patterns affecting gene expression without sequence change. Phenotypically manifests as changes in plant height, leaf shape, flowering time, yield components, disease resistance or metabolite profile. While undesirable when clonal fidelity required for micropropagation of elite genotypes, variation is exploited as source of novel useful traits generating new cultivars like sugarcane with Fiji disease resistance and tomato high lycopene lines selected from tissue culture derived population without transgenic intervention.
Ref: Larkin & Scowcroft 1981 somaclonal variation; NCBI Plants review somaclonal.
Cytoplasmic dynein requires adaptor complexes to achieve efficient cargo transport because motor alone exhibits low processivity and weak cargo binding. Dynactin is twenty three subunit complex containing short Arp1 filament, beta-spectrin adaptor, p150Glued subunit with CAP-Gly microtubule-binding domain and coiled-coil dimerization region that binds dynein intermediate chain. Interaction via extended CC1 fragment of p150Glued locks dynein-dynactin together, increasing run length from submicron to several microns by coordinating two motor domains and suppressing detachment. Preventing this interaction by mutating conserved residues in intermediate chain or depleting p50 dynamitin dissociates complex, leaving dynein catalytically active but unable to maintain association with vesicles such as endosomes, phagosomes and mRNA granules. Cellular outcome is reduced retrograde flux, peripheral accumulation, dispersed Golgi and impaired mitotic spindle alignment. Stronger microtubule binding or reversal to plus-end motion does not occur because directionality encoded in AAA ring and linker orientation, not adaptor identity, confirming dynactin primarily as processivity and cargo recruitment factor.
Ref: Schroer Annu Rev Cell Dev Biol; dynactin p150Glued enhances dynein processivity and cargo binding.
Iron uptake in proliferating cells depends on transferrin receptor mediated endocytosis tightly coupled to endosomal acidification. At extracellular pH 7.4 holo transferrin carrying two ferric ions binds transferrin receptor 1 homodimer with nanomolar affinity, clusters via AP2 clathrin adaptor and internalizes into early endosomes. Vacuolar ATPase acidifies lumen to about pH 5.5, protonation of transferrin histidines and nearby receptor residues induces conformational opening of transferrin lobes, reducing Fe3+ affinity by orders of magnitude and releasing ferric iron while apo transferrin remains bound to receptor due to retained high affinity at acidic pH. Liberated Fe3+ reduced to Fe2+ by ferrireductase STEAP3, exported through divalent metal transporter DMT1 into cytosolic labile iron pool for use in heme, Fe S clusters or storage in ferritin. Recycling vesicles return complex to plasma membrane where neutral pH dissociates apo transferrin for reuse. If receptor mutation prevents iron release at low pH, transferrin stays iron locked, endosomal iron export fails, cytosol becomes deficient, IRP IRE system upregulates receptor and represses ferritin, but iron does not accumulate in mitochondria nor cause rapid receptor degradation.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: Endocytosis and Transferrin Iron Transport.
Retinoblastoma protein Rb functions as central gatekeeper integrating mitogenic signaling with E2F transcription program controlling G1/S restriction point. In quiescent or early G1 cells, Rb exists in hypophosphorylated state, binding E2F1, E2F2, E2F3 through pocket domains A and B, simultaneously recruiting histone deacetylases, SWI/SNF chromatin remodelers, and Polycomb complexes to repress promoters of cyclin E, cyclin A, thymidine kinase, dihydrofolate reductase, and DNA polymerase subunits. Mitogens induce cyclin D-CDK4/6 synthesis downstream of Ras-MAPK and PI3K pathways, which initiates progressive phosphorylation of Rb at serine 780, 795, 807, 811. Subsequent cyclin E-CDK2 mediated hyperphosphorylation fully displaces Rb, liberating E2F to activate S-phase genes. Mutation eliminating CDK phosphorylation sites locks Rb in constantly bound conformation, maintaining E2F repression despite abundant cyclin-CDK activity. Consequently, cells fail to transcribe nucleotide biosynthesis enzymes and replication factors, arresting before S-phase commitment, illustrating how non-phosphorylatable Rb creates dominant negative barrier to proliferation. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.
Ref: Weinberg, Biology of Cancer, 2nd ed., Chapter 8: Rb Pathway. NCBI Bookshelf: Cell Cycle Control.
Functional p53 integrates stress signals, including DNA damage, oncogene activation, and hypoxia, to decide between cell cycle arrest, senescence, or apoptosis. Under normal conditions, wild-type p53 accumulates after ATM phosphorylates serine 15 and CHK2 phosphorylates serine 20, displacing E3 ubiquitin ligase MDM2 that normally targets p53 for proteasomal degradation. Transcriptional program includes CDKN1A encoding p21 inhibitor of CDK2-cyclin E and CDK4-cyclin D, causing Rb to remain hypophosphorylated and E2F-dependent S-phase entry genes repressed, imposing G1 arrest to allow repair. If TP53 gene is mutated, common missense mutations in DNA-binding domain producing dominant-negative tetramers, cells lose ability to transactivate p21 and other checkpoint targets, so G1 arrest fails despite DNA damage. Damaged templates proceed into replication, accumulating mutations and chromosomal aberrations driving tumor progression. Mutant p53 does not accelerate apoptosis or permanently block CDK1; instead it disables G1 surveillance and also compromises apoptosis via BAX and PUMA reduction. Consequently, p53 mutation abolishes G1 checkpoint stringency and genomic stability, explaining high prevalence in cancers.
Ref: Vogelstein et al., Nature 2000, Surfing p53 Network; Levine & Oren, Nature Rev Cancer 2009, p53 Mutations.
Saltation refers to a sudden and large genetic change, as proposed by Hugo de Vries in mutation theory.
Ref: NCERT Class 12 Biology Chapter 6: Evolution Mechanism of Evolution - Hardy-Weinberg and Natural Selection
A silent mutation changes a nucleotide sequence but does not alter the amino acid due to the degeneracy of the genetic code.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Human Genome Project and DNA Fingerprinting
A silent mutation changes a nucleotide sequence but does not alter the amino acid sequence due to redundancy in the genetic code.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Regulation of Gene Expression - Lac Operon
