#cellular biology

Cell Organelles

Mitochondrial DNA (mtDNA) is characterized by:

Mitochondrial DNA illustrates its bacterial endosymbiont origin and maintains partial organelle autonomy despite extensive nuclear contribution. Human mtDNA is a small, covalently closed circular double-stranded molecule about 16,569 base pairs in length, present in 2 to 10 copies per mitochondrion and organized into nucleoids with transcription factor A mitochondrial, TFAM, high mobility group proteins, and Twinkle helicase rather than histones. It is intron-less and extremely densely coded with limited noncoding displacement-loop control region containing promoters and origin of replication.

Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: Organization and Inheritance of Mitochondrial DNA.

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Eukaryotic Cell - Cell Wall and Cell Membrane - Fluid Mosaic Model

Which sequence correctly represents the layers of a typical bacterial cell envelope from the outermost to the innermost

The bacterial cell envelope is a tightly bound three-layered structure. Its outermost layer is the glycocalyx, followed by the cell wall and then the plasma membrane.

Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Eukaryotic Cell - Cell Wall and Cell Membrane - Fluid Mosaic Model

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Processing of rRNA and tRNA

Major role of rRNA processing is

Maturation of ribosomal RNA serves principal purpose of ribosome assembly rather than gene regulation or degradation alone. Processing cleaves polycistronic or 45S precursors, removes internal and external spacers, incorporates chemical modifications and coordinates binding of ribosomal proteins to produce functional 30S, 50S or 40S, 60S subunits. Modifications fine-tune decoding and peptidyl transferase function. Proper assembly guarantees translation competence, nucleolar surveillance, stress responses, and overall cellular growth control, linking rRNA biogenesis rate to cell proliferation d

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, rRNA processing and ribosome assembly pathway

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Repair Pathway

SOS response is regulated by

SOS response encompasses global induction of more than forty DNA repair and tolerance genes triggered by extensive single-stranded DNA at stalled replication forks. Central negative regulator is LexA repressor, homodimer binding SOS boxes with consensus CTGTN8ACAG in promoter regions, repressing transcription including own lexA gene and recA, uvr, umuDC operons. Under steady growth, LexA maintains low expression. After DNA damage, activated RecA nucleofilament stimulates LexA autocleavage at Ala84-Gly85, causing dissociation from operators and derepression. Temporal induction occurs as LexA af

Ref: Alberts Molecular Biology of Cell, 7th ed., Chapter 5: SOS regulon LexA repressor control; Scielo 2020 SOS regulation E coli

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Cancer

Intrinsic apoptotic pathway is mainly activated by

Intracellular stress, 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)

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