Cell Biology

Cell division doubles the number of cells with each mitotic division. Starting with 1 cell, after n divisions we have 2^n cells. Since 2^10 = 1024, it takes exactly 10 mitotic divisions to reach 1024 cells.

In animal cells, potassium concentration is significantly higher inside the cell (140 mM) compared to outside (4 mM). This gradient is maintained by the Na+/K+ ATPase pump and is essential for resting membrane potential and cellular function.

Histone proteins (H2A, H2B, H3, H4) are highly conserved across species and cell types. Their primary amino acid sequence is remarkably similar from yeast to humans, not characteristic of specific cell types.

Cell surface receptors include G protein-coupled receptors, enzymatic receptors (like receptor tyrosine kinases), and ligand-gated ion channels. Transcription factors are intracellular nuclear proteins that bind DNA - not cell surface receptors.

Endocrine signaling involves hormones secreted into the bloodstream that travel throughout the body to reach distant target cells - the definition of long-distance cellular communication.

To go from cytosol to mitochondrial matrix of a neighboring cell: exit first cell (plasma membrane), enter second cell (plasma membrane), cross outer mitochondrial membrane, cross inner mitochondrial membrane. Total: 4 membranes.

Colchicine inhibits microtubule polymerization, preventing spindle fiber formation during mitosis. Since chromosomes cannot attach to spindle fibers, they fail to separate and migrate to opposite poles during anaphase. DNA replication occurs normally in S phase, but chromosome segregation cannot complete without functional microtubules.

Ionophores are molecules that transport ions across lipid membranes. To function effectively, they must have a hydrophobic exterior that can dissolve in the lipid bilayer and a hydrophilic interior that can bind and transport ions like Ca2+. This amphipathic structure allows them to shuttle ions across the otherwise impermeable membrane.

Retinoblastoma follows Knudson's two-hit hypothesis, where both copies of the RB tumor suppressor gene must be inactivated for tumor development. A germinal (inherited) mutation in one allele predisposes cells, and a subsequent somatic mutation in the remaining allele leads to complete loss of tumor suppression, allowing uncontrolled cell growth.

When a differentiated nucleus (from red blood cell that doesn't normally replicate DNA) is transplanted into an enucleated egg, the egg's cytoplasm reprograms the nucleus. The cytoplasm contains factors that can reactivate dormant genes and initiate DNA replication and cell division, demonstrating that cytoplasmic factors control nuclear activity.

Gap junctions are intercellular channels that allow the passage of small molecules and ions between adjacent cells, but NOT proteins or larger macromolecules. They are crucial for metabolic cooperation, electrical coupling in heart muscle, and embryonic development. Option A incorrectly claims they allow protein passage.

Lysosomes are correctly marked by acid phosphatase, peroxisomes by catalase, and mitochondria by cytochrome oxidase. However, SER (smooth endoplasmic reticulum) is associated with detoxification enzymes and lipid synthesis, not amino acid permease, which is a plasma membrane transport protein. Therefore, only pair D is mismatched.

Telomeres are DNA sequences at chromosome ends (1-B). Kinetochores are protein structures where spindle fibers attach (2-D). Centromeres are DNA sites where kinetochores are found (3-A). Nucleolar organizers are DNA sequences that serve as templates for ribosomal RNA synthesis and amplification (4-C). Option B correctly matches these.

At the beginning of the cell cycle, a diploid cell has 2c DNA content and 2n chromosomes. After S phase (DNA replication), before mitosis begins, the cell has doubled DNA content (4c) but still has the same number of chromosomes (4n sister chromatids). Immediately before mitosis, after replication is complete, the cell contains 4c DNA and 4n chromatids.

Nucleosomes are fundamental units of chromatin consisting of histone octamers associated with DNA (A-2). Kinetochores are disc-shaped protein structures at centromeres where spindle fibers attach (B-3). Chromomeres are bead-like accumulations of chromatin visible along chromonema during certain phases (C-4). Option B correctly matches these structures.