Cell Biology - DNA, Chromatin, Cell Culture, and Microscopy Techniques

Comprehensive vocabulary flashcards covering nucleic acid structures, chromosome packaging, history of DNA discovery, cell culture methodologies, subcellular fractionation, and light, fluorescence, and electron microscopy techniques.

Nucleoside

A chemical combination composed of a nitrogen-containing base attached directly to a five-carbon sugar.

Nucleotide

A chemical building block of nucleic acids consisting of three components: a five-carbon sugar, a nitrogen-containing base, and a phosphate group.

Phosphodiester Bond

A $3'-5'$ chemical linkage formed between the $5'-P$ group of one nucleotide and the $3'-OH$ group of a neighboring nucleotide, imparting directionality to the polynucleotide chain.

Purines

Double-ring nitrogenous bases found in DNA and RNA, specifically adenine (A) and guanine (G).

Pyrimidines

Single-ring nitrogenous bases found in nucleic acids, consisting of cytosine (C), thymine (T) in DNA, and uracil (U) in RNA.

Kilobase (kb)

A unit of length measurement for double-stranded DNA corresponding to 1,000 base pairs.

Megabase Pair (Mb)

A unit of length measurement for double-stranded DNA corresponding to 1,000,000 base pairs.

Nuclein

An acidic, phosphorus-rich non-protein substance isolated from white blood cell nuclei by Friedrich Miescher in 1869, later recognized as DNA.

Chromatin

The complex of nuclear DNA, histones, and non-histone proteins that packs long linear DNA molecules into a compact structure inside eukaryotic cells.

Histones

Small eukaryotic proteins rich in basic amino acids (arginine and lysine) that bind to negatively charged DNA to form nucleosomes.

Karyotype

An ordered microscopic display of the full set of 46 human chromosomes (or an organism's full set) isolated during mitosis.

Gene

A functional unit of heredity defined as a segment of DNA containing instructions for making a specific protein or RNA molecule.

Genome

The total genetic information carried by a complete set of chromosomes present in a cell or organism.

Replication Origin

A specialized nucleotide sequence on a chromosome where DNA replication initiates.

Telomere

A specialized sequence at the ends of linear eukaryotic chromosomes, composed of simple repetitive DNA (e.g., TTAGGG in mammals) bound by shelterin proteins to prevent degradation.

Centromere

A specialized chromosomal region that holds sister chromatids together and facilitates mitotic spindle microtubule attachment via kinetochores during cell division.

Nucleolus

The most prominent interphase nuclear structure where genes encoding ribosomal RNAs (located on human chromosomes 13, 14, 15, 21, and 22) cluster and transcribe.

Nucleosome Core Particle

A structural unit of chromatin consisting of 147 base pairs of DNA wrapped 1.67 turns around an octamer core of histones (two each of H2A, H2B, H3, and H4).

Histone H1

The linker histone protein that binds to DNA as it enters each nucleosome core particle to aid in compacting chromatin fibers.

Heterochromatin

The most highly condensed, transcriptionally inactive form of interphase chromatin, often concentrated near centromeres and telomeres.

Euchromatin

The dominant, less condensed form of interphase chromatin that contains actively expressed genes.

ATP-Dependent Chromatin-Remodeling Complexes

Protein machines that use the energy of ATP hydrolysis to shift the position of nucleosomes along DNA, altering local chromatin structure.

X-Inactivation

The random, inherited transcriptional silencing of one of the two X chromosomes in female mammalian cells via heterochromatin formation during early development.

Epigenetic Inheritance

The transmission of chromatin structure and gene expression state from parent cell to daughter cells without altering the underlying DNA sequence.

Primary Culture

An initial cell culture established directly from isolated biological tissues before any sub-culturing or passaging occurs.

Secondary Culture

A culture formed by removing cells from a primary culture and replating them at a lower density (also referred to as sub-culturing or passaging).

HeLa Cells

The first established human immortal cell line, originally isolated from a cervical cancer tissue in 1951.

Differential Centrifugation

A subcellular fractionation technique that separates organelles and cell components based on size and density using sequential centrifugal force steps.

Svedberg Unit (S)

A unit of sedimentation rate during ultracentrifugation, where $1 S = 10^{-13}\,\text{seconds}$.

Isopycnic Centrifugation

Equilibrium density gradient centrifugation that separates particles based strictly on their buoyant density independent of size and shape.

Numerical Aperture (NA)

A measure of a microscope lens's light-gathering capability, defined mathematically as $\text{NA} = \eta \sin(\alpha)$, where $\eta$ is the refractive index.

Phase-Contrast Microscopy

An optical technique that converts small phase shifts in light (around $\frac{\lambda}{4}$) caused by cellular refractive index differences into visible contrast changes via destructive interference.

FRAP (Fluorescence Recovery After Photobleaching)

An imaging technique used to calculate the rate of protein movement within a live cell by monitoring fluorescence recovery in a laser-bleached region.

FRET (Fluorescence Resonance Energy Transfer)

A fluorescence technique used to monitor direct physical interactions and proximity between two protein molecules labeled with complementary fluorophores inside living cells.

Confocal Microscopy

A technique using a focused point laser and a spatial pinhole aperture to physically reject out-of-focus light, yielding sharp optical sections.

Multiphoton Microscopy

A live-cell imaging method where fluorophore excitation occurs only when two or more longer-wavelength photons hit simultaneously within $10^{-18}\,\text{seconds}$, eliminating out-of-focus damage without a pinhole.

STORM (Stochastic Optical Reconstruction Microscopy)

A super-resolution microscopy method that bypasses the light diffraction barrier down to 10–20 nm by stochastically activating individual fluorescent probes at different times.

Transmission Electron Microscopy (TEM)

A high-resolution microscopy technique where a beam of electrons passes through a thin specimen stained with heavy metal salts to form detailed 2D structural images.

Scanning Electron Microscopy (SEM)

An electron microscopy technique where electrons scan across a metal-coated sample surface to collect scattered electrons and build a 3D image.