Cellular Biology, Genetics, and Molecular Biology Review

Comprehensive vocabulary flashcards covering cell structure, organelles, endosymbiosis, DNA structure, packaging, replication, repair, PCR, transcription, pre-mRNA processing, translation, the genetic code, mutations, the eukaryotic/prokaryotic cell cycle, mitosis, checkpoints, cancer genetics, and operon gene regulation.

Prokaryotic Cell

A unicellular organism belonging to domain Bacteria or Archaea that lacks a membrane-bound nucleus and internal membrane-bound organelles, typically ranging in size from $0.1\text{--}5.0\,\mu\text{m}$ (or $1\text{--}10\,\mu\text{m}$) and containing circular double-stranded DNA.

Eukaryotic Cell

An organism belonging to domain Eukarya that possesses a true membrane-bound nucleus housing linear double-stranded DNA, extensive internal membrane-bound organelles, and a typical size of $10\text{--}100\,\mu\text{m}$.

Plant Cell Anatomy

Eukaryotic cell structure distinguished from animal cells by the presence of a rigid cellulose-containing cell wall, photosynthetic chloroplasts, and a large central vacuole.

Surface Area-to-Volume Ratio

The geometric principle constraining cell size; as a cell increases in dimension, its volume increases much faster than its surface area (e.g., a $1\,\text{mm}$ cube has a ratio of $6:1$, whereas a $2\,\text{mm}$ cube has a ratio of $3:1$), limiting diffusion efficiency in larger cells.

Cytoskeleton

A network of protein fibers within the cytoplasm comprising microfilaments (actin filaments), intermediate filaments, and microtubules that provides structural support, maintains cell shape, and facilitates intracellular movement.

Centrosome

An animal cell organelle consisting of two barrel-shaped centrioles surrounded by a protein-rich matrix that serves as the main microtubule-organizing center and directs mitotic spindle assembly.

Nucleolus

A darkly stained, dense region within the eukaryotic nucleus where ribosomal RNA (rRNA) is transcribed and combined with proteins to assemble ribosomal subunits.

Rough Endoplasmic Reticulum (RER)

A series of interconnected membranous tubules and flattened sacs studded with ribosomes on its cytoplasmic surface that folds and chemically modifies proteins before packaging them into transport vesicles.

Smooth Endoplasmic Reticulum (SER)

A region of the endoplasmic reticulum continuous with the RER that lacks ribosomes and functions in the synthesis of lipids, carbohydrate metabolism, and detoxification.

Golgi Apparatus

A eukaryotic organelle composed of a series of flattened membranous sacs that receives vesicles from the rough ER to modify, sort, package, and direct proteins and lipids to their destinations.

Lysosome

An enzyme-filled vesicle budding from the Golgi apparatus that acts as the animal cell's 'garbage disposal' by degrading proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles.

Peroxisome

A small, membrane-enclosed organelle independent of the endomembrane system that carries out oxidation reactions, fatty acid breakdown, and cellular detoxification.

Endomembrane System

A collaborative group of eukaryotic membranes and organelles—including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane—that coordinates protein and lipid synthesis, modification, and transport.

Mitochondria

The double-membrane 'powerhouse' organelle responsible for generating ATP; it possesses its own circular DNA and $70\text{S}$ ribosomes ($30\text{S}$ and $50\text{S}$ subunits) and replicates independently via binary fission.

Cilia and Flagella

Locomotor appendages extending from the plasma membrane; flagella are long structures used to propel an entire cell (e.g., sperm), while cilia are numerous, shorter projections that move cells or propel fluid and particles along cell surfaces.

Theory of Endosymbiosis

The evolutionary framework proposing that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells through phagocytosis and established a permanent symbiotic mutualism.

Nucleotide

The monomer building block of nucleic acids consisting of three parts: a five-carbon pentose sugar (deoxyribose in DNA or ribose in RNA), a negatively charged phosphate group attached to the $5'$ carbon, and a nitrogenous base attached to the $1'$ carbon.

Purines

Double-ringed nitrogenous bases found in nucleic acids, specifically Adenine (A) and Guanine (G).

Pyrimidines

Single-ringed nitrogenous bases found in nucleic acids, comprising Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA.

Phosphodiester Bond

The strong covalent linkage connecting the $3'\text{-OH}$ group of one nucleotide sugar to the $5'$ phosphate group of the adjacent nucleotide on the same strand, forming the sugar-phosphate backbone.

Nucleosome

The basic structural repeating unit of chromatin ($10\,\text{nm}$ fiber) consisting of approximately two turns of double-stranded DNA ($2\,\text{nm}$) wound around an octamer core of histone proteins ('beads on a string').

Euchromatin

Loosely packaged, accessible chromatin associated with histone acetylation, allowing high transcriptional activity ('ON').

Heterochromatin

Densely compacted, condensed chromatin associated with histone methylation ($\text{CH}_3$), preventing RNA polymerase access and resulting in transcriptional silencing ('OFF').

Homologous Chromosomes

A pair of matching chromosomes (one maternal and one paternal) that contain the same gene loci in the same order but may carry different versions (alleles) of those genes; humans possess 23 pairs (46 total).

Sister Chromatids

Two identical copies of a replicated chromosome joined together at a common centromere following DNA replication in S phase ($700\,\text{nm}$ each; forming a $1400\,\text{nm}$ metaphase chromosome).

Semi-Conservative Replication

The demonstrated model of DNA replication (Meselson-Stahl experiment) in which each parental strand serves as a template, producing daughter double helices composed of one original parent strand and one newly synthesized strand.

DNA Helicase

An ATP-dependent enzyme that breaks hydrogen bonds between complementary base pairs to unwind and separate the DNA double helix at the replication fork.

Topoisomerase

An enzyme that prevents excessive overwinding and relieves torsional strain ahead of the advancing replication fork by making transient cuts in the DNA strands and rejoining them.

Single-Strand Binding Proteins (SSBs)

Proteins that coat and stabilize separated single-stranded template DNA strands during replication to prevent them from reannealing or forming secondary structures.

DNA Primase

A specialized RNA polymerase enzyme that synthesizes a short RNA primer ($\sim 10\text{--}20$ nucleotides) complementary to the DNA template, providing the free $3'\text{-OH}$ group required by DNA polymerases.

DNA Polymerase III

The primary prokaryotic replication enzyme that elongates leading and lagging strands in the $5' \rightarrow 3'$ direction and proofreads newly incorporated bases via its $3' \rightarrow 5'$ exonuclease activity.

DNA Polymerase I

A prokaryotic enzyme that removes RNA primers using its $5' \rightarrow 3'$ exonuclease activity and replaces them with corresponding DNA nucleotides.

DNA Ligase

An ATP-requiring enzyme that catalyzes the formation of the final phosphodiester bond, sealing nicks between adjacent Okazaki fragments and completing the DNA backbone.

Okazaki Fragments

Short, discontinuously synthesized DNA segments formed on the lagging strand that grow in the $5' \rightarrow 3'$ direction away from the replication fork, each initiated by a distinct RNA primer.

Telomeres

Repetitive, non-coding hexanucleotide sequences (human: $\text{TTAGGG}$ repeated hundreds to thousands of times) capping the ends of linear eukaryotic chromosomes that prevent chromosome degradation during successive rounds of replication.

Telomerase

A ribonucleoprotein reverse transcriptase carrying an internal RNA template ($\text{AAUCCC}$) that elongates the $3'$ overhang of eukaryotic telomeres; it is active in germ cells, stem cells, and cancer cells, but largely inactive in somatic cells.

Nucleotide Excision Repair (NER)

A DNA repair pathway wherein a nuclease enzyme excises damaged bases and surrounding single-stranded DNA (such as UV-induced thymine dimers), followed by DNA polymerase filling the gap and DNA ligase sealing the nick.

Mismatch Repair

A post-replication DNA repair mechanism that scans newly synthesized daughter strands to identify, excise, and correct mispaired bases that escaped polymerase proofreading.

Polymerase Chain Reaction (PCR)

An in vitro DNA amplification technique devised by Kary Mullis utilizing a thermal cycler, template DNA, specific primers, Taq polymerase, dNTPs, and buffer through cycles of denaturation ($95^\circ\text{C}$), annealing ($50\text{--}65^\circ\text{C}$), and extension ($72^\circ\text{C}$) to double target DNA each cycle ($2^n \times X$).

Central Dogma

The fundamental framework of molecular biology describing the unidirectional flow of genetic information: DNA is copied into RNA via transcription, and mRNA directs polypeptide synthesis via translation.

Gene

A specific sequence of DNA nucleotides containing instructions required to synthesize a functional biological product (either a polypeptide or a functional RNA such as tRNA or rRNA).

Transcription

The enzymatic synthesis of an RNA transcript by RNA polymerase reading a DNA template strand in the $3' \rightarrow 5'$ direction and synthesizing complementary RNA in the $5' \rightarrow 3'$ direction.

Promoter

A specific regulatory DNA sequence upstream of the transcription start site where RNA polymerase and associated transcription factors bind to initiate transcription; it is not itself transcribed.

5' Cap

A chemically modified guanine nucleotide ($7\text{-methylguanosine}$) attached to the $5'$ end of eukaryotic pre-mRNA that protects the transcript from enzymatic degradation and serves as an attachment signal for the small ribosomal subunit.

Poly-A Tail

A sequence of hundreds of adenine nucleotides added enzymatically to the $3'$ end of eukaryotic pre-mRNA following a polyadenylation signal ($\text{AAUAAA}$) that enhances mRNA stability and assists in nuclear export.

Spliceosome

A large ribonucleoprotein complex composed of snRNAs and proteins (snRNPs) that recognizes splice sites, removes non-coding introns from pre-mRNA, and ligates coding exons together.

Alternative Splicing

A post-transcriptional process where pre-mRNA exons are selectively included or excluded in variable combinations, allowing a single gene to code for multiple distinct protein isoforms and explaining how $\sim 20{,}000$ human genes generate over $100{,}000$ proteins.

Aminoacyl-tRNA Synthetase

An amino-acid-specific enzyme that attaches an amino acid covalently to the $3'\text{-OH}$ terminal acceptor stem of its matching tRNA molecule using ATP, charging the tRNA for translation.

Codon

A trinucleotide mRNA sequence read in the $5' \rightarrow 3'$ direction during translation that specifies an amino acid or a termination signal (universal start: $\text{AUG}$; stop codons: $\text{UAA}$, $\text{UAG}$, $\text{UGA}$).

Wobble Effect

Non-standard, relaxed base pairing between the $3'$ base of an mRNA codon and the $5'$ base of a tRNA anticodon, allowing a single tRNA anticodon to recognize and pair with more than one synonymous codon.

Point Mutation Types

Single-base alterations in DNA classified by phenotypic consequence: silent mutations cause no amino acid change; missense mutations substitute one amino acid; nonsense mutations create a premature stop codon; and frameshift mutations alter the entire downstream reading frame via indels.

Binary Fission

The asexual reproductive mechanism in prokaryotes consisting of a growth phase (B), chromosomal replication (C), and cellular division (D) that yields two genetically identical daughter cells.

G0 Phase

A resting state outside the replicative cell cycle; cells may be quiescent (a reversible metabolic state prior to re-entering $\text{G}_1$), senescent (an irreversible non-dividing state due to DNA damage), or terminally differentiated.

Mitosis

The phase of the eukaryotic cell cycle dedicated to karyokinesis, consisting of prophase, prometaphase, metaphase, anaphase, and telophase to distribute identical chromosome sets into two nuclei.

Kinetochore

A specialized protein complex assembled on the centromeric region of each sister chromatid where spindle microtubules attach during prometaphase to pull chromatids apart.

Cytokinesis

The division of cellular cytoplasm; occurs in animal cells via an actin contractile ring forming a cleavage furrow, and in plant cells via the fusion of Golgi-derived vesicles into a cell plate.

Cell Cycle Checkpoints

Surveillance control mechanisms ensuring fidelity of division: the $\text{G}_1$ checkpoint assesses size, nutrients, and DNA damage; the $\text{G}_2$ checkpoint confirms complete, undamaged DNA replication; and the M checkpoint verifies proper kinetochore-microtubule attachment across all chromosomes.

Proto-oncogenes

Normal genes encoding positive cell cycle regulators (e.g., Cdks) that, when altered by gain-of-function mutations, become oncogenes driving uncontrolled division and tumor progression.

Tumor Suppressor Genes

Genes encoding negative cell cycle regulators (e.g., p53) that arrest cell division upon DNA damage, promote repair, or trigger apoptosis; loss-of-function mutations lead to unchecked cell proliferation.

Operon Anatomy

A prokaryotic gene cluster consisting of structural genes transcribed together into a single polycistronic mRNA, regulated by an upstream promoter (where RNA polymerase binds) and an operator (where repressors bind).

Trp Operon

A repressible prokaryotic operon active by default; when tryptophan levels are high, tryptophan binds the trp repressor as a corepressor, activating it to bind the operator and prevent RNA polymerase transcription.

Lac Operon

An inducible prokaryotic operon normally repressed by LacI; allolactose acts as an inducer by binding LacI and removing it from the operator, while low glucose triggers cAMP to bind CAP, accelerating transcription.

Catabolite Activator Protein (CAP)

A transcriptional activator in prokaryotes that forms a complex with cyclic AMP (cAMP) under low glucose conditions, binding upstream of the lac promoter to enhance RNA polymerase affinity and maximize transcription.