Molecular Genetics, Gene Expression, and Cell Division

Vocabulary study set detailing the central dogma, transcription, translation, chromosome mechanics during mitosis and meiosis, DNA structure and replication enzymes, and repair pathways.

Central Dogma

The framework of genetic information flow stating that DNA is transcribed into messenger RNA (mRNA), which is subsequently translated into proteins.

Codon

A three-nucleotide sequence in mRNA that specifies a particular amino acid or a termination signal during protein translation.

Reading Frame

The structural grouping of nucleotide triplets in mRNA determined by the starting nucleotide; each segment of DNA contains 6 possible reading frames.

Genetic Redundancy

The characteristic of the genetic code in which multiple different codons specify the same amino acid among the 64 possible triplets coding for 20 amino acids.

Frameshift Mutation

A mutation caused by the insertion or deletion of nucleotides that disrupts the triplet reading frame, fundamentally altering the downstream amino acid sequence or introducing a premature stop codon.

Promoter

A DNA sequence positioned upstream of the coding region that directs RNA polymerase where to bind and initiate transcription.

$\sigma$ (Sigma) Subunit

A prokaryotic RNA polymerase component that recognizes consensus sequences in the promoter region upstream of the transcription start site and dissociates following initiation.

Rho-Dependent Termination

A mechanism of bacterial transcription termination controlled directly by interaction with the Rho protein to liberate the mRNA strand.

Rho-Independent Termination

A mechanism of bacterial transcription termination governed by specific DNA sequences that cause the transcribed mRNA to fold into a hairpin loop.

Polyribosome

A structure formed when multiple ribosomes simultaneously translate a single bacterial mRNA strand while it is still being actively transcribed from DNA.

Spliceosome

A ribonucleoprotein complex composed of proteins and snRNAs that catalyzes the precise removal of non-coding introns from pre-mRNA by recognizing $5^\prime$ and $3^\prime$ intron splice sites.

Peptide Bond

A covalent amide linkage joining the carboxyl group of one amino acid to the amino group of another, formed by the condensation loss of a water molecule.

Aminoacyl-tRNA Synthetase

An enzyme that expends ATP energy to covalently attach the correct amino acid to its cognate tRNA containing the corresponding anticodon.

Signal Recognition Particle (SRP)

A protein complex that binds the N-terminal signal sequence of an emerging nascent peptide and targets the translating ribosome to the ER membrane translocation channel.

Gametes

Haploid ($1n$) reproductive cells, such as sperm and eggs, produced by gonads that fuse during fertilization to restore diploidy.

Synaptonemal Complex

A protein lattice that bridges paired homologous chromosomes along their lengths during prophase I of meiosis, maintaining them in synapsis.

Chiasmata

Visible X-shaped junctions between non-sister chromatids of homologous chromosomes where genetic crossing over occurs during prophase I.

Tetrad

The four-chromatid structure formed during prophase I of meiosis when a pair of replicated homologous chromosomes binds together.

Crossing Over

The reciprocal exchange of genetic segments between non-sister homologous chromatids during prophase I, generating recombinant chromosomes.

Cell Plate

A structure assembled from Golgi-derived vesicles along the equator of a dividing plant cell that coalesces into a new dividing cell wall during cytokinesis.

Binary Fission

The asexual cell cycle process in prokaryotes where the circular chromosome replicates and the cell constricts to produce two genetically identical daughter cells.

Diploid-Dominant Life Cycle

A reproductive strategy typical of animals where the organism exists as a multicellular diploid ($2n$) entity and produces gametes as its only haploid ($1n$) stage.

Nuclein

The phosphate-rich acidic substance isolated from the nuclei of white blood cells in pus by Friedrich Miescher in 1869, subsequently recognized as DNA.

Transforming Principle

The molecular component identified by Frederick Griffith that transfers hereditary pathogenic capability from heat-killed S strain to live R strain Streptococcus pneumoniae.

Chargaff's Rules

The principle stating that within double-stranded DNA, the relative concentration of adenine equals thymine ($\text{\% A} = \text{\% T}$) and cytosine equals guanine ($\text{\% C} = \text{\% G}$).

Antiparallel Strands

The arrangement of the two complementary strands in a DNA double helix, where one polymer runs in the $5^\prime \rightarrow 3^\prime$ orientation and the opposing strand runs $3^\prime \rightarrow 5^\prime$.

Sanger Dideoxynucleotide Sequencing

A chain-termination sequencing method utilizing dye-labeled dideoxynucleotides that halt DNA synthesis at variable points, yielding fragments resolved by capillary electrophoresis.

Gel Electrophoresis

A technique using an electrical gradient to pull negatively charged nucleic acid fragments through an agarose gel sieve, separating them inversely to molecular size.

Nucleoid

The non-membrane-bound cytoplasmic region of a prokaryotic cell where the main circular chromosome is located.

Nucleosome

The fundamental subunit of eukaryotic chromatin packaging, formed by winding the DNA double helix around an octamer core of histone proteins.

Semi-Conservative Replication

The mode of DNA copying demonstrated by Meselson and Stahl wherein the two parent strands unpair and each serves as an intact template for a newly synthesized daughter strand.

Helicase

The replication enzyme that unwinds and separates parental DNA strands at the origin of replication to establish the replication fork.

Topoisomerase

An enzyme that relieves torsional strain and prevents destructive overwinding in front of the advancing replication fork as DNA is unwound.

Single-Strand Binding Proteins (SSBs)

Proteins that adhere to exposed single-stranded template DNA to prevent premature reannealing into a double helix during replication.

Telomerase

A ribonucleoprotein reverse transcriptase carrying an internal RNA template that lengthens the $3^\prime$ unreplicated ends of eukaryotic chromosomes.

Mismatch Repair

A post-replication error-correction pathway in which specialized proteins identify mispaired bases, excise them via nuclease activity, and direct polymerase to insert the proper base.

Nucleotide Excision Repair

A repair mechanism that excises pyrimidine lesions—such as UV-induced thymine dimers—by cutting the damaged single strand on either side, resynthesizing DNA, and sealing it with ligase.