Comprehensive Genetics and Molecular Biology: From Meiosis to Gene Expression

Vocabulary practice flashcards covering cell division, classical Mendelian genetics, DNA structure and replication, gene expression (transcription and translation), and the regulation of gene expression.

Karyotype

An ordered display of the pairs of duplicated homologous chromosomes arranged by size and shape from a metaphase cell.

Homologous Chromosomes

A pair of chromosomes of the same length, centromere position, and staining pattern that carry genes controlling the same inherited characters at corresponding loci, one derived from each parent.

Alternation of Generations

A life cycle characteristic of plants and some algae that includes both multicellular diploid ($2n$) sporophyte stages and multicellular haploid ($n$) gametophyte stages.

Synapsis

The pairing and physical connection of duplicated homologous chromosomes along their lengths during prophase I of meiosis.

Crossing Over

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis, resulting in recombinant chromosomes.

Chiasmata

The X-shaped microscopic regions where crossing over occurred between homologous nonsister chromatids, holding homologous pairs together through sister chromatid cohesion until anaphase I.

Independent Assortment

The random and independent sorting of maternal and paternal homologous chromosome pairs into daughter cells at metaphase I, yielding $2^n$ possible chromosomal combinations (approximately $8.4 \times 10^6$ combinations in humans where $n = 23$).

Particulate Hypothesis

The genetic model proposed by Gregor Mendel asserting that parents pass on discrete heritable units (genes) that retain their separate identities across generations.

Law of Segregation

Mendel's first law stating that two alleles for a heritable character segregate during gamete formation and end up in different gametes, producing a $3:1$ phenotypic ratio in the $F_2$ generation of a monohybrid cross.

Testcross

The breeding of an individual displaying a dominant phenotype but an unknown genotype with a homozygous recessive individual to deduce the unknown genotype from the offspring's phenotypes.

Law of Independent Assortment

Mendel's second law stating that each pair of alleles segregates independently of any other pair of alleles during gamete formation, generating a characteristic $9:3:3:1$ phenotypic ratio in a dihybrid cross.

Incomplete Dominance

A pattern of inheritance in which heterozygotes exhibit an intermediate phenotype between the homozygous parental phenotypes, such as pink flowers in $F_1$ hybrids ($C^R C^W$) produced by crossing red ($C^R C^R$) and white ($C^W C^W$) snapdragons.

Codominance

An inheritance pattern in which the phenotypes of both alleles are distinctly and simultaneously expressed in a heterozygote, as seen in individuals with the $I^A I^B$ genotype in the human ABO blood group system.

Epistasis

A type of gene interaction wherein the phenotypic expression of one gene locus alters or masks that of a second independently inherited gene, producing modified phenotypic ratios such as $9:3:4$ in Labrador retriever coat colors.

Polygenic Inheritance

An additive genetic effect of two or more distinct genes on a single phenotypic character, causing quantitative, continuously graded variation such as human skin pigmentation.

Amniocentesis

A prenatal diagnostic procedure in which amniotic fluid containing fetal cells is sampled via a needle inserted into the uterus around the 15th week of pregnancy to conduct biochemical and karyotypic tests.

Chorionic Villus Sampling (CVS)

A prenatal technique in which a narrow tube is inserted through the cervix to suction a small sample of fetal tissue from the placenta's chorionic villi for rapid genetic testing and karyotyping.

Transformation

A permanent change in cellular genotype and phenotype resulting from the direct uptake and incorporation of foreign genetic material from the environment, discovered by Frederick Griffith using pathogenic S and nonpathogenic R strains.

Bacteriophage

A virus that infects bacterial cells, composed of DNA encapsulated within a protein head, tail sheath, and tail fibers.

Semiconservative Model

The model of DNA replication proven by Meselson and Stahl demonstrating that each newly synthesized double-stranded DNA molecule contains one original conserved parental strand and one newly assembled daughter strand.

Helicase

An enzyme that untwists the DNA double helix at replication forks, breaking hydrogen bonds between base pairs to separate the two parental strands.

Topoisomerase

An enzyme that relieves the torsional strain and supercoiling ahead of the replication fork by breaking, swiveling, and rejoining parental DNA strands.

Okazaki Fragments

Short, discontinuous segments of DNA synthesized along the lagging strand during replication, initiated with an RNA primer and elongated away from the replication fork in the $5' \rightarrow 3'$ direction.

DNA Ligase

An enzyme that catalyzes the formation of phosphodiester bonds to join the sugar-phosphate backbones of adjacent DNA fragments, sealing nicks between Okazaki fragments or repaired segments.

Nucleotide Excision Repair

A DNA repair pathway wherein a nuclease cuts out a damaged or distorted segment of a DNA strand, followed by DNA polymerase synthesizing replacement nucleotides and DNA ligase sealing the backbone.

Telomeres

Specialized, noncoding repetitive nucleotide sequences situated at the ends of eukaryotic linear chromosomes that prevent progressive gene loss and chromosome erosion during repeated rounds of replication.

Nucleosome

The fundamental unit of eukaryotic chromatin packaging, measuring approximately $10\,\text{nm}$ in diameter and consisting of double-stranded DNA wound around an octamer core of histone proteins.

Central Dogma

The molecular principle articulated by Francis Crick stating that genetic information flows sequentially from DNA to RNA via transcription, and from RNA to protein via translation.

TATA Box

A conserved promoter DNA sequence element located upstream of the eukaryotic transcription start site that is recognized and bound by general transcription factors to form the transcription initiation complex with RNA polymerase II.

Spliceosome

A large multi-component complex consisting of small nuclear ribonucleoproteins (snRNPs) and proteins that recognizes splice sites, removes noncoding introns, and splices coding exons together from pre-mRNA transcripts.

Aminoacyl-tRNA Synthetase

An enzyme that family-specifically couples an amino acid to its corresponding tRNA molecule via a covalent bond through an ATP-dependent activation reaction.

Signal-Recognition Particle (SRP)

A cytoplasmic ribonucleoprotein complex that recognizes the amino-terminal signal peptide of a nascent secretory polypeptide, temporarily halting translation and escorting the ribosome to a translocon on the endoplasmic reticulum membrane.

Silent Mutation

A point mutation involving a nucleotide-pair substitution that alters a codon but, due to the degeneracy of the genetic code, translates into the exact same amino acid, having no phenotypic consequence.

Missense Mutation

A nucleotide-pair substitution that alters a triplet codon to encode a different amino acid in the resulting polypeptide, such as the single-base change in hemoglobin causing sickle-cell disease.

Nonsense Mutation

A nucleotide-pair substitution that converts an amino acid-specifying codon into an early stop codon ($\text{UAG}$, $\text{UAA}$, or $\text{UGA}$), causing premature termination of translation and resulting in a truncated, nonfunctional polypeptide.

Frameshift Mutation

A genetic mutation caused by the insertion or deletion of a number of nucleotide pairs that is not a multiple of $3$, thereby altering the triplet codon reading frame for all downstream sequence.

Operon

A coordinated unit of bacterial gene expression comprising a cluster of functionally related structural genes, a promoter, and an operator sequence that controls their shared transcription as a single polycistronic mRNA.

Operator

A distinct regulatory DNA sequence positioned within or adjacent to a bacterial promoter that acts as an on/off switch by physically binding an active repressor protein to prevent transcription.

Repressible Operon

A bacterial operon that is continually transcribed under basal conditions but is repressed when a specific corepressor molecule binds to and activates an inactive repressor protein, exemplified by the trp operon.

Inducible Operon

A bacterial operon that is typically turned off by a constitutively active repressor but can be activated when an inducer molecule binds to and inactivates the repressor, exemplified by the lac operon.

Histone Acetylation

The covalent addition of acetyl groups to lysine residues in histone tails, which reduces the positive charge on histones, loosens chromatin architecture, and makes DNA accessible for transcription.

Enhancer

A segment of eukaryotic DNA composed of distal control elements located far from the gene that binds specific activator transcription factors to stimulate the rate of transcription initiation.

Alternative RNA Splicing

A eukaryotic post-transcriptional regulatory process wherein different combinations of exons from a primary transcript are selected for inclusion into the mature mRNA, enabling a single gene to encode multiple distinct protein variants.

Proteasome

A large cylindrical protein complex within eukaryotic cells that recognizes ubiquitin-conjugated target proteins and systematically hydrolyzes them into short peptide fragments.

microRNA (miRNA)

A small, noncoding, single-stranded RNA molecule that complexes with proteins to bind complementary sequences in target mRNAs, resulting in either transcript cleavage or translation inhibition.

Cytoplasmic Determinants

Maternally derived mRNAs and proteins heterogeneously partitioned within the egg cytoplasm that become differentially segregated into blastomeres to regulate gene expression during early development.

Bicoid Gene

A maternal effect gene in Drosophila encoding a morphogen gradient that specifies anterior body structures; embryos lacking maternal bicoid develop posterior tail structures at both ends.

Oncogene

A mutated, amplified, or abnormally active version of a normal proto-oncogene that stimulates excessive or unregulated cell division, contributing to tumor formation and malignancy.

p53 Gene

A tumor-suppressor gene encoding a transcription factor that halts the cell cycle, facilitates DNA repair, or triggers apoptosis when genomic DNA suffers damage.