Chromosome Structure, Abnormalities, and DNA Mutations

Comprehensive vocabulary flashcards covering chromosome structure, morphologies, banding patterns, numerical and structural aberrations, translocations, and molecular DNA point mutations.

Metacentric Chromosome

A chromosome morphology in which the centromere is positioned near the middle, resulting in chromosome arms of roughly equal length.

Submetacentric Chromosome

A chromosome morphology in which the centromere is located between the center and the tip, establishing a distinct short arm ($p$) and long arm ($q$).

Acrocentric Chromosome

A chromosome morphology in which the centromere is positioned very close to one end, producing a very short $p$ arm that often contains a satellite structure.

Telocentric Chromosome

A chromosome morphology in which the centromere is located at the terminal tip, resulting in the absence of a short arm (no $p$ arm).

Karyotype

An ordered photographic display of a complete set of chromosomes for a species or individual, organized into homologous pairs arranged in descending order of size, with sex chromosomes identified separately.

G (Giemsa) Banding

The standard cytogenetic staining method for human chromosomes that produces distinct, reproducible light and dark bands, providing a standardized address for regions along each chromosome.

Fluorescent In Situ Hybridization (FISH)

A molecular cytogenetic technique utilizing short, fluorescently labeled DNA probes that hybridize to specific genes or repetitive chromosome sequences (such as centromeres or telomeres) to visualize their physical positions.

Euchromatin

Chromatin regions that are less condensed during interphase and contain actively expressed genes.

Heterochromatin

Chromatin regions that remain highly condensed during interphase and contain relatively few expressed genes.

Facultative Heterochromatin

Heterochromatin that exhibits variable condensation states, being condensed in some cells or developmental stages and decondensed in others.

Constitutive Heterochromatin

Chromatin that remains permanently condensed across all states, is composed primarily of repetitive DNA sequences, and is prominently concentrated at centromeres and telomeres.

Chromosome Territory

The discrete, dedicated physical volume occupied by an individual chromosome within the interphase nucleus.

Interchromosomal Domain

The channel-like spaces located between chromosome territories in the nucleus that facilitate the transport and diffusion of proteins, enzymes, and RNA molecules.

CENP-A

A specialized histone $H3$ variant that binds centromeric DNA, whose N-terminal tail enables the recruitment and binding of kinetochore proteins to the centromere.

Euploidy

The state of possessing a chromosome number that is an exact multiple of the complete basic chromosome set for an organism (e.g., $n$, $2n$, $3n$).

Aneuploidy

A condition in which a cell or organism has an abnormal chromosome number that is not an exact multiple of the euploid set, resulting from the gain or loss of individual chromosomes.

Nondisjunction

The failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to abnormal chromosome numbers in daughter cells or gametes.

Meiosis I Nondisjunction

The failure of homologous chromosome pairs to separate during the first meiotic division, producing four aneuploid gametes: two with $(n+1)$ chromosomes and two with $(n-1)$ chromosomes.

Meiosis II Nondisjunction

The failure of sister chromatids to separate during the second meiotic division, resulting in two normal haploid gametes ($n$), one $(n+1)$ gamete, and one $(n-1)$ gamete.

Trisomy

An aneuploid condition in a diploid organism characterized by the presence of three copies of a specific chromosome ($2n+1$).

Monosomy

An aneuploid condition in a diploid organism characterized by the absence of one chromosome of a pair ($2n-1$).

Gene Dosage

The number of copies of a given gene present in a genome; imbalances in dosage disrupt relative levels of gene products and frequently cause severe phenotypic abnormalities in animals.

Down Syndrome Critical Region (DSCR)

A specific segment of human chromosome 21 that is correlated with the principal phenotypic manifestations and symptoms of Down syndrome (trisomy 21).

DYRK

A dosage-sensitive candidate gene located on chromosome 21 within the DSCR that causes learning defects when overexpressed and makes a major contribution to Down syndrome cognitive impairment.

DSCAM

A gene on chromosome 21 whose increased gene dosage is linked to congenital heart abnormalities observed in Down syndrome.

Klinefelter Syndrome

A human sex-chromosome aneuploidy with the karyotype 47, XXY, occurring at a frequency of approximately 1 in 1000 live male births.

Jacob Syndrome

A human sex-chromosome aneuploidy with the karyotype 47, XYY, occurring at a frequency of approximately 1 in 1000 live male births.

Triple X Syndrome

A human sex-chromosome aneuploidy with the karyotype 47, XXX, occurring at a frequency of approximately 1 in 1000 live female births.

Turner Syndrome

A human sex-chromosome monosomy with the karyotype 45, XO, occurring at an incidence of approximately 1 in 5000 live female births.

Chromosome Break Point

The specific nucleotide position along a chromosome where double-stranded DNA cleavage occurs during chromosome breakage.

Acentric Fragment

A chromosome fragment generated by breakage that lacks a centromere and is consequently lost during subsequent cell divisions due to inability to attach to spindle fibers.

Terminal Deletion

A chromosome mutation caused by a single double-stranded break that detaches and eliminates the tip of a chromosome arm lacking a centromere, such as in cri-du-chat syndrome.

Interstitial Deletion

A chromosomal aberration resulting from two breaks within a chromosome arm followed by the excision and loss of the intervening internal segment (e.g., deletions at band 11p1.3 in WAGR syndrome).

Unequal Crossing Over

An aberrant meiotic recombination event occurring between misaligned homologous chromosomes, yielding one recombinant chromosome with an insertion/duplication and the other with a deletion.

Paracentric Inversion

A chromosomal structural rearrangement in which two breaks occur within the same arm and the intervening segment is reinserted inverted $180^{\circ}$, without including the centromere.

Pericentric Inversion

A chromosomal structural rearrangement resulting from breaks in both arms where the intervening inverted segment includes the centromere.

Inversion Loop

A characteristic circular loop structure formed during synapsis in prophase I that allows homologous pairing between an inverted chromosome segment and its non-inverted homolog.

Dicentric Bridge

A temporary structure formed during anaphase I when a recombinant dicentric chromosome with two centromeres is pulled toward opposite spindle poles until it breaks at a random point.

Unbalanced Translocation

A chromosomal rearrangement in which a broken segment of one chromosome attaches to a nonhomologous chromosome without an exchange, altering total genetic content.

Reciprocal Balanced Translocation

A structural rearrangement in which fragments from two nonhomologous chromosomes break and trade places symmetrically, maintaining gene dosage balance.

Robertsonian Translocation

A chromosomal rearrangement (chromosome fusion) in which the long arms of two nonhomologous acrocentric chromosomes fuse at their centromeres, typically accompanied by the loss of their tiny short ($p$) arms.

Philadelphia Chromosome

A shortened derivative chromosome 22 generated by a reciprocal balanced translocation with chromosome 9 ($\text{t}(9;22)$) that creates an oncogenic $BCR\text{-}ABL$ fusion gene in chronic myelogenous leukemia (CML).

Hotspot of Mutation

A gene or chromosomal locus that exhibits a significantly elevated mutation frequency compared to the genomic background average, frequently correlated with large gene size.

Silent Mutation

A base-pair substitution in a protein-coding sequence that alters a codon but does not change the specified amino acid due to the degeneracy/redundancy of the genetic code.

Missense Mutation

A base-pair substitution mutation that changes a codon such that it encodes a different amino acid in the resulting polypeptide chain.

Nonsense Mutation

A base-pair substitution mutation that replaces an amino acid-specifying codon with a premature translation termination (stop) codon.

Transition Mutation

A base-pair substitution in which a purine is replaced by another purine ($\text{A} \leftrightarrow \text{G}$) or a pyrimidine is replaced by another pyrimidine ($\text{C} \leftrightarrow \text{T}$).

Transversion Mutation

A base-pair substitution in which a purine is replaced by a pyrimidine, or a pyrimidine is replaced by a purine ($\text{A} \leftrightarrow \text{C}$, $\text{A} \leftrightarrow \text{T}$, $\text{G} \leftrightarrow \text{C}$, or $\text{G} \leftrightarrow \text{T}$).

Frameshift Mutation

The insertion or deletion of one or more base pairs (not divisible by three) within an mRNA coding sequence, altering the reading frame and resulting in abnormal downstream amino acids and premature stop codons.