Chromosome Structure and Chromosomal Mutations

Vocabulary terms and definitions addressing viral, bacterial, eukaryotic, and archaeal chromosome packaging, chromatin levels, and structural chromosomal mutations including aneuploidies, deletions, inversions, and translocations.

Capsid

The protein shell of a virus that encloses its genetic material.

Directed viral assembly

The capsid assembly process observed in complex viruses (such as T4 Bacteriophage) that requires viral-encoded non-capsid scaffolding proteins rather than self-assembling.

Positive-sense single-stranded RNA virus

A virus possessing a single-stranded RNA genome that directly codes for protein and can be translated immediately upon entry into the host cell (e.g., SARS-CoV-2).

Negative-sense single-stranded RNA virus

A virus whose RNA genome cannot be translated directly and must first be transcribed into a complementary positive-sense RNA strand using RNA-dependent RNA polymerase (e.g., Rabies virus).

Reverse transcriptase

A viral polymerase enzyme that transcribes an RNA template into a complementary DNA strand.

Integrase

A viral enzyme that inserts and integrates reverse-transcribed viral DNA into the host cell's genome (e.g., HIV).

Nucleoid

The distinct, densely compacted intracellular region where the bacterial chromosome is localized without a surrounding nuclear membrane.

Nucleoid-associated proteins (NAPs)

Small bacterial proteins, including HU and H-NS, that bind and organize the bacterial chromosome by securing large DNA loops.

Structural maintenance of chromosome (SMC) proteins

Proteins responsible for securing small loop domains within larger looped regions during bacterial chromosome condensation.

DNA supercoiling

The over- or under-winding of the DNA helix that compacts bacterial chromosomes, altering relaxed DNA ($10.0\,\text{bp}/\text{helical turn}$) to a supercoiled state ($12.5\,\text{bp}/\text{helical turn}$).

Chromatin

The eukaryotic nucleoprotein complex constituting chromosomes, composed by weight of approximately half DNA and half protein.

Nucleosome core particle

An octameric protein complex composed of two copies each of histones H2A, H2B, H3, and H4, around which $146\,\text{bp}$ of DNA is wrapped.

Histone H1

A linker histone protein that binds to linker DNA adjacent to the core particle to stabilize chromatin into higher-order solenoid structures.

Solenoid structure

A compact, hollow, coiled cylindrical chromatin fiber approximately $34\,\text{nm}$ in diameter consisting of $6$ to $8$ nucleosomes per turn.

$300\text{-nm}$ fiber

A condensed interphase chromatin structure formed by chromatin loops ($20{,}000$ to $100{,}000\,\text{bp}$) anchored to a non-histone protein scaffold.

Karyotype

The complete visual profile of an individual's chromosomes, displaying chromosome number, relative sizes, centromere positions, and banding patterns.

Euchromatin

Relatively decondensed regions of chromatin that appear as light bands upon chemical staining and exhibit active gene expression.

Heterochromatin

Densely compacted chromatin appearing as dark staining bands that exhibits low transcriptional activity; classified as either constitutive (permanently condensed) or facultative (variable levels of condensation).

Centromeric DNA sequences (CEN)

Conserved, highly repetitive DNA elements located at the centromere that serve as the binding platform for kinetochore proteins and spindle microtubules.

Archaeal chromosome

A circular chromosome localized without a nucleus (bacterial feature) that is packaged with histone proteins structurally related to eukaryotic histones H3 and H4.

Nondisjunction

The failure of homologous chromosomes to segregate during meiosis I, or sister chromatids to separate during meiosis II, yielding gametes with abnormal chromosome numbers.

Euploidy

The normal chromosomal state in which somatic cells contain an exact integer multiple ($2n$) of the basic haploid gametic chromosome number ($n$).

Aneuploidy

An abnormal chromosomal condition marked by the gain or loss of individual chromosomes from the normal set, frequently producing gene dosage imbalances and reduced fertility.

Barr body

A densely condensed, transcriptionally inactive X chromosome (constitutive heterochromatin) present in somatic cells of mammalian females.

Klinefelter syndrome

A sex-chromosome aneuploidy in human males characterized by a $47, \text{XXY}$ karyotype, causing variable secondary sexual characteristics, infertility, frequent breast swelling, and normal mental capacity.

Turner syndrome

A sex-chromosome aneuploidy in human females characterized by a $45, \text{XO}$ karyotype, short stature, webbed neck, absence of secondary sexual characteristics, and infertility.

Jacob syndrome

A sex-chromosome aneuploidy in human males with a $47, \text{XYY}$ karyotype, commonly presenting with tall stature and possible reduction in fertility without affecting mental capacity.

Autopolyploidy

A polyploid condition in which an organism possesses three or more complete sets of chromosomes derived entirely from a single species.

Allopolyploidy

A polyploid state resulting from the hybridization of genomes from two distinct species, typically requiring chromosome doubling to produce fertile, viable gametes.

Terminal deletion

A chromosomal mutation involving a single break that removes an end terminal fragment of a chromosome, generating an acentric fragment that fails to segregate and is lost during division.

Cri-du-chat syndrome

A human congenital disorder resulting from a terminal deletion that causes infants to have a distinctive cat-like cry, high-pitched voice, and severe intellectual disability.

Interstitial deletion

The loss of an internal, centromere-lacking chromosomal segment caused either by two internal breaks or by unequal crossing over (e.g., WARGO syndrome on chromosome $11$).

Paralog

Homologous genes resulting from duplication that reside within the same genome, frequently acting as sites for unequal crossover events.

Unequal crossover

An aberrant meiotic recombination event between misaligned homologous chromosomes that produces a partial duplication on one chromatid and a partial deletion on the other (e.g., Williams-Beuren syndrome).

Paracentric inversion

A chromosomal structural rearrangement occurring when a fragment breaks and reattaches in reverse orientation entirely within one chromosome arm, excluding the centromere.

Pericentric inversion

A chromosomal inversion resulting from breaks occurring in both chromosome arms such that the inverted segment spans and includes the centromere.

Inversion loop

A loop-like pairing conformation formed during prophase I of meiosis to allow homologous alignment between an inverted chromosomal segment and its non-inverted homolog.

Dicentric chromosome

An abnormal recombinant chromosome possessing two centromeres, produced by a crossover event inside a paracentric inversion loop, which risks breakage during anaphase.

Acentric fragment

A broken segment of a chromosome that lacks a centromere and kinetochore assembly, causing it to fail to attach to spindle fibers and be lost during cell division.

Reciprocal balanced translocation

A chromosomal rearrangement in which two non-homologous chromosomes mutually exchange broken segments without net loss or gain of genetic material, resulting in approximately $50\%$ viable gametes.

Chromosome fusion

A large-scale chromosomal mutation where two ancestral chromosomes join into a single chromosome, as demonstrated in human chromosome $2$ by internal telomeric sequences and an inactivated vestigial centromere.