Chromosome Structure, Replication, and Genetic Variation

Heterogametic sex

The sex that produces two different kinds of sex-chromosome gametes (e.g., XY males produce X- and Y-bearing sperm).

Homogametic sex

The sex that produces only one type of sex-chromosome gamete (e.g., XX females produce only X-bearing eggs).

SRY gene

Y-linked gene encoding a SOX-family transcription factor whose expression triggers testis development in mammals.

Nondisjunction

Failure of homologous chromosomes or sister chromatids to separate in meiosis, generating gametes with missing or extra chromosomes (aneuploidy).

Dosage compensation

Mechanism that equalizes X-linked gene expression between sexes; in mammals it is achieved by X-inactivation in females.

Barr body

The condensed, transcriptionally inactive X chromosome visible in female somatic cell nuclei.

Aneuploidy

Numerical chromosome abnormality involving gain or loss of individual chromosomes, such as monosomy or trisomy.

Euploidy

Having an exact multiple of the haploid chromosome set (e.g., diploid 2n, triploid 3n).

Polyploidy

Condition in which a cell or organism possesses more than two complete chromosome sets; common and often viable in plants.

Deletion (Deficiency)

Chromosomal structural mutation in which a segment is missing, potentially causing haploinsufficiency (e.g., Cri-du-chat).

Duplication

Presence of a chromosomal segment in two copies on the same chromosome, altering gene dosage and occasionally giving rise to new functions.

Inversion (pericentric/paracentric)

Chromosome segment reversed end-to-end; pericentric includes the centromere, paracentric does not, and crossovers within can yield dicentric or acentric products.

Translocation

Exchange of segments between nonhomologous chromosomes; includes reciprocal swaps and Robertsonian fusions of two acrocentrics at their centric regions.

Fragile site

Specific chromosomal locus prone to breakage under replication stress (e.g., fragile X site) and often associated with disease.

Linkage

The tendency of genes located on the same chromosome to be inherited together because of limited recombination between them.

Crossing over

Physical exchange of chromosome arms between nonsister chromatids during meiosis I, producing genetic recombination.

Recombination frequency (RF)

Percentage of recombinant progeny calculated as (number of recombinants ÷ total progeny) × 100; used to estimate genetic map distance.

Map unit (cM)

CentiMorgan; unit of genetic distance corresponding to 1 % recombination frequency between two loci.

Interference

Phenomenon in which occurrence of one crossover reduces the probability of another nearby; quantified by the coefficient of coincidence.

Watson–Crick double helix

Right-handed, antiparallel DNA structure stabilized by complementary base pairing (A=T via 2 H-bonds, G≡C via 3) and base stacking.

Alternate DNA conformations (A-DNA / B-DNA / Z-DNA)

Three helical forms: A-DNA is short, wide, dehydrated; B-DNA is the common cellular right-handed form; Z-DNA is left-handed and favored by high salt or negative supercoiling.

Semiconservative replication

Mode of DNA duplication in which each daughter duplex contains one parental strand and one newly synthesized strand.

Leading strand

DNA strand synthesized continuously 5′→3′ toward the replication fork.

Lagging strand

DNA strand synthesized discontinuously away from the fork as multiple Okazaki fragments.

Okazaki fragments

Short DNA segments (∼1–2 kb in bacteria) produced on the lagging strand and later joined by ligase.

DNA helicase

Enzyme that unwinds the parental double helix at the replication fork using ATP hydrolysis.

Primase

Specialized RNA polymerase that synthesizes short RNA primers required to initiate DNA synthesis.

DNA polymerase III (bacteria)

Main bacterial replicative polymerase with high processivity and 3′→5′ exonuclease proofreading activity.

DNA polymerase I (bacteria)

Enzyme that removes RNA primers via 5′→3′ exonuclease activity, fills gaps with DNA, and proofreads with 3′→5′ exonuclease.

DNA polymerase II (bacteria)

Bacterial polymerase primarily involved in DNA repair, possessing 3′→5′ exonuclease proofreading.

DNA polymerase α (eukaryotes)

Primase-associated polymerase that lays down short RNA-DNA primers during replication initiation.

DNA polymerase δ (eukaryotes)

High-fidelity, 3′→5′ proofreading polymerase that elongates the lagging strand in eukaryotes.

DNA polymerase ε (eukaryotes)

High-fidelity, 3′→5′ proofreading polymerase that synthesizes the leading strand in eukaryotes.

Sliding clamp (β-clamp/PCNA)

Ring-shaped protein that encircles DNA and tethers DNA polymerase to the template, greatly increasing processivity.

Clamp loader

ATP-dependent multiprotein complex that opens and places the sliding clamp around DNA and later releases it.

Single-stranded DNA-binding proteins (SSBs)

Proteins that bind and stabilize unwound single-stranded DNA, preventing secondary structure formation.

Topoisomerase

Enzyme that relieves superhelical tension ahead of the replication fork by transiently cutting and rejoining DNA strands.

DNA ligase

Enzyme that seals nicks between adjacent DNA fragments by forming phosphodiester bonds.

Origin of replication (oriC/ORC)

Specific DNA sequence where replication begins; oriC in E. coli and ORC-bound sites in eukaryotes.

Telomere

Repetitive DNA sequence that caps linear chromosome ends (e.g., TTAGGG in vertebrates), protecting them from degradation.

Telomerase

Ribonucleoprotein reverse transcriptase that extends telomeres using its internal RNA template, solving the end-replication problem.