Unit 2: Chromosomal architecture, cell division and genetic principles

I. Orientation — Chromosomes as units of heredity and continuity

Chromosomes are organized DNA–protein structures that carry genes, replicate before cell division, and segregate into daughter cells. Their architecture connects molecular heredity with the cytological events of the cell cycle, mitosis, and meiosis.

• Defining properties:
  • Chemical composition: Eukaryotic chromosomes contain DNA, histone proteins, non-histone proteins, and small amounts of RNA.
  • Chromatin organization: DNA wraps around histone octamers to form nucleosomes; successive levels of folding compact long DNA molecules within the nucleus.
  • Chromosome constancy: Members of a species usually possess a characteristic chromosome number, size range, and morphology called the karyotype.
  • Replication and segregation: Each chromosome replicates once during S phase, producing sister chromatids that separate during division.
  • Genetic continuity: Mitosis largely preserves chromosome number, whereas meiosis halves it and generates variation.
  • Cytological convention: Chromosome number is written as n for one set and 2n for two homologous sets; DNA amount is represented by C.

II. Chromosomal Architecture — Structural organization of hereditary material

A. Architecture of chromosomes

Chromosomal architecture describes the hierarchical packaging and visible organization of a DNA molecule with its associated proteins.

• Nucleosome level: About 147 base pairs of DNA wrap approximately 1.7 turns around a histone octamer containing two copies each of H2A, H2B, H3, and H4.
• Linker organization: Linker DNA joins adjacent nucleosomes, while histone H1 helps stabilize higher-order compaction.
• Chromatin states:
  1. Euchromatin: Less condensed, generally gene-rich, transcriptionally accessible, and early-replicating.
  2. Heterochromatin: Highly condensed and generally transcriptionally inactive.
     • Constitutive heterochromatin occurs permanently at regions such as centromeres.
     • Facultative heterochromatin forms conditionally, as in the inactive mammalian X chromosome.
• Metaphase structure: A replicated metaphase chromosome consists of two sister chromatids joined at the centromere; each chromatid contains one continuous DNA double helix.
• Chromosome arms: The centromere divides each chromatid into a short arm, designated p, and a long arm, designated q.

B. Chromonemata

Chromonemata are thread-like, coiled structures historically observed within chromatids and correspond broadly to highly organized chromatin fibers.

• Organization: Each chromatid contains a chromonemal thread formed by DNA–protein material arranged into loops and progressively compacted domains.
• Coiling: Chromonemata undergo condensation during prophase and reach maximum compaction at metaphase; they decondense during telophase.
• Functional basis: Coiling shortens chromosomes, limits entanglement, and enables accurate movement through the mitotic or meiotic spindle.
• Modern interpretation: The older model of a single uniformly coiled thread has been refined by evidence for nucleosomes, loop domains, chromosome scaffolds, and condensin-mediated folding.

C. Chromosome matrix

The chromosome matrix refers to the non-DNA structural framework historically proposed to surround or support chromonemal material.

• Composition: It includes non-histone proteins such as topoisomerase II, condensin complexes, and other chromosome-associated proteins.
• Scaffold role: Chromatin loops attach to an axial protein framework, helping organize each chromatid as an elongated structure.
• Mechanical function: Scaffold proteins support chromosome condensation, rigidity, and resolution of intertwined sister DNA molecules.
• Conceptual limitation: The matrix is not a uniform inert substance; chromosome organization is dynamic and depends on protein-mediated loops and molecular interactions.

D. Chromomeres

Chromomeres are bead-like, locally condensed regions visible along chromosomal threads, especially during meiotic prophase and in giant chromosomes.

• Appearance: Alternating condensed chromomeres and less-condensed interchromomeric regions produce a beaded chromosome pattern.
• Observation: Chromomeres are conspicuous during leptotene and zygotene of meiosis and along lampbrush chromosomes.
• Organization: Each chromomere represents a domain of compacted chromatin rather than necessarily a single gene.
• Cytological value: Their reproducible patterns can help identify corresponding regions of homologous chromosomes.

E. Centromere

The centromere is the primary constriction where sister chromatids remain associated and spindle attachment is organized.

• Kinetochore formation: A multiprotein kinetochore assembles on centromeric chromatin and binds spindle microtubules.
• Segregation function: Correct bipolar attachment permits sister chromatids or homologous chromosomes to move toward opposite poles.
• Centromere position:
  • Metacentric: Central centromere; arms are approximately equal.
  • Submetacentric: Off-center centromere; arms are unequal.
  • Acrocentric: Centromere lies close to one end, producing a very short p arm.
  • Telocentric: Centromere is terminal; this form does not occur in the normal human karyotype.
• Failure of function: Incorrect attachment can cause nondisjunction and aneuploid daughter cells.

F. Secondary constriction and telomere

Secondary constrictions mark specialized non-centromeric regions, whereas telomeres protect the physical ends of linear chromosomes.

1. Secondary constriction:

   • Nucleolar organizer region: Many secondary constrictions contain tandemly repeated ribosomal RNA genes and organize nucleolus formation.
   • Satellite: A distal chromosome segment separated by a secondary constriction is called a satellite; human acrocentric chromosomes carry satellites.
   • Distinction: Unlike the primary constriction, a secondary constriction does not normally serve as the principal kinetochore site.

2. Telomere:

   • Protective sequence: Vertebrate telomeres contain repeated TTAGGG DNA sequences bound by shelterin proteins.
   • End protection: They prevent chromosome ends from being recognized as DNA breaks and inhibit end-to-end fusion.
   • Replication problem: Conventional DNA polymerases cannot completely copy linear DNA ends, causing progressive shortening in many somatic cells.
   • Telomerase: This RNA-dependent DNA polymerase extends telomeres in germ cells, many stem cells, and most cancer cells.

III. Specialized Chromosomal Forms — Adaptations in structure and function

A. Special types of chromosomes

Special chromosomes display unusual size, replication, morphology, or inheritance adapted to particular cellular functions.

• Polytene chromosomes: Repeated DNA replication without chromatid separation produces giant, banded chromosomes, as in Drosophila larval salivary glands; transcriptionally active regions form visible puffs.
• Lampbrush chromosomes: Large diplotene chromosomes in growing amphibian oocytes possess lateral loops where intense RNA synthesis occurs.
• B chromosomes: Supernumerary chromosomes occur in addition to the normal A-chromosome set; they are often dispensable, heterochromatic, and irregularly inherited.
• Holocentric chromosomes: Kinetochore activity extends along much of the chromosome rather than occupying one localized centromere, as in some nematodes and insects.
• Sex chromosomes: X, Y, Z, or W chromosomes participate in sex determination and may differ markedly in gene content and morphology.
• SAT chromosomes: These possess a satellite separated from the main chromosome body by a secondary constriction associated with nucleolar organization.

IV. Chromosomal Basis of Heredity — Linking genes with chromosome behavior

A. Chromosomal theory of inheritance

The chromosomal theory of inheritance, developed independently by Walter Sutton and Theodor Boveri in 1902–1903, states that genes reside on chromosomes whose meiotic behavior explains Mendelian inheritance.

• Parallel with Mendel’s factors: Homologous chromosomes occur in pairs in diploid organisms, just as hereditary factors occur as allele pairs.
• Segregation: Homologous chromosomes separate during anaphase I, placing only one member of each pair into a gamete.
• Independent assortment: Different homologous pairs orient independently at metaphase I when their genes are on different chromosomes or sufficiently far apart.
• Fertilization: Fusion of haploid gametes restores the diploid chromosome number and combines maternal and paternal chromosomes.
• Morgan’s evidence: Thomas Hunt Morgan’s work on X-linked white eye colour in Drosophila connected a specific trait with a specific chromosome.
• Linkage qualification: Genes on the same chromosome tend to be inherited together, but crossing over can produce recombinant combinations; recombination frequency is therefore used to estimate gene distance.

V. Cell-Cycle Control — Ordered growth, replication, and division

A. Cell cycle

The cell cycle is the regulated sequence through which a cell grows, duplicates its genome, and divides.

• Interphase:
  • G1 phase: Cellular growth and biosynthesis occur; a diploid cell has 2n chromosomes and approximately 2C DNA.
  • S phase: DNA replication creates sister chromatids; chromosome number remains 2n, but DNA content rises from 2C to 4C.
  • G2 phase: The cell checks replicated DNA and synthesizes proteins needed for division.
• M phase: Nuclear division is followed by cytokinesis, distributing genetic material and cytoplasm.
• G0 state: Some cells leave the active cycle temporarily or permanently; mature neurons commonly remain in G0.
• Regulation: Cyclins activate cyclin-dependent kinases at specific stages, while checkpoint pathways delay progression if conditions are unsuitable.
• Major checkpoints: The G1/S checkpoint assesses growth and DNA damage; G2/M checks replication; the spindle checkpoint verifies chromosome attachment before anaphase.

VI. Mitosis — Equational division for cellular continuity

A. Mitosis

Mitosis separates replicated chromosomes into two genetically similar nuclei while normally preserving chromosome number.

• Prophase: Chromatin condenses, nucleoli disappear, and the spindle begins forming as centrosomes separate.
• Prometaphase: The nuclear envelope breaks down, and spindle microtubules attach to kinetochores.
• Metaphase: Chromosomes align at the metaphase plate with sister kinetochores attached to opposite spindle poles.
• Anaphase: Cohesin is cleaved, allowing sister chromatids to separate and move poleward; each chromatid becomes an independent chromosome.
• Telophase: Chromosomes arrive at the poles, decondense, and become enclosed by newly formed nuclear envelopes.
• Cytokinesis: Animal cells use an actin–myosin contractile ring; plant cells construct a cell plate from Golgi-derived vesicles.
• Significance: Mitosis supports growth, tissue repair, cell replacement, and asexual reproduction while maintaining the parental karyotype.

VII. Meiosis — Reduction division and genetic variation

A. Meiosis

Meiosis consists of one DNA replication followed by two nuclear divisions, producing haploid cells from a diploid precursor.

• Meiosis I:
  • Prophase I: Homologues pair by synapsis, forming bivalents or tetrads.
  • Leptotene initiates condensation; zygotene begins synapsis; pachytene permits crossing over; diplotene reveals chiasmata; diakinesis completes condensation.
  • Metaphase I: Homologous pairs align independently at the equator.
  • Anaphase I: Homologues separate while sister centromeres remain together, reducing 2n to n.
• Meiosis II: Without another S phase, chromosomes align individually and sister chromatids separate in an equational division resembling mitosis.
• Variation mechanisms: Crossing over exchanges DNA between non-sister chromatids, while independent assortment produces different maternal–paternal chromosome combinations.
• Outcome: One diploid cell generally forms four haploid products; spermatogenesis yields four sperm, whereas oogenesis commonly yields one ovum and polar bodies.
• Biological significance: Meiosis maintains chromosome number across sexual generations and generates genetically diverse gametes.