Introduction
Chromosomes are the fundamental structural and functional units of heredity in eukaryotic cells. The term chromosome comes from the Greek words chroma (colour) and soma (body) because they readily take up certain stains and appear as distinct colored bodies under the microscope during cell division. They are composed primarily of deoxyribonucleic acid (DNA) wrapped around histone proteins, forming a complex called chromatin. Each chromosome bears many genes arranged linearly; each gene occupies a specific position called a locus.
Chromosomes perform several essential roles: they store and protect genetic information, help regulate gene expression, ensure faithful segregation of DNA during mitosis and meiosis, and enable recombination that generates genetic variability. The field that studies chromosomes and their behaviour is called cytogenetics, which intersects with genetics, molecular biology, and evolutionary biology.
Chromosome Shape
The visible shape of a chromosome is most evident during metaphase of mitosis when chromatin is highly condensed. The shape is determined primarily by the position of the centromere (primary constriction), which divides the chromosome into two arms. Based on centromere position chromosomes are classified into several morphological categories used in karyotype analysis.
Types of chromosome shapes
- Metacentric — Centromere is approximately central; both arms (p and q) are nearly equal in length. These chromosomes often look like an inverted V or X shape during anaphase. Example: some human autosomes.
- Sub-metacentric — Centromere is off-center, producing one long arm and one short arm. These appear slightly L-shaped during anaphase.
- Acrocentric — Centromere lies very close to one end; one arm is very short and may be associated with a satellite separated by a secondary constriction. Human chromosomes 13, 14, 15, 21 and 22 are acrocentric.
- Telocentric — Centromere is terminal (at the extreme end), giving the appearance of a rod with a single arm. Telocentric chromosomes are absent in humans but occur in some animals and plants.
Practical note: Centromere position is useful in cytogenetic identification, diagnosing chromosomal abnormalities, and arranging chromosomes during karyotyping.
Chromosome Size
Chromosome size varies widely among organisms and between chromosomes within the same genome. Size is usually measured in micrometres (µm) when observed cytologically; it also correlates with the amount of DNA contained (measured in base pairs or megabases at the molecular level).
Examples and key points:
- Human chromosomes range from relatively large (chromosome 1 ~ 10 µm in condensed metaphase state) to small (chromosome 21 ~ 2–3 µm).
- Plants often have larger chromosomes than animals; however, there are exceptions depending on genome size and ploidy level.
- Some special chromosomes (e.g., polytene chromosomes in Drosophila) can be extremely large — visible as thick, banded structures under low magnification.
Chromosome size affects their visibility in cytogenetic preparations and the choice of staining/banding techniques used for detailed study.
Chromosome Numbers
Each species has a characteristic and usually stable chromosome number called the chromosome complement. The count may be expressed as haploid (n) for gametes or diploid (2n) for somatic cells. Variation in chromosome number (aneuploidy, polyploidy) has major biological consequences.
Common examples
- Human — 2n = 46 (n = 23)
- Fruit fly (Drosophila melanogaster) — 2n = 8
- Pea (Pisum sativum) — 2n = 14
- Dog — 2n = 78
- Wheat (common bread wheat) — hexaploid, 2n = 6x = 42
Important concepts
- Homologous chromosomes: Pairs of chromosomes that carry genes for the same traits at corresponding loci; one homolog is inherited from each parent.
- Polyploidy: Condition of having more than two complete sets of chromosomes (common in plants; significant in evolution and breeding).
- Aneuploidy: Gain or loss of individual chromosomes (e.g., trisomy 21 in humans causes Down syndrome).
Chromosome number is a key taxonomic and evolutionary character and is extensively used in genetics and breeding programs.
Chromosome Morphology
Chromosomes are not simple rods: they contain multiple identifiable regions and structures that have distinct roles during the cell cycle. Below we discuss each part separately.
1. Chromatids
A chromosome in the duplicated state (after S-phase) consists of two identical sister chromatids joined at the centromere. Each chromatid contains one DNA molecule. During anaphase of mitosis and meiosis II, sister chromatids separate and migrate to opposite poles, ensuring each daughter cell receives an identical set of genetic information.
2. Centromere (Primary constriction)
The centromere is a specialized region of the chromosome that plays a central role in chromosome movement. It is the site where the kinetochore (a protein complex) assembles and attaches to spindle microtubules. Centromeres are also important for chromosome stability and are typically composed of repetitive DNA sequences and specific centromeric proteins.
3. Secondary constriction and Nucleolar Organizer Regions (NORs)
Some chromosomes show a secondary constriction separate from the centromere. Nucleolar organizer regions (NORs), located at or near certain secondary constrictions, contain ribosomal RNA (rRNA) gene clusters and are responsible for forming the nucleolus. Chromosomes bearing NORs often display satellite bodies beyond the constriction.
4. Telomeres
Telomeres are repetitive DNA sequences at the ends of linear chromosomes (e.g., the human repeat: TTAGGG). They protect chromosome ends from exonuclease degradation, prevent recognition as DNA breaks, and avoid end-to-end fusions. Telomeres shorten with each round of cell division in most somatic cells — a process linked to cellular ageing. Specialized enzyme telomerase can elongate telomeres in germ cells, stem cells, and many cancer cells.
5. Satellite DNA and Satellites
Some chromosomes show a small segment separated from the main body by a secondary constriction; this segment is called a satellite. Satellites commonly contain rDNA and contribute to nucleolus formation.
Summary: Each morphological component — chromatids, centromere, secondary constrictions, telomeres, and satellites — has a dedicated role in maintaining genome integrity and facilitating correct chromosome behaviour during the life cycle of the cell.
Heterochromatin and Euchromatin
Chromatin exists in two basic forms that differ in compaction state, staining properties, and transcriptional activity.
Euchromatin
Euchromatin is the less condensed, transcriptionally active portion of chromatin. It appears lightly stained in cytological preparations and contains most of the genes that are actively transcribed in a given cell type. Euchromatic regions are accessible to transcription factors and the transcriptional machinery, enabling gene expression necessary for cellular function and differentiation.
Heterochromatin
Heterochromatin is highly condensed and stains darkly. It is usually transcriptionally inactive and serves structural roles. Two main forms exist:
- Constitutive heterochromatin: Permanently condensed, found at centromeres and telomeres; enriched in repetitive DNA sequences.
- Facultative heterochromatin: Can switch between condensed and decondensed states depending on developmental stage or cell type (example: the inactive X chromosome or Barr body in female mammals).
Heterochromatin contributes to chromosome stability, suppresses recombination in repetitive regions, and plays a role in regulating gene expression through chromatin modifications.
Classification of Chromosomes
Multiple classification schemes are used depending on the context — morphological, functional, or cytogenetic.
1. Based on Centromere Position (Morphology)
Metacentric, Sub-metacentric, Acrocentric, and Telocentric (discussed earlier under Shape).
2. Based on Function
- Autosomes: Chromosomes that are not involved in determining the sex of the organism; they carry the majority of genetic information responsible for somatic traits.
- Sex chromosomes: Chromosomes that determine sex (e.g., X and Y in mammals, Z and W in birds). These chromosomes often show unique inheritance patterns and dosage compensation mechanisms.
3. Based on Size
- Macrochromosomes: Large chromosomes easily visualized under a light microscope.
- Microchromosomes: Very small chromosomes, common in birds and some reptiles; often gene-rich despite their small size.
4. Based on Staining/Banding Patterns
Banding techniques used in karyotyping (G-banding, C-banding, Q-banding, R-banding) help to identify individual chromosomes, detect structural rearrangements (deletions, duplications, translocations), and locate genes. These patterns are critical in clinical cytogenetics and evolutionary studies.
Special Types of Chromosomes
Certain chromosomes differ drastically from the classical metaphase chromosome and are adapted to specific biological functions or arise from unusual replication patterns.
1. Polytene Chromosomes
Polytene chromosomes are giant chromosomes formed by repeated rounds of DNA replication without cell division (endoreduplication). Each polytene chromosome is a bundle of many aligned chromatids (hundreds to thousands) that produce conspicuous banding patterns. They are famously found in the salivary glands of Drosophila larvae and were historically important for gene mapping and cytogenetic studies. The dark and light bands represent regions of condensed and less condensed chromatin respectively, and puffs indicate active transcription.
2. Lampbrush Chromosomes
Lampbrush chromosomes occur in the oocytes of amphibians, birds, and some fish during prolonged prophase I of meiosis. They are characterized by numerous lateral loops that are sites of intense transcriptional activity, providing RNA and proteins needed for oocyte growth and early embryonic development. Their appearance resembles a brush used to clean lamp chimneys — hence the name.
3. B-Chromosomes (Accessory or Supernumerary Chromosomes)
B-chromosomes are additional chromosomes found in some species beyond the normal set (A-chromosomes). They are usually smaller, vary in number between individuals, and are often heterochromatic and gene-poor. Although non-essential, B-chromosomes can influence phenotype, fertility, and genome evolution in certain contexts.
4. Giant/Endopolyploid Chromosomes in Plants and Specialized Cells
Many plant tissues and specialized animal cells undergo endoreduplication, producing nuclei with multiple chromosome sets and unusually large chromosomes. These cells often exhibit increased metabolic activity (e.g., secretory or storage tissues) and the enlarged chromosomes reflect greater gene dosage.
Application note: Special chromosome types are valuable tools in cytogenetics and developmental biology; for example, lampbrush and polytene chromosomes reveal chromosomal organisation and gene activity at a resolution not seen in condensed metaphase chromosomes.
