Chromosomes

1. Chromosomes, DNA and genes

A chromosome is a DNA-containing structure that stores and transmits genetic information. In eukaryotes, including animals, plants and fungi, nuclear chromosomes consist of long, linear DNA molecules associated with proteins. This DNA–protein complex is called chromatin. During most of the cell cycle, chromatin is relatively dispersed; during cell division, it condenses into individually distinguishable chromosomes. Chromosomes therefore exist even when they are not visible as compact bodies under an ordinary light microscope.

A gene is a DNA sequence that contributes to a functional product, such as a protein or functional RNA. Its position on a chromosome is its locus; alternative forms of a gene are called alleles. A chromosome contains many genes as well as regulatory and other non-coding sequences. The genome is an organism’s complete genetic material, whereas a karyotype describes its chromosome complement, including chromosome number and morphology.

Most bacteria have a principal circular chromosome located in a nucleoid rather than a membrane-bound nucleus. However, some bacteria have linear chromosomes or multiple chromosomes, so universal claims are incorrect. Plasmids are independently replicating DNA molecules distinct from the principal bacterial chromosome. Mitochondria and chloroplasts also possess their own genomes, illustrating that not all DNA in a eukaryotic cell occurs in the nucleus.

• Chromatin describes the DNA–protein material; chromosome describes an organised hereditary unit made of that material.
• Homologous chromosomes carry corresponding gene loci but may carry different alleles.
• A larger chromosome count does not necessarily imply more genes, greater intelligence or greater biological complexity.

2. Structure and packaging

In eukaryotic chromatin, DNA wraps around histone protein cores to form nucleosomes, the basic units of packaging. Further folding allows very long DNA molecules to fit inside a microscopic nucleus. Packaging is not merely mechanical: chromatin organisation helps regulate access to genes. Euchromatin is generally less condensed and more accessible for transcription, while heterochromatin is generally more condensed and less transcriptionally active.

The centromere is a specialised chromosome region where the kinetochore forms. Spindle microtubules attach to the kinetochore during division, enabling chromosome movement and segregation. The centromere divides a chromosome into a short p arm and a long q arm. Based on centromere position, chromosomes may be described as metacentric, submetacentric, acrocentric or telocentric. Normal human chromosomes include the first three categories, but not telocentric chromosomes.

Telomeres are repetitive DNA–protein structures at the ends of linear chromosomes. They help protect chromosome ends from degradation and inappropriate fusion. Telomerase can extend telomeric DNA and is active in germ-line cells, certain stem cells and many cancers. Telomere shortening is associated with replicative ageing, but it is not a complete explanation of organismal ageing. After DNA replication, a chromosome consists of two sister chromatids held together until segregation; the familiar X-shaped drawing represents this replicated, condensed condition, not a chromosome’s permanent shape.

• Centromere: essential region for accurate segregation through kinetochore formation.
• Telomere: protective structure at a linear chromosome’s end.
• Sister chromatids: replicated copies of one chromosome, initially nearly identical except for replication errors.

3. Chromosomes during mitosis and meiosis

Diploid cells contain two chromosome sets, represented as 2n; haploid cells contain one set, represented as n. In humans, 2n equals 46 and n equals 23. A typical diploid cell in G1 has 46 chromosomes and 46 chromatids. During the S phase of interphase, DNA replicates. By G2, it still has 46 chromosomes but now has 92 chromatids. Thus, chromosome number, chromatid number and DNA quantity are related but distinct measurements.

Mitosis normally produces two daughter cells with the same chromosome number as the parent cell. It supports growth, tissue repair and many forms of asexual reproduction. Sister chromatids separate during anaphase and are then counted as daughter chromosomes. Consequently, before cytokinesis, the undivided human cell temporarily contains 92 chromosomes, with 46 moving towards each pole. Each resulting daughter cell receives 46.

Meiosis involves one round of DNA replication followed by two divisions. Homologous chromosomes pair during prophase I, and crossing over occurs between non-sister chromatids of homologous chromosomes. Homologues separate in meiosis I, reducing chromosome sets from diploid to haploid. Sister chromatids separate in meiosis II. Crossing over and independent assortment generate genetic variation. In humans, spermatogenesis generally produces four functional sperm from one primary spermatocyte, whereas oogenesis produces one large functional ovum and polar bodies.

• Using C for DNA quantity in one unreplicated haploid genome: human G1 is 2n, 2C; G2 is 2n, 4C.
• After meiosis I, each daughter cell is n, 2C; after meiosis II, each product is n, 1C.
• There is no intervening S phase between meiosis I and meiosis II.

4. Sex chromosomes and chromosomal abnormalities

In the usual human XX–XY system, ova carry an X chromosome, while sperm carry either X or Y. The sperm’s sex-chromosome contribution therefore establishes the usual XX or XY combination at fertilisation. The SRY gene, normally on the Y chromosome, commonly initiates testis development. However, sex development also depends on other genes, hormones and tissue responses; chromosomal patterns alone do not explain every variation. Other organisms use different systems: birds generally have ZZ males and ZW females.

Failure of homologous chromosomes or sister chromatids to separate properly is called nondisjunction. It can produce aneuploidy, the gain or loss of individual chromosomes. Down syndrome usually results from trisomy 21; Edwards syndrome from trisomy 18; and Patau syndrome from trisomy 13. Turner syndrome commonly involves 45,X, while Klinefelter syndrome commonly involves 47,XXY. Mosaic forms occur when different cell populations within one individual possess different chromosome complements.

Polyploidy differs from aneuploidy because it involves additional complete chromosome sets. It is common in plants and important in crop evolution and breeding; bread wheat is hexaploid, with 42 chromosomes. Structural changes include deletion, duplication, inversion and translocation. A balanced rearrangement may preserve the overall amount of genetic material and cause no obvious symptoms, yet still affect reproduction. A rearrangement can also disrupt a gene or alter its regulation.

• Trisomy means three copies of a particular chromosome, not three complete chromosome sets.
• Most Down syndrome cases involve nondisjunction, but translocation-associated and mosaic cases also occur.
• Not every mutation changes chromosome number or produces a visible chromosome abnormality.

5. Diagnosis, biotechnology and Indian relevance

Conventional karyotyping examines chromosome number and large structural changes, usually using stained metaphase chromosomes. It can identify common aneuploidies but generally cannot detect single-base mutations or very small deletions. Fluorescence in situ hybridisation, or FISH, uses labelled DNA probes to detect particular chromosome regions. Chromosomal microarrays identify smaller gains and losses of DNA, although balanced rearrangements may escape detection. DNA sequencing examines nucleotide order and complements rather than simply replaces these techniques.

Prenatal screening estimates the chance of certain chromosomal conditions; it does not by itself establish a diagnosis. Non-invasive prenatal testing analyses cell-free DNA in maternal blood, with the pregnancy-associated component arising mainly from the placenta. A high-risk result generally requires counselling and consideration of diagnostic confirmation through chorionic villus sampling or amniocentesis. Test interpretation must account for factors such as placental mosaicism and the condition’s prevalence.

In India, the Pre-Conception and Pre-Natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994, regulates relevant diagnostic techniques and prohibits sex selection and communication of fetal sex. It does not impose a blanket ban on legitimate prenatal diagnosis of genetic or chromosomal disorders. Chromosome science also informs cancer treatment, assisted reproduction, conservation genetics and crop improvement. Genetic counselling should explain uncertainty and reproductive options without coercion, stigma or discrimination against persons with disabilities.

• A screening result indicates risk; a diagnostic result provides stronger condition-specific evidence.
• A normal karyotype does not rule out all genetic diseases.
• Genetic disorders are not necessarily inherited: some arise from new mutations or chromosome-segregation errors.

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