Chromosome
A chromosome is a long, thread-like structure composed of DNA and associated histone proteins (chromatin) that carries genes in a linear sequence. In humans, the diploid somatic cells contain 46 chromosomes (23 pairs), with each chromosome harbouring thousands of genes. Chromosomes are visible under a light microscope during cell division and are essential for the accurate segregation of genetic material to daughter cells.
Human Chromosome Summary
| Category | Number | Type | Notes |
|---|---|---|---|
| Somatic (diploid) | 46 | 22 autosome pairs + 1 sex pair | Standard body cells |
| Gametes (haploid) | 23 | 22 autosomes + 1 sex chromosome | Sperm or egg cells |
| Sex chromosomes (female) | 2 | XX | Homologous pair |
| Sex chromosomes (male) | 2 | XY | Non-homologous pair |
| Largest chromosome | 1 | Chromosome 1 | ~249 million bp, ~2000 genes |
| Smallest chromosome | 1 | Chromosome 22 | ~51 million bp, ~700 genes |
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Related Terms
Gene
A gene is a specific sequence of DNA (or RNA in some viruses) that encodes the information required to produce a functional product, most commonly a protein or functional RNA molecule. Genes occupy defined loci on chromosomes and are the fundamental units of heredity, transmitted from parent to offspring. The modern molecular definition encompasses not only coding sequences (exons) but also regulatory elements such as promoters and enhancers that control when, where, and how much a gene is expressed.
DNA
Deoxyribonucleic acid (DNA) is the double-stranded helical polymer that stores the genetic information of virtually all living organisms. It is composed of nucleotide monomers, each containing deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C), paired by complementary hydrogen bonds (A–T and G–C). The sequence of bases encodes instructions for building proteins and regulating cellular functions, passed faithfully to daughter cells through semiconservative replication.
Allele
An allele is one of two or more alternative forms of a gene that occupy the same locus (position) on homologous chromosomes. In diploid organisms, each individual carries two alleles for each gene — one inherited from each parent — and these alleles may be identical (homozygous) or different (heterozygous). The interaction between alleles (dominance, co-dominance, incomplete dominance) determines the phenotype expressed by an individual.
From Greek "chroma" (colour) + "soma" (body). The term was coined by Heinrich Wilhelm Waldeyer in 1888, referring to the structures' strong affinity for certain histological stains (chromatin dyes).