A genome is the complete complement of genetic material in an organism, encompassing all of its genes and non‑coding sequences. The genome includes every molecule of DNA (or RNA in some viruses) that carries hereditary information. In eukaryotes, the genome consists of the nuclear chromosomes and may also include mitochondrial and chloroplast DNA. In bacteria and archaea, the genome is typically a single circular chromosome along with plasmids. Viruses may have genomes composed of either single‑stranded or double‑stranded RNA or DNA. Genome size varies widely; some viruses have genomes of just a few thousand nucleotides, whereas the human haploid nuclear genome contains about 3.2 billion base pairs. Genome sequencing and analysis have revealed that much of the genome consists of non‑coding regions such as introns, regulatory sequences, repetitive elements and intergenic DNA. The organisation, gene content and repetitive structures of genomes provide insight into evolutionary history and cellular function. Genomes are inherited as units but can undergo mutation, recombination and rearrangements. Comparative genomics examines similarities and differences among genomes to infer phylogenetic relationships and identify conserved genes. The field of genomics applies high‑throughput sequencing, assembly and annotation to characterise genomes in diverse organisms. Information about genomes underpins modern biology, medicine, agriculture and biotechnology; it guides gene discovery, disease diagnosis, breeding and conservation.
Genome Variation and Examples
The bacterium Escherichia coli strain K‑12 has a single circular chromosome of about 4.6 million base pairs. The model plant Arabidopsis thaliana has a nuclear genome of roughly 135 million base pairs across five chromosomes, while the bread wheat genome is enormous, spanning more than 15 billion base pairs due to polyploidy. Humans possess 23 pairs of chromosomes with a total of about 20 000 protein‑coding genes. Mitochondrial genomes are much smaller; the human mitochondrial genome is a circular molecule of 16 570 nucleotides encoding 37 genes. Viral genomes illustrate the extremes of genome size: influenza A virus has eight segments totaling around 13 5 0 nucleotides, whereas Megavirus has a genome exceeding one million base pairs. The concept of the genome encompasses all hereditary information within an organism. Studying genome structure and variation enables researchers to understand evolution, identify gene function and develop applications in medicine and agriculture. Related Terms: DNA, Chromosome, Genomics, Gene, Nucleotide