Genome Organisation: Human

Abstract

The human nuclear genome is organised into a highly complex arrangement of two sets of 23 chromosomes comprised of various types of deoxyribonucleic acid (DNA) sequences. With less than 2% of the human genome consisting of protein‐encoding DNA sequence, the remainder of the 3 × 10 9 bp of the haploid genome consists of a multifaceted assortment of DNA sequence classifications. At the broadest level, the genome can be divided into single‐copy protein‐encoding genes, repetitive sequences and spacer DNA. Notably, the categories of repetitive sequences are profoundly intricate and may be further classified into functional or functionally related groups such as gene families and superfamilies, or groups with no known function such as satellite DNA and transposable elements. Their genomic organisation may be dispersed, within localised regions, and/or tandemly repeated. The generation of repetitive sequences results from a variety of mechanisms continually promoting a genome that is highly dynamic. Key Concepts The human genome is highly complex and dynamic. Only 2–4% of the three billion base pairs of the human genome consists of protein‐encoding genes, of which only a portion of that is translated into amino acids with some of the sequence being involved in regulation of gene expression. The human genome consists of single or low‐copy genes, moderately repetitive sequences and highly repetitive sequences initially determined utilising reassociation kinetics, with all three categories of DNA found throughout the genome. G‐band staining of chromosomes yield karyograms that define characteristic properties and organisation of the genome. Repetitive DNA sequences comprise a large proportion of the genome and may be dispersed and/or in tandem arrays. The repetitive sequences in the genome may be functional or serve no known function. Functional repetitive sequences may encode protein or RNA molecules. A large fraction of DNA sequences in the human genome have been generated via mechanisms such as unequal crossing over yielding either functional, or nonfunctional pseudogene, copies typically adjacent to each other, as well as by reverse transcription which yields retrocopies dispersed throughout the genome. Functional tandem repeats of DNA sequences are localised in certain chromosomal regions, for example the rRNA genes within nucleolar organiser regions (NORs), providing numerous copies of genes typically having general functions (e.g. protein synthesis). Telomeres are the sequences at the ends of the chromosomes that consist of tandemly repeated sequences associated with stability and maintenance of chromosome size after cell division.