Genes: Types


A gene is a segment of DNA that contains in its nucleotide sequence the information to make protein or RNA molecules. Genes are passed on from parent to offspring and therefore constitute the basic units of heredity. The human genome contains more than 30000 protein‐coding genes, several thousand genes for noncoding RNA molecules and many thousands of pseudogenes – the nonfunctional by‐products of gene evolution.

Keywords: gene structure and organization; protein‐coding genes; noncoding ribonucleic acid; pseudogenes; gene numbers

Figure 1.

Structure and expression of a typical human protein‐coding gene. Exons contain the DNA sequence that encodes the protein. The 5′ and 3′ UTRs are located in exons and form part of the mRNA, but they do not encode amino acids. The 5′ flanking region (5′ FR) is the sequence immediately adjacent to the transcription initiation point, the start of the actual gene. The 5′ FR contains the promoter and other regulatory sequences that are involved in initiating and controlling transcription.

Figure 2.

Overview of types of gene and pseudogene with estimated numbers in the human genome in parentheses.

Figure 3.

Principle of gene rearrangements in immunoglobulin genes. A germ‐line copy of an immunoglobulin heavy chain gene comprises four groups of coding segments, variable (V), diversity (D), joining (J) and constant (C), each with up to 100 segments. In developing B cells, the DNA is rearranged so that individual (randomly chosen) V, D and J segments are joined together. Once transcribed, these segments are joined to a C segment by RNA splicing.


Further Reading

Blumenthal T (1998) Gene clusters and polycistronic transcription in eukaryotes. BioEssays 20: 480–487.

Cooper DN (1999) Human Gene Evolution. Oxford, UK: Bios Scientific.

Eddy SR (1999) Noncoding RNA genes. Current Opinion in Genetics and Development 9: 695–699.

Edwalds‐Gilbert G, Veraldi KL and Milcarek C (1997) Alternative poly(A) site selection in complex transcription units: means to an end? Nucleic Acids Research 25: 2547–2561.

Epp CD (1997) Definition of a gene. Nature 389: 537.

Lander ES, Linton LM, Birren B, et al. (2001) Initial sequencing and analysis of the human genome. Nature 409: 860–921.

Mann JR, Szabo PE, Reed MR and Singer‐Sam J (2000) Methylated DNA sequences in genomic imprinting. Critical Reviews in Eukaryotic Gene Expression 10: 241–257.

Modrek B, Resch Grasso C and Lee C (2001) Genome‐wide detection of alternative splicing in expressed sequences of human genes. Nucleic Acids Research 29: 2850–2859.

Venter JC, Adams MD, Myers EW, et al. (2001) The human genome. Science 291: 1304–1351.

Wright FA, Lemon WJ, Zhao WD, et al. (2001) A draft annotation and overview of the human genome. Genome Biology 2: research0025.1–research0025.18.

Web Links

A collection of genomic resources

Database of RNA families

Nature Genome Gateway. A full breakdown of predicted protein function and evolutionary conservation from the draft sequence of the human genome

Noncoding RNAs Database. A database of noncoding RNAs with putative riboregulator function

Science Special Issue. A full breakdown of predicted protein function and evolutionary conservation from the draft sequence of the human genome

UCSC Genome Bioinformatics. A primary source for the draft sequence of the human genome, gene locations and linked information

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Berg, Lutz‐Peter, and Cooper, David N(Sep 2005) Genes: Types. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0005016]