Gene
Gene is the basic physical and functional unit of heredity in living organisms. It consists of a sequence of nucleotides in DNA, or in RNA in some viruses, that carries the instructions for making a functional product. Most often that product...
Gene is the basic physical and functional unit of heredity in living organisms. It consists of a sequence of nucleotides in DNA, or in RNA in some viruses, that carries the instructions for making a functional product. Most often that product is a protein, but many genes instead specify functional RNA molecules such as tRNA and rRNA. The complete set of genetic material in an organism is its genome.
The definition is less clear-cut than it appears. Genes can overlap one another, a single gene can give rise to several different proteins, and some stretches of DNA produce RNA that is never translated. Biologists therefore use the term somewhat flexibly, depending on the context.
Structure: What Functional Sequence Elements Make Up a Gene?
Promoters, Exons, Introns, and Untranslated Regions
A gene includes more than the instructions for building a product. It also contains sequences that control when, where, and how strongly it is used. The arrangement differs between prokaryotes and eukaryotes, but a typical eukaryotic gene has the following parts:
- Promoter. Situated before the rest of the gene, the promoter acts as a switch. It provides the binding site for RNA polymerase and for transcription factors, which together begin the process of copying the gene into RNA.
- Exons. These are the segments retained in the finished RNA transcript. They include the coding sequence, which specifies the order of amino acids in the protein, together with the untranslated regions described below.
- Introns. These lie between the exons and are removed from the transcript before it is used. They vary enormously in length, from under a hundred nucleotides to hundreds of thousands. Most are removed by a large complex of RNA and protein called the spliceosome, in a process known as RNA splicing, though some introns splice themselves out and others are removed by dedicated enzymes. Prokaryotic genes rarely contain introns.
- Untranslated regions. The mature mRNA carries sequences at each end that are not translated into protein. These help determine how stable the transcript is, how it is transported, and how efficiently the ribosome translates it.
| Structural Feature | Prokaryotic Genes (Bacteria & Archaea) | Eukaryotic Genes (Multicellular & Unicellular) | Primary Functional Consequence |
|---|---|---|---|
| Transcriptional Unit Organization | Often polycistronic (operons encoding multiple proteins) | Almost exclusively monocistronic (one gene per promoter) | Co-regulated metabolic pathway expression in prokaryotes |
| Intron-Exon Architecture | Continuous open reading frames; introns rare | Split genes with ~8–10 exons interrupted by long introns | Enables alternative splicing and protein isoform diversity |
| Typical Gene Length | ≈ 1,000 bp (≈ 1 kb) | ≈ 10,000–50,000 bp (up to 2.4 Mb for human DMD) | High coding density (≈88%) in bacteria vs. ≈1.5% in humans |
| Regulatory Elements | Proximal promoter (−10 / −35 boxes) and operator sites | Core promoter plus distal enhancers and silencers (up to >1 Mb away) | Complex tissue-specific 3D chromatin looping in eukaryotes |
| Post-Transcriptional Processing | Minimal; coupled co-transcriptional translation | 5′ m7G cap, spliceosomal splicing, 3′ poly(A) tail | Nuclear quality control and regulated cytosolic half-life |
Expression: How Is Genetic Information Transcribed and Translated?
Transcription, Translation, and the Central Dogma
The process by which a cell uses the information in a gene is called gene expression. It proceeds in two main steps:
- Transcription. RNA polymerase reads the DNA of an active gene and builds a complementary RNA copy.
- Translation. At the ribosome, the sequence of codons in the mRNA is read three bases at a time and used to assemble a chain of amino acids, which folds into a working protein.
In eukaryotes these steps are separated: transcription occurs in the nucleus and the processed mRNA is exported to the cytoplasm for translation. Prokaryotes have no nucleus, so ribosomes can begin translating a transcript while it is still being made.
This flow of information, from DNA to RNA to protein, is often associated with the central dogma of molecular biology. Francis Crick's original statement was narrower: that sequence information, once it has passed into protein, cannot be transferred back out again. Information does move from RNA to DNA in some circumstances, for example by reverse transcriptase in retroviruses.
Inheritance and Alleles: How Do Chromosomal Loci and Allelic Variants Determine Phenotype?
Chromosomal Loci, Ploidy, and Genotype-Phenotype Mapping
Genes are arranged along chromosomes. The position a gene occupies is called its locus. Not all of an organism's genes lie on nuclear chromosomes: mitochondria and, in plants, chloroplasts carry small genomes of their own.
Humans and many other organisms are diploid, inheriting one set of chromosomes from each parent, and so carry two copies of most genes. The exception is the sex chromosomes: a human male has only one copy of the genes on the X chromosome.
Different versions of the same gene are called alleles. A difference of even a single nucleotide can alter the protein produced and, in turn, an observable characteristic. The particular combination of alleles an organism carries is its genotype; the characteristics that result, which depend on the environment as well as on the genes, are its phenotype.
Mutation and Evolution: How Do Sequence Changes Generate Heritable Variation?
Replication Errors, Mutagens, and Natural Selection
DNA sequences are not fixed. They change through mutation, which can arise from errors made by DNA polymerase during replication or be caused by agents such as ultraviolet light and certain chemicals.
Most mutations have no noticeable effect. Some are harmful, causing genetic disorders or contributing to cancer. A few are beneficial, introducing advantageous new alleles into a population. This variation is the raw material on which natural selection acts, and over long periods it underlies adaptation and the origin of new species.
Genomics: How Many Genes Are in the Human Genome?
The Human Genome Project and Non-Coding DNA
Before the human genome was sequenced, estimates of the number of human genes ran to 100,000 or more. The Human Genome Project, completed in 2003, showed the true figure to be far lower. Current estimates put it at roughly 19,000 to 20,000 protein-coding genes, comparable to the number in a millimetre-long nematode worm. The number of genes therefore says little about an organism's complexity. Part of the explanation is that a single human gene can yield many different proteins, because its exons can be combined in different ways during splicing.
These protein-coding genes account for less than 2% of human DNA. The rest includes sequences that regulate gene activity, sequences that give chromosomes their structure, genes for non-coding RNAs, and large numbers of repeated and mobile elements. How much of this DNA has a function is debated.
CRISPR-Cas9 Genome Editing
Techniques developed since the 1970s allow genes to be studied and altered directly. The most widely used today is CRISPR-Cas9, which can be directed to cut a chosen DNA sequence, enabling genes to be disabled, corrected, or inserted. These methods are applied in gene therapy, agriculture, and synthetic biology.
Frequently Asked Scientific Questions
What is the difference between a gene and an allele?
A gene is a specific functional hereditary unit occupying a defined chromosomal coordinate (locus) that encodes a particular RNA or protein product (such as the HBB gene on human chromosome 11 encoding beta-globin). An allele is one of two or more alternative nucleotide sequence variants of that same gene within a population (such as the wild-type HbA allele versus the sickle-cell HbS allele caused by a single glutamate-to-valine point mutation).
Why do humans have roughly the same number of protein-coding genes as a microscopic nematode worm?
Both Homo sapiens and the nematode Caenorhabditis elegans possess approximately 19,000–20,000 protein-coding genes, resolving the misconception that organismal complexity scales linearly with gene count. Vertebrate anatomical and neurological complexity instead arises from extensive alternative pre-mRNA splicing (which affects >95% of human multi-exon genes to generate diverse protein isoforms), combinatorial _cis_-regulatory enhancer networks, and expanded non-coding RNA repertoires.
Can a functional gene encode a product other than a protein?
Yes. While roughly 20,000 human genes encode polypeptides, thousands of additional genes are transcribed into functional non-coding RNAs (ncRNAs) that are never translated by ribosomes. These include ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs) essential for protein synthesis, small nuclear RNAs (snRNAs) that form the catalytic core of the spliceosome, microRNAs (miRNAs) that silence target mRNAs, and long non-coding RNAs (lncRNAs) that guide chromatin remodeling.