In biology, the word gene (from Greek: γένος, génos;[1] meaning generation[2] or birth[1] or gender) can have several different meanings. The Mendelian gene is a basic unit of heredity and the molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protein-coding genes and noncoding genes.[3][4][5][6]
During gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the intermediate template for a protein that performs a function. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits. These genes make up different DNA sequences called genotypes. Genotypes along with environmental and developmental factors determine what the phenotypes will be. Most biological traits are under the influence of polygenes (many different genes) as well as gene–environment interactions. Some genetic traits are instantly visible, such as eye color or the number of limbs, and some are not, such as blood type, the risk for specific diseases, or the thousands of basic biochemical processes that constitute life.
Genes can acquire mutations in their sequence, leading to different variants, known as alleles, in the population. These alleles encode slightly different versions of a gene, which may cause different phenotypical traits. Usage of the term "having a gene" (e.g., "good genes," "hair color gene") typically refers to containing a different allele of the same, shared gene.[7] Genes evolve due to natural selection / survival of the fittest and genetic drift of the alleles.
The concept of gene continues to be refined as new phenomena are discovered.[8] For example, regulatory regions of a gene can be far removed from its coding regions, and coding regions can be split into several exons. Some viruses store their genome in RNA instead of DNA and some gene products are functional non-coding RNAs. Therefore, a broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affect an organism's traits by being expressed as a functional product or by regulation of gene expression.[9][10]
The term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1909.[11] It is inspired by the Ancient Greek: γόνος, gonos, that means offspring and procreation.
There are lots of different ways to use the term "gene." Richard Dawkins, for example, wrote a book called "The Selfish Gene"[12] where 'gene' simply meant any part of the chromosome that was subject to natural selection. The problem with this definition is that natural selection (or evolution) acts on the phenotype; the organism. It is the organism that lives or dies as a result of natural selection. This 'gene' is often referred to as the "Mendelian gene" whereas the physical gene described in this article is called the "molecular gene."[3]
Chromosome
(107 - 1010 bp)
DNA
Gene
(103 - 106 bp )
Function
A gene is a region of DNA that encodes function. A chromosome consists of a long strand of DNA containing many genes. A human chromosome can have up to 500 million base pairs of DNA with thousands of genes.
Gregor Mendel
The chemical structure of a four base pair fragment of a DNA double helix. The sugar-phosphate backbone chains run in opposite directions with the bases pointing inwards, base-pairing A to T and C to G with hydrogen bonds.
Micrographic karyogram of human male, showing 23 pairs of chromosomes. The largest chromosomes are around 10 times the size of the smallest.[48]
Schematic karyogram of a human, with annotated bands and sub-bands. It shows dark and white regions on G banding. It shows 22 homologous chromosomes, both the male (XY) and female (XX) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (at bottom left).
Further information: Karyotype
Regulatory sequence
Regulatory sequence
Enhancer
/silencer
Promoter
5'UTR
Open reading frame
3'UTR
Enhancer
/silencer
Proximal
Core
Start
Stop
Terminator
Transcription
DNA
Exon
Exon
Exon
Intron
Intron
Post-transcriptionalmodification
Pre-mRNA
Protein coding region
5'cap
Poly-A tail
Translation
MaturemRNA
Protein
The structure of a eukaryotic protein-coding gene. Regulatory sequence controls when and where expression occurs for the protein coding region (red). Promoter and enhancer regions (yellow) regulate the transcription of the gene into a pre-mRNA which is modified to remove introns (light grey) and add a 5' cap and poly-A tail (dark grey). The mRNA 5' and 3' untranslated regions (blue) regulate translation into the final protein product.[52]
Polycistronic operon
Regulatory sequence
Regulatory sequence
Enhancer
Enhancer
/silencer
/silencer
Operator
Promoter
5'UTR
ORF
ORF
UTR
3'UTR
Start
Start
Stop
Stop
Terminator
Transcription
DNA
RBS
RBS
Protein coding region
Protein coding region
mRNA
Translation
Protein
The structure of a prokaryotic operon of protein-coding genes. Regulatory sequence controls when expression occurs for the multiple protein coding regions (red). Promoter, operator and enhancer regions (yellow) regulate the transcription of the gene into an mRNA. The mRNA untranslated regions (blue) regulate translation into the final protein products.[52]
Schematic of a single-stranded RNA molecule illustrating a series of three-base codons. Each three-nucleotide codon corresponds to an amino acid when translated to protein
Protein coding genes are transcribed to an mRNA intermediate, then translated to a functional protein. RNA-coding genes are transcribed to a functional non-coding RNA. (PDB: 3BSE, 1OBB, 3TRA)
Inheritance of a gene that has two different alleles (blue and white). The gene is located on an autosomal chromosome. The white allele is recessive to the blue allele. The probability of each outcome in the children's generation is one quarter, or 25 percent.
A sequence alignment, produced by ClustalO, of mammalian histone proteins
Evolutionary fate of duplicate genes.
Depiction of numbers of genes for representative plants (green), vertebrates (blue), invertebrates (orange), fungi (yellow), bacteria (purple), and viruses (grey). An inset on the right shows the smaller genomes expanded 100-fold area-wise.[107][108][109][110][111][112][113][114]
Gene functions in the minimal genome of the synthetic organism, Syn 3.[124]
Comparison of conventional plant breeding with transgenic and cisgenic genetic modification.
