Gene
From 1857 to 1864, Gregor Mendel studied inheritance patterns in 8000 common edible pea plants within the garden of an Augustinian monastery in Brno, Austrian Empire. He tracked distinct traits from parent to offspring and described these mathematically as 2n combinations where n is the number of differing characteristics in the original peas. Although he did not use the term gene, he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured Wilhelm Johannsen's distinction between genotype and phenotype decades later. Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance which suggested that each parent contributed fluids to the fertilization process. Charles Darwin developed a theory of inheritance he termed pangenesis from Greek pan meaning all or whole and genesis meaning birth. Darwin used the term gemmule to describe hypothetical particles that would mix during reproduction.
Deoxyribonucleic acid DNA was shown to be the molecular repository of genetic information by experiments conducted during the 1940s to 1950s. The structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography. James D. Watson and Francis Crick published a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication. Two chains of DNA twist around each other to form a DNA double helix with the phosphate-sugar backbone spiralling around the outside. The bases point inward with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two hydrogen bonds whereas cytosine and guanine form three hydrogen bonds. In 1965, the lab of Max Birnstiel was the first to isolate single genes which were ribosomal RNA genes from Xenopus laevis. Walter Fiers and his team determined the sequence of a gene in 1972 which was that of bacteriophage MS2 coat protein.
The expression of genes encoded in DNA begins by transcribing the gene into RNA which is a second type of nucleic acid very similar to DNA but whose monomers contain the sugar ribose rather than deoxyribose. RNA also contains the base uracil in place of thymine. Genes that encode proteins are composed of a series of three-nucleotide sequences called codons which serve as the words in the genetic language. The genetic code specifies the correspondence during protein translation between codons and amino acids. The genetic code is nearly the same for all known organisms. Translation is the process by which a mature mRNA molecule is used as a template for synthesizing a new protein. Translation is carried out by ribosomes which are large complexes of RNA and protein responsible for carrying out chemical reactions to add new amino acids to a growing polypeptide chain. Each tRNA has three unpaired bases known as the anticodon that are complementary to the codon it reads on the mRNA. During and after synthesis, most new proteins must fold to their active three-dimensional structure before they can carry out cellular functions.
A gene can acquire mutations in its sequence leading to different variants known as alleles in the population. These alleles encode slightly different versions of a gene which may cause different phenotypic traits. Genes evolve due to natural selection and genetic drift of the alleles. When a gene is duplicated, the new copy can evolve a new function. Some biological traits occur under the combined influence of polygenes which are a set of different genes and gene-environment interactions. Some genetic traits are instantly visible such as eye color or the number of limbs while others are not such as blood type or the risk for specific diseases. R.A. Fisher, Sewall Wright and J.B.S. Haldane integrated Mendelian genetics with Darwinian evolution in what became known as the modern synthesis. The development of the neutral theory of evolution by Motoo Kimura in the late 1960s led to the recognition that random genetic drift is a major player in evolution.
The discovery of introns in the 1970s meant that many eukaryotic genes were much larger than the size of the functional product would imply. Typical mammalian protein-coding genes are about 62,000 base pairs in length transcribed region and since there are about 20,000 of them they occupy about 35-40% of the mammalian genome including the human genome. Eukaryotic genes often have introns that are much larger than their exons and those introns can even have other genes nested inside them. The chromosomes are packed within the nucleus in complex with storage proteins called histones to form a unit called a nucleosome. DNA packaged and condensed in this way is called chromatin. The manner in which DNA is stored on the histones as well as chemical modifications of the histone itself regulate whether a particular region of DNA is accessible for gene expression. Prokaryotes typically store their genomes on a single large circular chromosome while some eukaryotic organelles contain a remnant circular chromosome with a small number of genes.
Essential genes are the set of genes thought to be critical for an organism's survival assuming abundant availability of all relevant nutrients and absence of environmental stress. In bacteria, an estimated 250-400 genes are essential for Escherichia coli and Bacillus subtilis which is less than 10% of their genes. Half of these genes are orthologs in both organisms and are largely involved in protein synthesis. In the budding yeast Saccharomyces cerevisiae the number of essential genes is slightly higher at 1000 genes which represents about 20% of their genes. Gene nomenclature was established by the HUGO Gene Nomenclature Committee HGNC which is a committee of the Human Genome Organisation. Symbols are chosen to be unique and each gene has only one symbol although approved symbols sometimes change. The synthetic organism Syn 3 has a minimal genome of 473 essential genes and quasi-essential genes necessary for fast growth although 149 have unknown function.
Since the 1970s, a variety of techniques have been developed to specifically add remove and edit genes in an organism. Recently developed genome engineering techniques use engineered nuclease enzymes to create targeted DNA repair in a chromosome to either disrupt or edit a gene when the break is repaired. Genetic engineering is now a routine research tool with model organisms. For example, genes are easily added to bacteria and lineages of knockout mice with a specific gene's function disrupted are used to investigate that gene's function. Many organisms have been genetically modified for applications in agriculture industrial biotechnology and medicine. For multicellular organisms typically the embryo is engineered which grows into the adult genetically modified organism. However, the genomes of cells in an adult organism can be edited using gene therapy techniques to treat genetic diseases. The total number of protein-coding genes the Earth's proteome is estimated to be 5 million sequences.
Common questions
What did Gregor Mendel study in the garden of an Augustinian monastery in Brno between 1857 and 1864?
Gregor Mendel studied inheritance patterns in 8000 common edible pea plants within the garden of an Augustinian monastery in Brno, Austrian Empire. He tracked distinct traits from parent to offspring and described these mathematically as 2n combinations where n is the number of differing characteristics in the original peas.
When was the structure of DNA determined by Rosalind Franklin and Maurice Wilkins using X-ray crystallography?
Experiments showing that deoxyribonucleic acid DNA was the molecular repository of genetic information were conducted during the 1940s to 1950s. James D. Watson and Francis Crick published a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication.
Who first isolated single genes and when did this occur according to the text about Max Birnstiel's lab?
In 1965, the lab of Max Birnstiel was the first to isolate single genes which were ribosomal RNA genes from Xenopus laevis. Walter Fiers and his team determined the sequence of a gene in 1972 which was that of bacteriophage MS2 coat protein.
How many essential genes are estimated to be critical for Escherichia coli and Bacillus subtilis bacteria?
In bacteria, an estimated 250-400 genes are essential for Escherichia coli and Bacillus subtilis which is less than 10% of their genes. Half of these genes are orthologs in both organisms and are largely involved in protein synthesis.
What percentage of the mammalian genome do typical mammalian protein-coding genes occupy including the human genome?
Typical mammalian protein-coding genes are about 62,000 base pairs in length transcribed region and since there are about 20,000 of them they occupy about 35-40% of the mammalian genome including the human genome. Eukaryotic genes often have introns that are much larger than their exons and those introns can even have other genes nested inside them.