2.3 Mutations and evolution
Just as there are many factors influencing the expression of a particular DNA strand, there are many ways to have genetic mutations.
For example, natural variations within regulatory sequences appear to underlie many of the heritable characteristics seen in organisms. The influence of such variations on the trajectory of evolution through natural selection may be as large as or larger than variation in sequences that encode proteins. Thus, though regulatory elements are often distinguished from genes in molecular biology, in effect they satisfy the shared and historical sense of the word. Indeed, a breeder or geneticist, in following the inheritance pattern of a trait, has no immediate way to know whether this pattern arises from coding sequences or regulatory sequences. Typically, he or she will simply attribute it to variations within a gene.
Errors during DNA replication may lead to the duplication of a gene, which may diverge over time. Though the two sequences may remain the same or be only slightly altered, they are typically regarded as separate genes (i.e. not as alleles of the same gene). The same is true when duplicate sequences appear in different species. Yet, though the alleles of a gene differ in sequence, nevertheless they are regarded as a single gene (occupying a single locus).
3 History
The existence of genes was first suggested by Gregor Mendel, who studied inheritance in pea plants and hypothesized a factor that conveys traits from parent to offspring. Although he did not use the term gene, he explained his results in terms of inherited characteristics. Mendel was also the first to hypothesize independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, and the difference between what would later be described as genotype and phenotype. Mendel's concept was finally named when Wilhelm Johannsen coined the word gene in 1909.
In the early 1900s, Mendel's work received renewed attention from scientists. In 1910, Thomas Hunt Morgan showed that genes reside on specific chromosomes. He later showed that genes occupy specific locations on the chromosome. With this knowledge, Morgan and his students began the first chromosomal map of Drosophila. In 1928, Frederick Griffith showed that genes could be transfered. In what is now known as Griffith's experiment, injections into a mouse of a deadly strain of a bacteria that had been heat-killed transferred genetic information to a safe strain of the same bacteria, killing the mouse.
In 1941, George Wells Beadle and Edward Lawrie Tatum showed that mutations in genes caused errors in certain steps in metabolic pathways. This showed that specific genes code for specific protiens, leading to the "one gene, one enzyme" hypothesis. Oswald Avery, Collin Macleod, and Maclyn McCarty showed in 1944 that DNA holds the gene's information. In 1953, James D. Watson and Francis Crick demonstrated the molecular structure of DNA. Together, these discoveries established the central dogma of molecular biology, which states that proteins are transcribed from RNA which is translated from DNA. This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses.
4 See also
- Genetics, Gene expression, Gene therapy, Homeobox, Human Genome Project, Genomics, DNA, Protein, Gene family
- Genetic programming, Genetic algorithm
- Meme, Memetics
