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Molecular Genetics. By- Jahin Akhand Red = Important. Charles Darwin’s Theory of Evolution.
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Molecular Genetics By- Jahin Akhand Red = Important
Charles Darwin’s Theory of Evolution • On his voyage on the H .M .S . Beagle from 1831 to 1836, Charles Darwin performed extensive examinations of many different species on the Galapagos Islands, including some 13 species of finches from different islands and habitats that revealed distinct traits. • After his observations Darwin published his landmark book On the Origin of Species by Means of Natural Selection in 1859.
Major Observations that Darwin Made • Individuals within a species vary in their heritable traits and this is referred to as descent with modification. • In every generation, more offspring are produced than can be supported by the environment. • Individuals with the most favorable variations survive and go on to reproduce and are selected naturally.
Darwin's Ideas • His ideas received wide acceptance from the scientific community, but also generated long-lasting debates with other people within his field. • One major weakness of Darwin’s thesis was that he could not explain the genetic base of his observations because of how the heritable traits were determined and subsequently passed down to descendantswhich was poorly understood during his time period.
Post Darwin Era Genetic Basis of Evolution by Natural Selection • In 1878, a few years after Darwin’s death, aGerman cytologist(study of cells) Walther Flemming described the distribution and behavior of chromosomes during different stages of mitosis. • A few years later, biologist August Weismann also from Germany described the crossing over during meiosis and suggested the role this step plays in genetic variability and natural selection.
Post Darwin Era Genetic Basis of Evolution by Natural Selection (Continued) • In one of his experiments, Weismann removed the tails of mice to see if this acquired trait would be passed down to their offspring. The result of this experiment was that no offspring displayed this trait over five generations . This suggested to Weismann that a somatic trait (expressed by the body) would not be transmitted to the offspring and led him to propose the germ-plasm theory of heredity and this theory states that germ plasm is the essential component of germ cells and is the hereditary material that gets passed on from one generation to the next.
Post Darwin Era Population Genetics • Thomas Morgan, a scientist who rediscovered Mendel’s, work began to breed large populations of fruit flies in an attempt to reproduce Mendel’s results. • During his experiments, Morgan noticed that the mutation of virtually all traits occurred in fruit flies over generations and this research along with other discoveries that linked Mendel’s physical factor with chromosomes helped Morgan develop the chromosome theory of inheritance which explains the fact that chromosomes (and later genes) were identified as the physical unit that determines heritable traits provided the genetic basis for both Mendel’s principles of heredity and Darwin’s descent with modification observations.This realization led to the creation of a new branch of genetics and biometry (statistical study of biological data) called population genetics = the study of genes and genotypes in a population.
Post Darwin Era The Hardy-Weinberg Equilibrium • Mathematician G .H . Hardy and German physician Wilhelm Weinberg proposed the principle known as the Hardy Weinberg theorem in 1908 which states that the *allele frequencies in a population will not change generation after generation if the equilibrium is reached. • A simple Hardy-Weinberg equilibrium would be p + q = 1 *Allele- is an alternative form of a gene that is located at a specific position on a specific chromosome or like the trait blue eyes.
Post Darwin Era The Hardy-Weinberg Equilibrium (Cont.) • To use p+q=1, you would need to understand allele frequency which is the percentage of certain alleles in two genotypes. Example: Bb and bb The allele frequency for B would be 25% The allele frequency for b would be 75% • How it relates to p+q=1 p= the allele frequency of the first allele like B q= the allele frequency of the second allele like b So, 25%+75%=1 or 100%
Post Darwin Era Theodosius Dobzhansky Ernst Mayr The Modern Synthesis • In the early twentieth century, major publications surfaced in scientific areas such as embryology, biogeography, population genetics, and paleontology. • Theodosius Dobzhansky, a biologist who worked at Morgan’s lab in the 1930s merged genetics and evolutionary biology and wrote the book, Genetics and the Origin of Species. He also defined evolution as “a change in allele frequency within a gene pool” • Evolutionary biologist Ernst Mayr proposed the “biological species concept,” which defined species as groups of organisms that are able to interbreed naturally to produce viable and fertile offspring of both sexes. He also wrote The Evolutionary Synthesis.
Genetic Material Mononucleotides • Between 1885 and 1901, German chemist Albrecht Kossel studied the nuclear material nuclein, which is a crude mixture of nucleic acids complexed with the many proteins that make up a chromosome, identified the four nitrogenous bases that make up the major part of DNA, adenine (A), cytosine (C), guanine (G), and thymine (T), and one that is part of RNA, uracil (U). • His discovery was not that important to scientists during the time he discovered it but in 1910 he was awarded the Nobel Prize.
Genetic Material Polynucleotides • Russian-American biochemist Phoebus Levene, who had worked with Kossel, tried to identify other components of the nucleic acid • Levene identified the monosaccharide deoxyribose as part of nucleic acid, which led to its name DNA, deoxyribonucleic acid • In 1919, by breaking DNA into its constituent building blocks, Levene concluded that DNA is a long chain polynucleotide made of individual mononucleotides with three major components: phosphate, sugar, and one of four nitrogenous bases.
