Chemical nature of nuclei acid investigated. It was thought to be a tetranucleotide composed of one unit each of adenylic, guanylic, thymidylic and cytidylic acids
Date: 2015-10-07; view: 637.
Nucleic acid found to be a major component of the chromosomes
Nucleic acid is found to be a major component of chromosomes but its molecular structure is thought to be too simple to carry genetic information. William Bateson investigates T. H. Morgan's chromosome theory by experiments on peas. He tests Morgan's assumption that the number of independent elements cannot exceed the number of chromosomes. For the next few years Bateson also continues to work on physiological aspects of plants and forms of reproduction where segregation of chromosomes plays no part. He studies root cuttings, chimaeras (plants composed of tissues of two or more genetically distinct types), variegated plants, and rogue peas (peas that do not come true from seed), all in the hope of finding an alternative to Morgan's chromosome theory. Bateson's objections to the chromosome theory stemmed from his requirement that it should not only explain heredity but also differentiation and development in plants. Other genetic research at JIHI focuses on studies of linkage.
(1930)
DNA was discovered in 1869 by Friedrich Miescher, a Swiss physician. He foudn an acidic substance in the nuclei fo cells that he named nucleic acid. It was not until 1930 that Albrecht Kossel and Phoebus Levene established that nucleic acids were in fact DNA which consisted of repeating molecules containing sugar, nitrogenous bases and phosphate. The combination of a sugar with a any one of five different nitrogen bases creates a nucleoside. The five bases are divided into two categories based on the structure of their molecules; purines have two ring structures (adenine and guanine) while pyrimidines have one (thymine, cytosine and uracil). Adenine, guanine, thymine and cytosine are found in DNA whereas RNA replaces thymine with uracil. If a phosphate molecule is added to a nucleoside it becomes known as a nucleotide. Nucleotides with a ribose sugar are therefore ribonucleotides, and nucleotides with a deoxyribose sugar are deoxyrobonucleotides. Each nucleotide's name can be shortened to a single letter, A for adenine, C for cytosine, G for guanine, T for thymine and U for uracil. The chemical bond that links one nucleotide to another is formed between the phosphate group of one nucleotide and the sugar group of the next nucleotide via an ester bond between a carbon atom and an oxygen atom. Two things remain the same no matter how many nucleotides are added to the growing polynucleotide chain; one end of the chain has a free phosphate group and the other end has a free hydroxyl (-OH) group. These ends are called 5' ("five prime") and 3' ("three prime") respectively. This naming system comes from the way we present a sugar structure when we draw it on paper. We start at the "top, right-hand" carbon and count in a clock-wise manner. The phosphate group of the previous nucleotide is linked to carbon number 5, and the phosphate group of the next nucleotide is linked to carbon number 3. The naming system, 5' to 3' is used to describe the order of the nucleotides in the DNA strand. Think of the system as being similar to the way European people are taught to read and write - from the left side of a page to right side. Nucleic acids are one of several macromolecules found in the body (others include proteins and carbohydrates) which are formed by lots of individual molecules (nucleotides) strung together to form a polynucleotide. Each nucleotide consists of a sugar, a nitrogen base and a phosphate group. In RNA the sugar is called ribose (how the name ribonucleic acid comes about), and in DNA it is called deoxyribose which means that it is missing ("deoxy") a carbon atom compared to ribose.
(early 1940s)