However, if a mixture of live R strain and heat-killed S strain is injected into a mouse, the mouse will die, and live S strain can be isolated from the blood. ![]() If the S strain is injected subcutaneously into mice, the mice die, whereas, if either live R strain is injected or heat-killed S strain is injected, the mouse lives. The encapsulated so-called S strain is virulent, whereas the non-capsulated R strain is nonvirulent. Griffith was working with two strains of the bacterium Streptococcus pneumoniae. In 1928, Griffith showed that living cells could be transformed by extracts from heat-killed cells and that this transformation had the potential to permanently change the genetic makeup of the recipient cell. At the other end the chain terminates with a 3′ hydroxyl.Īlthough many scientists, including Miescher, had observed that prior to cell division the amount of nucleic acid increased, it was not believed to be the genetic material until the work of Fredrick Griffith, Oswald Avery, Colin MacLeod and Maclyn McCarty. The deoxyribose at the top of the diagram in Figure 1B is not linked to another deoxyribose it terminates with a 5′ phosphate group. This linkage is called a phosphodiester bond. In the DNA chain ( Figure 1B), the phosphate residue forms a link between the 3′-hydroxyl of one deoxyribose and the 5′-hydroxyl of the next. The phosphate group is acidic, hence the name nucleic acid. Within each monomer the phosphate is linked to the 5′ carbon of deoxyribose and the nitrogenous base is linked to the 1′ carbon, this is called an N-glyosidic bond. The carbon within the deoxyribose ring are numbered 1′ to 5′. There are four different bases in DNA, the double-ring purine bases: adenine and guanine and the single-ring pyrimidine bases: cytosine and thymine ( Figure 1B). The 5′ end is at the top and the 3′ end at the bottom. The 3′ carbon of one nucleotide is linked to the 5′ carbon of the next via a phosphodiester bond. ( B) A DNA strand containing four nucleotides with the nitrogenous bases thymine (T), cytosine (C), adenine (A) and guanine (G) respectively. A nucleotide is a nucleoside with one or more phosphate groups. A nucleoside is a base linked to a sugar. The nitrogenous base (guanine in this example) is linked to the 1′ carbon of the deoxyribose and the phosphate groups are linked to the 5′ carbon. ( A) A nucleotide (guanosine triphosphate). ![]() The building blocks for DNA synthesis contain three phosphate groups, two are lost during this process, so the DNA strand contains one phosphate group per nucleotide. So, for example, the human genome contains 20% C, 20% G, 30% A and 30% T.ĭNA is a polymer made of monomeric units called nucleotides ( Figure 1A), a nucleotide comprises a 5-carbon sugar, deoxyribose, a nitrogenous base and one or more phosphate groups. Erwin Chargaff had found that DNA molecules from a particular species always contained the same amount of the bases cytosine (C) and guanine (G) and the same amount of adenosine (A) and thymine (T). Work in the 1930s from many scientists further characterised nucleic acids including the identification of the four bases and the presence of deoxyribose, hence the name deoxyribonucleic acid (DNA). Kossel went on to show that nucleic acid contained purine and pyrimidine bases, a sugar and phosphate. Miescher, Richard Altmann and Albrecht Kossel further characterised ‘nuclein’ and the name was changed to nucleic acid by Altmann. Almost all molecular bioscience graduates would have repeated a form of this experiment in laboratory classes where DNA is isolated from cells. ![]() Although initially interested in all the components of the cell, Miescher quickly focussed on the nucleus because he observed that when treated with acid, a precipitate was formed which he called ‘nuclein’. He wanted to determine the chemical composition of leucocytes (white blood cells), his source of leucocytes was pus from fresh surgical bandages. ![]() We will learn later how to change the base of any logarithm before condensing.Discovery and chemical characterisation of DNAĭNA was discovered in 1869 by a Swiss biochemist, Friedrich Miescher. It is important to remember that the logarithms must have the same base to be combined. We can use the rules of logarithms we just learned to condense sums and differences with the same base as a single logarithm. In the following video, we show another example of expanding logarithms.
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