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THE NUCLEIC ACIDS Introduction Nucleotides The DNA Differences between DNA and RNA What is a nucleic acid? Nucleic acids were molecules first discovered in the nucleus of cells. These molecules are essential to life, are composed of 4 repeating units which, unlike proteins, are always the same: the nucleotides. There are 2 types of nucleic acids: the DNA and the RNA. The nucleotides The nucleotides are the units that are covalently linked forming a long chain. They comprise a nitrogenous base, a pentose (a 5 C sugar) and a phosphate group. Two types of bases Purines Pyrimidines Big bases Small bases (2 rings) (1 ring) What’s so special about DNA? DNA is one of the most chemically boring macromolecules imaginable - its made of only four building blocks and has a perfectly monotonous structure. DNA just sits there - it doesn’t catalyze reactions or build the cell or organism. So, what’s so good about DNA? The answer lies in DNA’s ability to store and copy information. A race to fame and glory THE PLAYERS IN THE UK IN THE USA CAMBRIDGE Watson & Crick (models) CALIFORNIA (Caltech) LONDON Franklin & Wilkins – (X-ray diffraction) Linus Pauling (protein expert) Rosalind Franklin’s famous DNA image “Chargaff’s rule” A = T & C = G And the winners are… The DNA structure was finally deduced by James Watson and Francis Crick in 1953, based on studies carried out by Dr. Rosalind Franklin and Dr. Pauling. This discovery was a revolution in biology. Knowledge has grown exponentially ever since, from merely knowing the DNA to its replication and link to protein synthesis; to cloning organisms and the deciphering of the human genome, among other current achievements, in less than 60 years. So what is the DNA? The deoxyribonucleic acid is a huge molecule composed of 2 long strands of thousands of repeating monomers called NUCLEOTIDES. What is the DNA for? The DNA stores and passes on the genetic information; it has the instructions to make all the proteins a cell needs and at the same time it is what gives the characteristics to an organism. DNA STRUCTURE It is like a spiral staircase, with the sides made by sugars linked to phospahte groups and the rungs made by the bases paired up in a specific way. What holds the two strands together are the H bonds that form between complementary bases. Complementary bases Bases on opposite strands pair up according to a rule: A pairs up with T C pairs up with G This simple rule always applies in nucleic acids and it is key for replication and transmission of genetic information to the next generation. 3 weak H–bonds 2 weak H–bonds G and C are complementary as are A and T The other nucleic acid: RNA The ribonucleic acid plays a crucial role in protein synthesis. It is also made by nucleotides as the DNA but with some differences and it is found both in the nucleus and in the cytoplasm of a cell. Unlike the DNA, there are three types of RNA in a cell: mRNA, tRNA and rRNA Main differences between RNA and DNA Characteristics DNA RNA Nº of strands 2 1 Name of sugar deoxyribose ribose Bases A, T, C, G A, U, C, G Shape Double helix Linear or folded Types 1 3 nucleus Nucleus and cytoplasm Location in eukaryotes