The Nuclecule Of Life: The Structure Of DNA

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1.1 The structure of DNA
Deoxyribonucleic acid (DNA) is often referred to as “the molecule of life”. It encodes the instructions that are used in the development and functioning of all known living organisms. The molecule itself was first discovered in 1860 by Friedrich Miescher (reviewed in Dahm 2005). Further work was performed by other chemists, including Phoebus Levene who identified the components of the molecule, including the presence of ribose sugars and phosphate groups, as well as four nucleotide bases; the pyrimidines, cytosine (C), and thymine (T); and the purines, guanine (G), and adenine (A) (Levene 1919). Erwin Chargaff showed that in DNA the amount of adenine nucleotides equalled the number of thymine nucleotides and the number
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These single strands then act as templates for nucleotides to be incorporated into a new complementary strand as shown in Figure 1.3b. DNA replication occurs from the 5’ end to the 3’ end, so on one strand it can proceed continuously, while on the lagging strand it must stop and start repeatedly, creating Okazaki fragments (Figure 1.3b) (Sakabe and Okazaki 1966). The terms upstream and downstream are used to refer to the 5’ and 3’ ends respectively of the molecule, due to this unidirectionality of DNA synthesis.
A polymerisation reaction can then take place using the enzyme DNA polymerase, to create two new identical double helices from these single stranded templates. The complementary nature of the base pairing allows faithful reproduction of the genetic material during cell divisions, and between generations of organisms (Kornberg et al. 1956).
This natural ability of DNA to replicate was later used to develop the polymerase chain reaction (PCR) (Mullis and Faloona 1987) which selectively amplifies a DNA sequence using oligonucleotide primers, DNA polymerase, and deoxynucleotides in a reaction mixture that is cycled through various temperatures, allowing the DNA to sequentially denature, then bind primers and finally extend a complementary strand (Saiki et al.

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