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 of cytosine nucleotides equalled the number of guanine nucleotides. He also showed that the composition of DNA varied between species. These became known as “Chargaff’s rules” and hinted at the pairing structure of DNA, as well as suggesting that DNA rather than protein is the molecule of heredity (Chargaff 1951).
Chargaff’s observation was subsequently found to be due to complementary base pairing formed by hydrogen bonds between adenine and thymine, and between cytosine and guanine. DNA molecules are made up of …show more content…
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