In this post, we will discuss the second universal feature of cells, according to Alberts et al.:
The structure of the double-stranded DNA molecule is essential to making life possible. A nucleotide is an organic molecule that serves as a monomer in a single DNA stand. Nucleotides consist of two parts: (1) a sugar (deoxyribose) with a phosphate group attached to it, and (2) any of the four bases of either A, G, C, or T as discussed in the previous post. Sugars are linked together using phosphate groups, and a polymer chain is created, which has a sugar-phosphate backbone that is repetitive, with a number of bases protruding from it. Adding monomers to one end of a DNA polymer extends it. In isolation, rather than in a living cell, monomers are able to be added in any order hypothetically, since each monomer is linked to the next in the same fashion, through the piece of the molecule that they all share. Because things are much different in the living cell, DNA is synthesized on a template that is created from a preexisting DNA strand. The bases (A, G, C, or T in possibility) that protrude from the existing strand are able to bind to bases of the new strand that is being formed, or synthesized, following a rule that it is essential to remember: A binds to T, and C binds to G. This “law” governing the process of base-pairing keeps fresh monomers in place. It also controls for which of the four monomers ends up being added to the growing strand next. This results in the double-stranded structure of DNA, in which two exactly complementary sequences of As, Cs, Ts, and Gs exist. The double helix is formed by the twisting of the two strands around each other (Alberts et al. 2014).
While the bonds between the sugar-phosphate links are strong, the bonds between base pairs are fairly weak in comparison So, when the two strands of the double helix are separated from one another, their backbones do not break, conveniently, allowing each strand to be used in the synthesis of yet another DNA strand that will be complementary to it. Though differences exist among cells of different organisms in, for example, the rate at which this process of DNA replication occurs, the controls that start or stop it, and the auxiliary molecules that help the process along, one thing always remains the same: that DNA stores the information about an individual pertaining to heredity, and through the process of templated polymerization, this information continues to be replicated throughout the living world (Alberts et al. 2014).
If some of this seemed a bit confusing to you, don't worry. In an upcoming post I will either draw, design, or cite a diagram that makes some of this a little easier to understand through visualization.
A prospective medical student, looking to help others succeed.