Today, in a post entirely inspired and based upon the Alberts et al. text, I would like to go over a few things that may be easier to understand visually. Above is a diagram that I spent much of my day yesterday drawing. It is a bit of a sloppy sketch. (I'll have to get better and quicker at these.) I will explain what each letter of the figure is showing. I would like to emphasize that when reviewing this diagram, you should not focus specifically on how each thing is particularly connected (such as to which carbon the phosphate is connected to on the sugar if you have some organic chemistry knowledge), but rather, focus on what connectivities are being made (such as the sugar phosphate to a base).
In the sketch above, DNA and its building blocks are shown. In part A of the sketch, the building blocks of DNA are shown, where you can see that a sugar and a phosphate combine to create a sugar-phosphate that binds to a base, creating a DNA nucleotide. Obviously, this does not just happen with guanine; it is the case with adenine, cytosine, and thymine as well. In other words, the sugar-phosphate that you can see in part A has a nitrogen-containing side group, otherwise known as a base, which may be of the A, C, G, or T type (Alberts et al. 2014).
In part B of the sketch above, what is detailed is a single DNA strand. This is depicted by a set of nucleotides joined together by sugar-phosphate linkages. Individual sugar-phosphate linkages are not symmetric, notably, which means that the backbone of the DNA strand has a given and definite directionality, or polarity. The directionality of the backbone serves as a guide to the molecular processes that occur, allowing the interpretation of and copying of DNA within cells. Just as how we read from left to right in English, the information contained in DNA must always be read in a particular direction as well (Alberts et al. 2014).
In part C of the sketch, you can see that nucleotides join together in a new DNA stand in a particular sequence, as controlled by the sequence of nucleotides in an existing DNA strand, through the process of templated polymerization. As discussed, T pairs with A, and G pairs with C. The nucleotide sequence of the new strand is complementary to the old strand. The backbone of the new strand has a directionality opposite to that of the old strand, corresponding to the base pairs. For instance, a sequence of GTAA on the original strand now has TTAC on the complementary strand (Alberts et al. 2014).
In part D, a normal and complete double-stranded DNA molecule is shown, with two connected complementary strands. What links the nucleotides of each strand is strong, covalent chemical bonds. However, what links nucleotides on opposite strands is a force that is much weaker, known as a hydrogen bonding force (Alberts et al. 2014).
And finally, in the sloppy sketch in part E, a DNA double helix is shown, wherein the two strands of DNA twist around one another. This structure is very sturdy in its constructiona and can accommodate any sequence of nucleotides without changing its basic structure (Alberts et al. 2014).
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