Like the first chapter, the 3rd chapter is quite entertaining and easy to read. However, like the first chapter, it also seems to jump around topics a bit. But I can see the underlying connection between the sections; that the molecular world must be understood with statistical laws due to the thermal motion of the world.
I really enjoyed reading section 3.3, which describes the history and development of the field of genetics. I had heard of many of the scientists mentioned, but I had never read their stories in a chronological order, which meant I failed to realise how each one affected each other until now. I also didn’t realise how much physics (and physicists) helped deduce the hereditary material. Because it is what I have been taught as I grew up, it now seems so obvious that the hereditary material in cells is a macromolecule. But it seems that in a number of points in history, if the scientists weren’t so lucky, it would have taken even a few decades more to understand that DNA is the hereditary molecule. It makes me wonder what things that we don’t understand now that we will look back on and feel foolish for not realising straight away.
We know now though that DNA is not the only mechanism of inheritance. I read a few parts of the 2nd chapter that caught my eye, and one part discussed other sources of hereditary information. Naturally DNA is the only source that is both necessary and (almost) sufficient for producing a daughter cell, but the others can pass on other characteristics of the parent. When a cell splits, the daughter cell takes some of the cytoplasm with it. This includes some of the proteins and other macromolecules in the cytoplasm. This is necessary for the daughter cell to survive, since DNA with no other biomolecular infrastructure, like RNA polymerases, cannot produce a living cell. However, this means if there are unwanted molecules in the cytoplasm, then they will also be passed on to the next generation. While a particular unwanted molecule may not coalesce to a stage which is detrimental to a cell during its life, daughter cells of this cell will be generated with a higher concentration of these unwanted molecules. Diseases like Huntington’s disease exist because of this hereditary mechanism. It is also for this reason that identical twins are even more identical than clones, since identical twins originate from the same cytoplasm.
Cell differentiation is another inherited characteristic that is not encoded by DNA. Once a stem cell differentiates into another cell, all the offspring of that cell are will be that kind of cell, despite having the same DNA as the original cell. More details can be found in Nelson, 2.3.4’ Track 2.