DNA and DNA replication
Hey, (probably non-existent) everyone, I’m back. This time I’m going to talk about DNA. I even made my own images! As you can imagine, DNA is essential for molecular biology. Actually, the main idea, or, more accurately, the central dogma of molecular biology, dictates the directional flow of genetic information within living cells: DNA makes RNA, and RNA makes protein.
There are four main types of macromolecules in cells: proteins, lipids, nucleic acids and sugars. DNA (as well as RNA) is a nucleic acid. Nucleic acids monomers are called nucleotides and are made of a sugar (ribose in RNA, deoxyribose in DNA), a phosphate group and a nitrogen base.
There are two types of a nitrogen base: purines and pyramidines. Adenine and Guanine are purines, Cytosine and Thymine (in RNA instead of Thymine we have Uracil) are pyramidines. Further I will be using their first initials (A,G,C,T and U) instead of the full name, that’s what everyone does anyway Bases and sugars together are called nucleosides, if you add a phosphate group, you get a nucleotide. Each strand of DNA is a strand of nucleotides, where the sugar is bonded with the phosphate group, thus creating a sugar-phosphate backbone. It is directional - there is a 5' (phosphate) and 3' (OH) ends. The strands are located antiparallel, meaning they look in different directions. The interesting thing about the nucleotide, is that each base can be paired only with a certain other: Adenine is paired with Thymine or Uracill (depends on what we have - DNA or RNA), and Guanine is paired with Cytosine. A—T, G—C
Actually, there is a whole story behind the discovery of the DNA structure. For many years, the discovery of the DNA structure was attributed solely to Francis Crick and James D. Watson, 2 male scientists who received a Nobel prize for that discovery. It was later pointed out that the key contribution to that discovery was made by a woman named Rosalind Franklin, a physical chemist who made the clearest at the time X-Ray image of the DNA (called photo 51) with her student Raymond Gosling. She also determined the sugar-phosphate backbone location. However, her colleague shared her findings with Watson and Crick without her consent, and they used her unpublished data to determine the DNA double helix.
Anyway, Rosalind Franklin's contribution was forgotten for a while and only resurfaced later. Although Watson said that she should have shared the Nobel prize if she hadn't died earlier (at age 37)
Anyway, this is her.
And this is the image (photo 51)
Another important woman in the history of molecular biology is Tsuneko Okazaki - a japanese molecular biologist who, together with her husband Reiji, discovered Okazaki fragments. She is still alive, by the way
To understand what those are, we need to talk a bit about DNA replication.
There were a few theories about the nature of DNA replication. There were three main approaches that were considered by the scientist. Does the DNA copy itself, making both strands of a new copy from a new material, leaving the old copy unchanged? (that's the conservative model) Or, maybe, there is a mix of old and new material in each strand? (dispersive model) Or is there a strand that is made out of new material and the other of an old one? (semiconservative model) The semiconservative model was proven to be right by Frank Stahl and Matthew Meselson. Turns out, DNA replicates using a structure called a replication fork - a localized region of replication that moves along the parental DNA strand. It has the form of a Y, hence the name "fork"
Anyway, what happens during replication? Replication always happens from 5" to 3" end. Which means, that if we use both strand to replicate DNA, we need to figure something out. That's where Okazaki fragments come in. They move the opposite way inside a replication fork, making fragments of new DNA I think, it'll make sense in a picture:
Anyway, that's the gist of it.
