# DNA --- **DNA** or **deoxyribonucleic acid** is a self-replicating [[nucleic acid]] that stores all the information needed in order to make complex life possible. The study of DNA and genes is [[genetics]]. It is a polymer of the four [[nucleotide#Dideoxynucleotides|deoxyribonucleotides]], adenine, guanine, cytosine and thymine. *Every organism on Earth* has DNA. ![[DNA.png]] DNA can *store an enormous amount of information*. It's also basically a library dedicated to [[evolution]], holding a record of all the mutations that that organism has undergone. In one human nucleus there is ~2 meters of DNA. [[vaccine#DNA Vaccines|DNA vaccines]] are vaccines that use DNA as a vector of getting safe pathogenic parts into a host system. ## Structure [[DNA structure|The structure of DNA]] is made up of two very long strands of [[nucleotide]] chains bound to each other in a distinctive "double helix" spiral. The two strands our bound together sugar to phosphate, sugar to phosphate all the way down, forming two long "backbone" structures. The two strands are bond to *each other* by hydrogen bonds between their complementary nitrogenous bases. This arrangement is very simple and very secure. ## Function An (vastly) oversimplified overview of DNA's function is that, for every [[protein]] that a cell or organism makes in order to live its life, there is a segment of DNA called a [[gene]] that is arranged in such a way that forms a [[codon|code]] that [[RNA polymerase|other molecules]] can [[transcription|read]] and then those molecules send [[mRNA|a copy]] of that code to yet *another* [[ribosome|molecule]] that can then [[translation|make the protein]]. All the proteins ultimately serve the purpose to allow the DNA time, space and energy to replicate again. This overarching process is called the [[central dogma of biology]]. ## Why is DNA So Special? The continuity of life depends on its genetic information being accurately [[DNA replication|replicated]] over and over and over and over again. DNA is so good at this because despite the complexity and vastness of information that it holds, it is *very simple and very stable*. It may not seem simple—a [[chromosome]] of DNA is the largest molecule in our bodies—yet it is comprised of only 4 different types of [[nucleotide|building blocks]]. And those building blocks are arranged in a perfectly repetitive pattern—meaning there is no variation in the chain of one nucleotide after another all the way down. DNA is also incredibly stable. When not being actively [[DNA replication|replicated]] or [[transcription|transcribed]], it just sort of sits there. ## DNA Replication **[[DNA replication]]** is one of, if not *the* most fundamental process in the whole of biology. It amazingly complex and *stunningly accurate*. Consistently (mostly) accurate replication is just about the single strongest [[evolution|evolutionary imparative]] there is (for certain organisms, like humans). The replication process only makes about one mistake per *billion* nucleotides. Mistakes do, of course, happen but—despite being potentially deadly—they are not always a bad thing. This is how random [[mutation|mutations]] come about, and these are a necessary function during [[evolution]]. DNA replicates by **semiconservative replication**. This means that each strand is used as a template to make one new strand, resulting in two new whole molecules of DNA, each one with an "original" parent strand and one "new" daughter strand. ![[semiconservative replication.png]] ## Prokaryotic vs Eukaryotic DNA Every organism has DNA, but there are significant differences in between how prokaryotic and eukaryotic genes are coded into the DNA. [[Prokaryotic DNA]] is circular, and the [[gene|genes]] are sometimes found in groups that are all transcribed together called [[prokaryotic DNA|operons]]. Eukaryotic DNA is linear and arranged in [[chromosome|chromosomes]] %%#verify all eukaryotes%%. They also have a lot of "junk" DNA, and the DNA is split over a lot of space. %%add link%% ### DNA Methylation **DNA methylation** is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to [[gene regulation#Negative Control|repress gene transcription]]. ### DNA Vaccines > [!experiments] The **Hershey-Chase experiment** proved that it is the [[DNA]] molecules found in chromosomes, not the [[protein]], that carries the genetic information. > >They did this by observing a T2 [[virus]] infect an *[[Escherichia coli|E. coli]]* bacterium. >[!experiments] The **Meselon-Stall experiment** determined that DNA replicates using [[#DNA Replication|semiconservative replication]]. They used [[bacteria#E Coli|e. coli]] and tracked the DNA strands over generations. >[!people] American **James Watson** and Englishman **Francis Crick** are most well known for their discovery of the double helix structure of DNA building on (or stealing from, depending on your definition) the discoveries and data of Rosalind Franklin. **Rosalind Franklin** was an English chemist and x-ray crystallographer whose groundbreaking work informed Watson and Crick's discovery of DNA structure. She was ineligible to share their Nobel Prize as she had passed away before it was awarded in 1962. ___