There are certain topics in science that do not allow you to know “basics”. These are the topics that you either have vast knowledge about, or have no knowledge at all. Typically, these are the topics that are categorized as complicated and difficult. I’m not going to lie, this post will divulge into one of these topics – and it won’t be easy. But rest assured that, if you hang in there, and concentrate, this will be comprehendible. As I’m sure you are well aware by now, this post is on the Central Dogma.
What exactly is the Central Dogma, might you ask? For those who are unfamiliar, the Central Dogma is – basically – the processes involving genetic information. These processes are known as transcription and translation. The goal of the Central Dogma is to go from DNA, to Protein. This requires an intermediate step of turning into RNA. While this seems basic now, there is a lot more to it than meets the eye. Shall we begin?
The entire process starts with DNA. As you know, DNA is located in the nucleus of a cell, and permanently contains genetic material. It is comprised of nucleotides that form the pairs A-T, and G-C. The first step in Central Dogma is transcription. This is, basically, the process that forms RNA. Transcription occurs in three major steps: Initiation, Elongation, Termination. The former step, Initiation, is when an RNA Polymerase – an enzyme that specializes in creating RNA polymers – is attracted to what is known as a TATA box. The TATA box is a location on the DNA where there is a pattern of T-A-T-A nucleotides. This is where the new strand of genetic material will start to be copied from. Initiation continues when other factors are assembled around the DNA. These other factors include an RNA transcript, which temporarily separate the hydrogen bonds that run down the middle of DNA’s double helix.
The secondary step of transcription is Elongation. This is when the RNA polymerase slides down the DNA, using RNA nucleotides to “match” the DNA strand being replicated. The RNA nucleotides still follow the A-T and G-C pattern of pairing together. However, in RNA, the T nucleotide (Thymine) is replaced with a U (Uracil) nucleotide. A diagram of elongation can be seen below. Take notice of how the RNA nucleotides (which are red in picture) match with the blue DNA nucleotides (which are blue in picture) inside of the RNA Polymerase structure (the white “bubble” looking figure).
The final step of transcription is known as Termination. The process of Elongation creates what is known as Pre-mRNA. In termination, the elongation process ceases (the RNA polymerase received a message from a codon to stop replicating the DNA). Then, the Pre-mRNA is given a 5′ cap and a 3′ tail. The 5′ cap goes at the “head”/”front” of the RNA strand. The purpose of this cap is to ensure that nothing can be added to this side of the genetic strand. The 3′ tail, known as a Poly-A tail as it is primarily comprised of the nucleotide A (Adenine), is where any additions to the strand are made. Termination is also the process where splicing happens. Splicing is when any unneeded segments are removed from the strand (we will touch on that a little later). After the Termination process is complete, Pre-mRNA becomes mRNA, otherwise referred to as messenger RNA.
Ok wow! That is a lot of steps! Here’s what we’ve done so far: we’ve taken DNA in the nucleus, undergone the process of Transcription, and made RNA. The next step is that of Translation (which occurs in the cytoplasm of a cell). This is the step that actually creates the proteins that will be used throughout the body. The process uses what is known as a tRNA to create proteins. The tRNA is an entity that contains an amino acid (which is used to make proteins) on one side. The other side has what is known as an anticodon. An anticodon is the exact opposite of a codon. So what then is a codon? A codon is a segment of three nucleotide codes.
(Time for a mini lesson on codons! Pretend you have an mRNA with the following nucleotides: AGUCCAGUG. The codons for this selection would be AGU, CCA, and GUG. A codon is a segment of three nucleotides. Understand? Good!)
Going back to translation, now: tRNA is comprised of an amino acid on one side and an anticodon on the other (see the picture above). The anticodon is the “opposite” of the codon. This is the case so that the codon and anti-codon are able to pair together. (If a codon was CAC, then the anti-codon would be GTG). This process of pairing occurs inside of a Ribosome, which slides down the strip of RNA. The Ribosome, for purposes of this post only, has the same “job” as the RNA polymerase – it “houses” the process as it occurs (if that helps you any in visualizing what is happening). As tRNA pair up with the mRNA, long strips of amino acids start to form. These strips are known as polypeptides, and are what make up proteins. The picture below demonstrates this process of Translation. Take special notice of the tRNA (light blue) and the Ribosome (brown).
There you have it! That’s the Central Dogma! We’ve gone from DNA, through transcription to RNA, through translation to a Protein! That about does it for this blog post. I hope you lear-…
…just kidding! We aren’t done yet! Absolutely not! There is still so much more of the Central Dogma to discuss! The next topic on the table is that of alternate splicing. As we just talked about above, DNA makes RNA, which makes protein. The concept of alternate splicing is relatively simple. This process says that DNA makes RNA, and RNA is capable of producing multiple types of protein. As a result, one gene is able to make multiple proteins!
First thing’s first, however: we need to learn what exactly is splicing? Splicing is the process of removing a “segment” of RNA that is not needed for a specific protein synthesis. The removed segment, known as an intron, is recycled for its nucleotides. The pieces that are then joined together are known as exons. The process of splicing occurs by a splicesome – which is comprised of RNA and protein. The diagram below most accurately represents splicing.
So now we know how splicing works. Take the same general concept and apply it to the transcription process we talked about earlier. By using the process of alternate splicing, each piece of mRNA can end up with different combinations of exons. These different combinations will lead to different proteins. Check out the diagram. Take notice of how the transcription process occurs the same regardless of which exons are selected.
Still with me, so far? Like I said, this was not going to be an easy blog post! If you have made it this far, though, then you definitely deserve some credit! You are doing really well! Hang in there, because we are almost done! The final part of the Central Dogma that we are going to discuss is the idea that transcription is not a one way street. That’s right, we’re now going to talk about reverse transcription!
The process of reverse transcription is exactly how you picture it to be: starting with an mRNA, and ending with DNA. The process is virtually the same as normal transcription. This time, however, the enzyme reverse transcriptase starts at the Poly A tail and moves “up” the strand of mRNA. As it goes, it uses DNA nucleotides to make pairs with the mRNA nucleotides. The end result is a cDNA strand (complimentary DNA).
What exactly is the purpose of reverse transcription? The most common use is that of viruses. A normal virus works by implanting its genetic material into a cell’s DNA. The cell then copies the genetic material of the virus, turning it into mRNA, which in turn makes more of the virus. The virus then spreads to other cells, which also copy and duplicate the virus. Sometimes, however, the virus starts with RNA. Known as a retrovirus, these viruses have to use reverse transcription, using the cell’s genetic material as a template, to create DNA. This viral cDNA is then used to make more viruses within the cell. A common retrovirus, by the way, is the HIV/AIDS virus.
Well there you go! Now we are done with Central Dogma! And I mean it this time too! We have covered a lot. I hope that you have been paying attention, and have learned a lot about the processes of our genetic material. Now, at least, you can say that the Central Dogma is a topic in science that you really understand!