DNA to RNA

The Central Dogma of Molecular Biology

The central dogma of molecular biology states that DNA leads to RNA and then leads to proteinsLarge molecules made of amino acids with various functions in the body..  What does this mean, though?  Your cellsThe basic structural and functional units of life. carry chromosomes that are made of DNA.  The DNA only holds the instructions for making a protein.  That’s all, just info storage.  There is a process called protein synthesis. During this process, the DNA is accessed for its instructions. These instructions are then used to build a protein. That explains the blue DNA on the picture and the purple amino acidThe building blocks of proteins, consisting of an amino group, carboxyl group, and side chain. chain, but what’s the pink?  In this process, an intermediary carries the information from the nucleusThe control center of the cell that contains DNA and directs cellular activities. where the DNA is. It is then transported to the place in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol.. There, the protein will be assembled according to the instructions in the DNA.

All right, so, I have a Swedish lineage and I have my great-great-grandmother’s cookbook.  It’s in Swedish.  I needed it translated, and doing it myself was quite slow going.  I found a woman on the internet that did translationThe process of converting mRNA into a protein. services for Swedish to English.  She was on the west coast. I didn’t care where she was. I was NOT sending her my great-great-grandmother’s cookbook.  I made a copy, send her the copy, she translated it, and sent me back the English version.  I then made wonderful Swedish recipes like curried egg salad and salted cod.  Yeah, great.  But, the point is that I retained the original, like the DNA stays in the nucleus.  I sent out a copy to be translated into this new language. It is similar to how RNA, the pink on the diagram, carries the instructions for making the protein. The instructions are then translated from nucleic acidA substance that releases hydrogen ions (H⁺) in solution. to amino acids. 

What does this all mean for you?  Your DNA holds the instructions for the proteins that make you you.  For example, there are pigmentation genes in your chromosomes. They carry instructions for making a pigment of a certain color. That pigment can be made and then inserted into your eyes for expression for the world to see.

Sequence of Events

Taking the instructions in DNA and making it into RNA and then into an amino acid sequence is a two part process.  First, we have to make a disposable copy of the DNA. This copy can travel out of the nucleus. It can get destroyed, and we don’t care.  This is like me making the copy of the cook book.  This is called transcription.  This word just means “to copy.”  This is what you do when you copy someone’s notes if you miss class; you transcribe their notes.  More specifically, we are going to make something called mRNA, m for messenger.  This is the disposable copy that is produced in transcription.  This is what is taking place in the top part of this picture, in the nucleus of the cell.

Transcription requires that we know the base pairingThe specific hydrogen bonding between complementary bases (A-T, C-G in DNA). rules for RNA. Just as in DNA guanine matches with cytosineA pyrimidine nitrogenous base that pairs with guanine in DNA and RNA.. You can see their pointy shapes in the picture above. In DNA, adenineA purine nitrogenous base found in DNA and RNA, pairs with thymine (DNA) or uracil (RNA). pairs with thymine. This is untrue with RNA. Adenine pairs with a nucleotideThe basic building block of DNA and RNA, consisting of a sugar, phosphate group, and nitrogenous b known as uracil. The baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). pairing rules needed to complete transcription are guanine matches cytosine, adenine matches uracil, and thymine matches adenine.

Transcription

This is a picture of transcription.   That big blob there is not the nucleus. We are going to assume that this process we see here takes place in the nucleus.   The big blob is an enzyme called RNA polymeraseAn enzyme that synthesizes RNA from a DNA template..  Just like DNA polymeraseAn enzyme that synthesizes new DNA strands by adding nucleotides to a template., RNA polymerase makes the RNA polymer.  It knows the base pairing rules too. However, it believes adenine will always connect with uracil. In contrast, DNA polymerase thinks that adenine and thymine always pair up. 

Transcription occurs in three steps according to this slide. In other classes i teach it it goes through many more steps than three. But there are three general steps and they all have something to do with what the RNA polymerase is doing.

The first step is called initiation. This is not as simple as it sounds. In short, RNA polymerase attaches to the DNA. This DNA holds the gene we ultimately want to make into a protein. There are many things that can determine whether or not RNA polymerase will attach to a gene. It is beyond us in this class to delve into those things but I will give you one example. When blood glucoseA simple sugar that is the main source of energy for cells. is high, certain moleculesGroups of atoms bonded together. assist. They help RNA polymerase attach to the genes holding the instructions for insulin in the cells of your pancreasA gland that produces digestive enzymes and hormones like insulin and glucagon.. Whether RNA polymerase attaches depends more on the environment in this example. The need for insulin to bring blood glucose down is a determining factor.

The second step of transcription is called elongationThe phase of DNA replication, transcription, or translation in which the molecule is lengthened.. And this is the process of adding nucleotidesThe building blocks of nucleic acids.. This looks deceptively like DNA replication with one significant difference. We are making M RNA and will use uracil.

The last step is called termination. This process again is not as simple as it sounds. It is when mRNA polymerase releases itself from the DNA. This action stops transcription of the mRNA.  Let’s look into more detail about elongation.

Transcription

In this picture, RNA polymerase has broken the hydrogen bondsWeak attractions between hydrogen and electronegative atoms like oxygen or nitrogen. in DNA. The DNA has unwound in a small spot. This is where we know there is a gene we want to make a protein from.  When we unwind the DNA, we have two strands of DNA. We aren’t going to take the sequence from both strands, just one.  The strand holding the gene sequence is called the template or coding strand.  The other strand is called the non-coding or non-template strand and just kinda hangs out.  RNA polymerase is assembling the teal colored strip of mRNA by calling over free nucleotides.  When it gets to an adenine, it pairs I with uracil.  When it gets to a thymine, it pairs it with adenine.

mRNA Travel and Safety

Before leaving the nucleus mRNA receives a cap and a tail. These are just extra nucleotides on either side of the transcripted gene. The cytoplasm of the cell contains many enzymesProteins that speed up chemical reactions in the body. that are ready to breakdown DNA. The cap and the tail allow these enzymes to start chewing on the mRNA without damaging the gene inside. The coding sequence is protected by the cap and the tail on either side. The mRNA is ready to leave the nucleus through one of those nuclear pores. It then travels to a ribosome for the second step of protein synthesis.

In the ribosome, we will take the sequence of DNA. We will translate it into a sequence of amino acids.  To do this, you need a decoder ring.  I, for my cook book, needed a translator, some who knows both Swedish and English. Here in the ribosome, we need a molecule. This molecule must know both the nucleotides of a DNA sequence and the amino acids that correspond to them.  This is the second process and is called translation.