Protein Synthesis: Transcription

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.  But, the DNA only holds the instructions for making a protein.  That’s all, just info storage.  There is a process called protein synthesis in which the DNA is accessed for its instructions and then the instructions are used to build a protein.  So that explains the DNA on the picture and the amino acidThe building blocks of proteins, consisting of an amino group, carboxyl group, and side chain. chain, but what’s the RNA?  In this process, an intermediary is used to carry the information from the nucleusThe control center of the cell that contains DNA and directs cellular activities. where the DNA is located to the place in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. where the protein will be assembled according to the instructions in the DNA.

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, but I didn’t care where she was because I was NOT sending her my great-great-grandmother’s cookbook.  I made a copy, sent 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.  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, just as RNA carries the instructions for making the protein to be 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, and that pigment can be made and then inserted into your eyes to be expressed for the word to see.

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 that can travel out of the nucleus and 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.  There is an enzyme called DNA helicaseAn enzyme that unwinds and separates DNA strands during replication. that gets in there and breaks the hydrogen bondsWeak attractions between hydrogen and electronegative atoms like oxygen or nitrogen. between the nitrogenous bases of the nucleotidesThe building blocks of nucleic acids..  This bond breakage allows the DNA to unwind.  This is a lot like DNA replication, up to this point.  But instead of using the enzyme DNA polymeraseAn enzyme that synthesizes new DNA strands by adding nucleotides to a template., an enzyme named RNA polymeraseAn enzyme that synthesizes RNA from a DNA template. inserts itself into the DNA bubble.  It’s this enzyme that will make the mRNA.

This is a picture of transcription.   That big blob there is not the nucleus, although we are going to assume that this process we see here is taking place in the nucleus.   The big blob is an enzyme called RNA polymerase.  Just like DNA polymerase, RNA polymerase makes the RNA polymer.  It knows the base pairingThe specific hydrogen bonding between complementary bases (A-T, C-G in DNA). rules too, but it thinks that adenineA purine nitrogenous base found in DNA and RNA, pairs with thymine (DNA) or uracil (RNA). will always connect with uracil whereas DNA polymerase thinks that adenine and thymine always pair up. Unlike DNA replication that wants to copy the entire length of DNA that makes up one of those long pieces of DNA called a chromosome, transcription targets only certain areas on the DNA.  These areas are genes.  They contain sequences of nucleotides that hold the recipe for making one protein – or at, least, that is what we are going to leave it at right now. 

When we unwind the DNA, we have the two strands of DNA, but we aren’t going to take the sequence from both of them, just one.  The strand holding the gene sequence is called the template or coding strand.  This is the red strand on the picture above.  The top strand is called the non-coding or non-template strand and just kinda hangs out.  But, notice that if you know the sequence of a non-coding strand, you could use the baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). pairing rules for DNA to determine the sequence of the coding strand. 

OK, so RNA polymerase gets in there and starts reading the DNA from the 3’ end headed toward the 5’ end of the coding sequence, or the specific strip of DNA holding the gene in question. RNA polymerase does the same thing that DNA polymerase does, with one small difference.  RNA polymerase encounters a guanine and yells out for the match – cytosineA pyrimidine nitrogenous base that pairs with guanine in DNA and RNA..  It does the opposite when it encounters a cytosine by yelling out for a guanine.  When it encounters a thymine, it yells out for an adenine, but when it encounters an adenine, it calls out for uracil.  This is because RNA polymerase knows the base pairing rules for RNA. 

RNA transcripts can be long, like one long word.  But it’s not, it is many three-letter words.  We will find out later why this is the case, but we call a sequence of three nucleotides on mRNA codons.  It’s important to remember that codons are on the mRNA. 

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 it’s when RNA polymerase attaches to the DNA that is holding the gene we would ultimately like to make into a protein. There are many things that can determine whether or not RNA polymerase will attach to a gene. When blood glucoseA simple sugar that is the main source of energy for cells. is high there are moleculesGroups of atoms bonded together. that help RNA polymerase attach to the genes holding the instructions for insulin in cells of your pancreasA gland that produces digestive enzymes and hormones like insulin and glucagon.. So whether or not RNA polymerase attaches is actually determined more by the environment in this example the need of insulin to bring blood glucose down.

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 nucleotides. This looks deceptively like DNA replication with one significant difference. We are making M RNA and will use uracil. We will go into more detail about elongation in the next slide. .

The last step is called termination. This process again is not as simple as it sounds but it is when mRNA polymerase releases itself from the DNA and stops transcription of the mRNA. 

Transcription is actually much more complicated than this.  How does RNA polymerase know where to attach?  How does it know when to detach? 

Learn more about Gene Regulation.