Mutations

DNA to RNA to Protein

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 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 is used to carry the information from the nucleusThe control center of the cell that contains DNA and directs cellular activities. in the DNA 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.

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, but I didn’t care where she was because 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.  And I sent out a copy to be translated into this new language, just as RNA, the pink on the diagram, 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.

Mutations

Recall from the DNA Replication mini-lecture that we define a mutationA change in DNA sequence that can affect gene function. as a permanent variation in DNA that happens because of damage to the DNA.  Again, yes, these CAN result in cancer, if they target the right genes in the cell.   There are different kinds of mutations.  I have you consider the effect of these different types in the DNA transcription and translation lab.  In that lab, we consider the effect of each type of mutation on the protein for which a gene codes.  We previously described these general types of mutations.  First, we had point mutations, where a nucleotideThe basic building block of DNA and RNA, consisting of a sugar, phosphate group, and nitrogenous b is substituted for another.  Second, we had frameshift mutations where we inserted or deleted a nucleotide.  Without even describing these, I’m sure that you can see that mutations change the original DNA sequence, which changes the codon, the anticodon, and POSSIBLY the protein.

Now, these terms: point mutation and frameshift mutation refer to what happens to the DNA.  There are different terms for describing what happens to the resulting protein. 

Silent Mutations

No matter what type of mutation occurs, substitution or deletion, if the mutation doesn’t cause any difference in the resulting protein, it is called a silent mutation.  How can this happen?  Take a look at the genetic codeThe set of rules by which DNA sequences are translated into proteins..  In general, if we change the last nucleotide on a codon, it probably doesn’t change the amino acid.  See….changing CUU to CUC doesn’t change the factA statement based on direct observation that is repeatedly confirmed. that Leucine will be the amino acid in the protein chain. You are accumulating silent mutations right now.  You get more as you age.

But what if it DOES change the amino acid?  If it does change it, no matter how, it is called either a missense or a nonsense mutation.

A nonsense mutation simply means that no viable protein is produced.  This most often happens when we change the codon to a stop codonA codon that signals the end of translation (UAA, UAG, UGA)..  Again, take a look at the genetic code.  If we change UAC to UAA, we will stop making the protein. 

Nonsense Mutations

But what if it DOES change the amino acid?  If it does change it, no matter how, it is called either a missense or a nonsense mutation.

A nonsense mutation simply means that no viable protein is produced.  This most often happens when we change the codon to a stop codon.  Again, take a look at the genetic code.  If we change UAC to UAA, we will stop making the protein. 

Missense Mutations

Let’s do a missense mutation.  Just think of it like using wheat flour to make bread instead of white flour.  You get bread, but it’s not what  you intended. 

Every human has a gene that says: make a red blood cell.  Actually, it’s bunch of genes, but to keep things simple, let’s just say it’s one gene that determines if your red blood cells are nice and plump, like the one in the top of the picture, and able to carry oxygen, or if your red blood cells are sickle shaped and not so great at carrying oxygen.  There is a variant of the gene in which one nucleotide has been changed.  One nucleotide.  Somewhere in the sequence of the DNA, a thymine got replaced by an adenineA purine nitrogenous base found in DNA and RNA, pairs with thymine (DNA) or uracil (RNA)..  Oooops.  But that changes the codon, which changes the anticodon, which changes the amino acid to which the codon matches.  In people with sickle cell anemiaA condition characterized by a deficiency of red blood cells or hemoglobin, leading to reduced oxyge, there is one amino acid, out of thousands, that is the genesis of their disease.  But what if the nucleotide changed and it didn’t lead to a different amino acid?  Great, fine, that’s called a silent mutation.

The MAGY protein

Let’s do some mutations with the MAGY protein.  The lab gives you this sequence of DNA that will make this MAGY protein that has only four amino acid in it.  Let’s figure out what those four amino acids are by transcribing and translating this DNA sequence. 

We can use those base-pairing rules to figure out the RNA sequence that starts with AUG.  Don’t forget that when you transcribe, you can’t use T in the RNA.  Right??  Don’t forget those base-pairing rules. 

Let’s translate the first two amino acids using the genetic code.  Don’t try to read this little tiny one  have on this slide. To start, let me give you a hint.  Methionine is the first amino acid in every protein. Kinda…sometimes it can get cut out of the protein upon final folding.  But, look at your genetic code!  AUG matches with the Met amino acid.  Keep going.  Find GCC on the genetic code.  What is its corresponding amino acid?  Keep doing this until you get to the UAA codon.  That corresponds to a signal to stop making the protein.  So, what is the GCA at the end???  Ummm….junk?  Kinda?  That’s explained in a future mini-lecture.

Referring back to what I said at the beginning of this slide, that the MAGY protein has four amino acids in it, it seems as though we’ve got that.  M   A   G  and tyrosine.  I feel like someone said MAGT doesn’t sound right, let’s name it MAGY. 

The MAGY protein

Let’s apply the MAGY protein to the different kinds of mutations.  In the lab, you are given these variations of the MAGY DNA sequence.  The first one listed is the one that we just translated into Met Ala Gly and Tyr amino acids.  Look at that silent mutation there where that one amino acid circled in red is substituted for that original A.  Translate this DNA sequence, using the process lightly described in the previous slide, and see what happens to the protein.  Remember what a silent mutation means…that there will be no difference in the resulting protein.  Hint. Hint.

Check out the missense mutation that is also just one nucleotide, but this time, it makes a difference.  And then we have the frameshift mutation that causes complete rearrangement of the codons.  This example here is when one nucleotide is inserted, but remember that frameshifts can have deletions as well.

For the lab, you have to translate these four DNA sequences.  Remember to make the RNA first and then use the genetic code to get the amino acids.