The effects of mutations

Voiceover: Today we're going to talk about the overall effects of a genetic mutation and how mutations impact the organism as a whole. But first, I want to review the central dogma of molecular biology, and how genetic information in a cell is stored in the form of DNA, which is then transcribed to form RNA, which is then translated to form protein. Nucleotides from DNA are transcribed to their complementary forms on RNA, which are then read as codons, or groups of three, to code for specific amino acids in a larger protein. If you mutate one of the nucleotides on DNA, like turn this thymine base into an adenine base, then that will affect the RNA sequence, and ultimately the protein that follows. We say that mutations are generally mistakes in a cell's DNA that lead to abnormal protein production. So are mutations good, or are the bad, and what kind of effect do they have on the affected organism? Well, there really isn't a good answer to this question at all. There are many, many different types of mutations out there that can result in big structural changes, like the little pictures I've drawn out here, or may result in little, subtle changes, that might go completely unnoticed. It's very difficult to call a mutation good or bad though, since it really depends on a huge number of things, including the environment that the organism lives in. Let's look at an example of a good mutation. The bacteria Streptococcus Pneumoniae is the bacteria that you typically see associated with pneumonia. One of the more popular treatments for pneumonia is giving the infected person an antibiotic like penicillin, which would help kill all of the bacteria and get rid of the disease. But sometimes, you can find some mutated Streptococcus bacteria, that will have a special trait that makes them resistant to penicillin, and now penicillin won't kill them as easily as it'll kill the bacteria without the mutation. We call this a good mutation because the bacteria are living in a human host, where they are likely to encounter this deadly penicillin. Being resistant to antibiotics like penicillin would then be beneficial to the bacteria. Just to clarify, I'm calling this a good mutation for the bacteria, not really for the human infected, since it'll be harder for them to get rid of the bacteria that are resistant to certain antibiotics. Let's look at an example of a bad mutation. The disease cystic fibrosis is usually caused by a mutation in the CFTR gene. I'm not really going to go into detail about how this mutation actually hurts you, but I'll leave you with the idea that what it does is it makes the mucus that you would find in a person's lungs really, really thick, which makes it really hard for people affected with the disease to breathe. In general, we can say that a mutation causing cystic fibrosis would be a "bad" mutation. But mutations aren't strictly good or strictly bad. In fact, there are some mutations that can cause some favorable and some disadvantageous effects. Sickle-cell disease results from a mutation in a protein called hemoglobin, that you'd find in red blood cells. This mutation turns hemoglobin into a much less functional form, which we'll call "HbS". It's much less efficient at moving oxygen around the human body. Another effect of sickle-cell disease is it makes the diseased person less susceptible to malaria. Malaria is a parasite that grows and multiplies in red blood cells, and can have a lot of nasty effects on the host organism. The malaria parasite can't really grow as well in red blood cells that are affected with sickle cell disease. In this case, the mutation associated with this disease has one bad effect, which is that the HbS isn't as good at carrying oxygen around the body, but also a good effect, in that it makes it less likely that the diseased person will be affected by malaria, since they can't grow as well in the human's red blood cells. What did we learn? First we learned that the effects of a mutation will usually, but not always, appear at the protein level. There are some exceptions to this rule. Second, we learned that genetic mutations can have advantageous, deleterious, or neutral effects, depending on the type of mutation, the environment that the infected organism lives in, as well as a multitude of other factors.