M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
Genetic mutation is a change in DNA caused by random errors in DNA replication or by the presence of mutagens which leads to increased genetic variation. A point mutation is a single change, either through base substitution or insertion/deletion. Chromosomal mutation on the other hand, affects large portions of DNA through deletion/insertion, duplication, inversion, translocation or non-disjunction.
Mutations are very important concept in Biology. They're the process by which new genes can be created creating new opportunities for evolution to occur. Now how do mutations occur? There's two major ways that you can have a mutation occur.
One is during the normal DNA replication process of the cell cycle right before cell division mistakes can happen now you got 6 billion nucleotides that your cell has to copy in a human cell that is but your cell is really good at catching any errors and in fact you only have out of every billion nucleotides you only get one error sneaking in but that doesn't mean that out of your 6 billion you're going to get somewhere around six-ish or so mistakes every time you copy your cells DNA and if you think about it you've been copying your cells DNA since you were wee little cell inside your mummy's uterus so you're much bigger now you copy your DNA a whole bunch more times and that's one of the reason why as people age the chances of giving cell cancers et cetera increases because there's just accumulated genetic damage.
Another common source is things called mutagens which are chemicals or other environmental factors that can cause mistakes either doing DNA replication or damage to the DNA that later on when it gets repaired mistakes are made by the repairing enzymes for example ultra violet radiation can cause a damage because it causes two neighboring thymines in the DNA sequence to get joined to each other instead of joining with the adenines across from them and then when the repair enzymes cut out those two joined together thymines or thymine dimer they can readily make some mistakes well good reason not to, so there's two major categories of mutations. Point mutation which is talking about when you're changing maybe a single base versus chromosomal mutation that's where you're changing large portions of the DNA.
Within the point mutations you can have base substitutions which fall under several categories. There are silent base substitutions, missense and nonsense base substitutions. That's a little bit different from insertions or deletions where they pretty much have one big effect.
Chromosomal mutations, when you're changing large portions of the DNA you can have huge effects on the overall genetic function of that cell. Now there's deletions and insertions, there's duplications, inversions where you're swapping things around translocation where you're taking DNA from one chromosome putting on another and then something that's very different called non-disjunction which I'll get into a more depth in just a moment. But let's first start off with the point mutations.
Now one kind of substitution is something called a silent mutation and this is one reason why a lot of mutations seem relatively benign they don't have any real bad effects because of something called redundancy within the genetic code.
Here's a sequence of codons caa, guc, cga, aau now these brackets do indicate the beginning and end of a of each codon which was set up for the ribosome by the start codon. And in green I put the 3 letter abbreviation for the various amino acid names otherwise I just have too much to write because some of these names can be pretty long like Phenylalanine, and Threonine and such so that's what these three letter abbreviations are. Now in this particular instance I'm going to change the a in this codon to a g now cga, cg cga I can see translates to Arg the amino acid cgg if I look hey! Cga cgg they are both the same Arg or Arginine amino acid so that's why this is called silent because it doesn't have any real effect on how the protein looks so you could have two sequences of RNA or DNA being different but they still have no real difference in their phenotype.
That's different from a missense mutation where again I'm going to change my cga codon but this time I'm changing not the last letter but I'm changing my first one and now if I take a look instead of being up here I'm down there I'm changing it Gly which can have some significant effects on the overall protein if it's in a part of the active side or an important forwarding location of the protein for example the disease known as sickle cell anemia is caused by single missense mutation.
A nonsense mutation is typically something that's bad it's not good for the cell in that what happens is you randomly create a new stop codon so as cga if we change that first c to a u, uga is one of the stop codons that tells the ribosome stop making the protein and so it will come along du du du and just stop so sometimes this is often called a sorry a chain terminating mutation because it terminates the chain of amino acids early.
Now a insertion or deletion, you won't have silent insertion or deletions because you're going to change things and if you notice by adding in a g in front of that c now I've shifted which three amino acids sorry which three nucleotides I'm reading so my my reading frame has been changed that's why this is called a reading frame shift or simply a frame shift mutation and it causes every amino acid after the insertion or after the deletion to be screwed up and that's because you delete or insert three pairs and then you'll have one additional amino acid but still you can have really big effects from this kind of mutation.
Now chromosomal mutations as I said are talking about changing the entire chromosome. Here's an example of deletion and insertion, here is a chromosome with my genes abcd and so on. If I delete gene d then the chromosome is being altered and then the cells looses the effect of whatever that d gene works. Now that's different than say a duplication, here during copying sometimes mistakes can happen and I can wind up with two additional copies of b and c so now the cell will windup having depending on perhaps more of those proteins called for by those genes or may influence other factors during say meiosis because of them pairing up during prophase and prometaphase et cetera you can have lots of effects. Inversion here I have my 3 genes and I have not lost any in DNA but instead I flipped it around so again this can cause some problems during the pairing up that happens during the first step of meiosis but it can also have some effects if say there's a regulatory sequence of DNA right here now it's supposed to be controlling b but now it's controlling d so weird things can happen in the cell.
Translocation, I hate some of the names that scientists come up with, rather than just saying a it moved they say translocation see a and b have been swapped over from the long chromosome to the shorter chromosome and mno has been swapped over to the big one.
Now I had mentioned previously that non-disjunction. This is something that is not caused by a mistake during DNA replication or whatever, this can be caused during the process of making a gamete either sperms or eggs called meiosis and here we see rather than having the appropriate sequence of events happening during meiosis I meiosis II we wind up having some of our chromosomes not being separated whether during the pairing process or during meiosis II when the chromatids gets separated and you wind up with gametes these two are normal but this one has an extra chromosome this one here is missing a chromosome. As and to give you an idea of kind of effects they can have, having an additional chromosome number 21 called trisomy 21 is the disease known as down syndrome, so as you can see gene mutations can happen in many different ways sometimes they're effects are benign they have no effect sometimes they're effects can be bad and sometimes occasionally they're good like if you gain super powers.