Microbiology of Microbial Genetics

[Music] [Music] in this lecture we're going to discuss microbial genetics this lecture is going to be a little longer than the other ones simply because the material is complex and far-reaching the concepts are essential to microbiology and it may be that you'll need to listen to this lecture a few times but you will definitely get great rewards from understanding the material very well so we're going to start off the discussion by bringing back some important definitions the first of which is what is a gene and there have been several changes in the definition of a gene most recently a gene now is thought of as a section of the chromosome or a section actually of DNA that encodes the primary sequence of some final gene product which can be either a polypeptide or RNA but initially back in the 60s gene was thought of to be a segment of genetic material that encoded for one enzyme the theory was called one gene one enzyme hypothesis then we broadened the definition to include one gene one polypeptide and now we've also included RNA as part of the definition so genes can also include regulatory sequences and these are parts of the DNA that are actually there to regulate the transcription of other genes and these sequences provide signals that denote the beginning or the end or some kind of influence of the transcription itself or the regulatory sequence functions as a place where transcription is begun or a place where replication occurs or where recombination occurs so it's also been broadened to include several different things but basically the true underlying function is in some kind of way it regulates transcription so the genetic code we've alluded to this genetic code in a couple of our discussions already and today we're actually going to clearly define what it means so in the DNA the way it works is that each DNA sequence and each subsequent RNA sequence encodes for one amino acid and generally it takes three base pairs to encode for one amino acid and depending on the order of arrangement of these nucleotide bases that will determine the arrangement of the or the ultimate arrangement of these amino acid sequences and each of these amino acids are joined by a peptide bond and depending on which amino acid actually occurs in these various positions will also influence how the protein is shaped after post translation so transcription and replication as we discussed in earlier lectures is basically very similar to transcription and replication in eukaryotes so bacterial transcription and replication as I said is very similar to eukaryotic to the eukaryotic processes so we have transcription and translation occurring here and we have recombination and replication occurring here but the thing that is very different about bacteria that we're going to see in the next couple of panels is we can also have gene transfer occurring between individuals of the same generation so we certainly get inheritance of the parental DNA into the daughter cells but we can also exchange material genetic material between the daughter cells and that's actually what makes microbial genetics so complex so here we have replication okay remember that word replication of bacterial DNA it's very similar to replication of eukaryotic DNA basically we need to copy both sides of the DNA strand so we get this unfolding or unwinding of the DNA and we have this replication fork and then we actually have a copying of both sides of the DNA and the circular motion and then we basically flip the circles and then we have a complete copy of the DNA in the daughter cell now DNA transcription and bacteria also work rather similarly to eukaryotic cells and that's where the DNA ultimately produces proteins right according to the arrangement of the nucleotide bases where we talked about in the genetic code slide so we take DNA and it gets transcribed to RNA and that RNA polymerase binds to the promoter sequence we get transcription beginning and then it stops when we reach that stop codon or that Terminator sequence so here is a picture bacterial transcription so you can see over here this is very similar to eukaryotic transcription and translation however there is a major difference what is that difference there is no nucleus okay so all this needs to go on in the cytoplasm and you know biochemically its different of course but in terms of functionally it's not really that much different at all so here we have another slide of the actual process of bacterial transcription so you can see the RNA molecule is assembling these nucleotides to match up in a complementary way with the information on the DNA strand and the synthesis it moves along the DNA and and then the RNA kind of comes off in the opposite direction and it moves you know the the transcription machinery moves in one direction and the RNA moves in the other so then we get translation what is translation remember translation is assembling the amino acids into the protein based on the codes that were transcribed by the messenger RNA so messenger RNA is translated into codons remember those three nucleotides and the messenger RNA you know starts and then it stops based on that regulatory sequence so the protein in a bacteria is used for cell metabolism and growth so gene regulation how can we regulate all of those processes we certainly have different kind of genes so we have the constitutive genes that are just expressed at a fixed rate they're just continually made at some rate that's fixed but other genes are expressed only when they are needed and those genes are called repressible genes and inducible genes and we're going to have a little in-depth discussion about what that means just right after this slide so regulation of transcription is a way in which we can figure out and initiate or stop sequencing are actually not sequencing but synthesizing certain protein so for instance if you have a bacteria that produces a toxin right when it infects a organism a human host or an animal host then that toxin requires the production of potentially some sort of protein substance and that regulatory sequence on that bacterial DNA will need to get initiated in order for the toxin to produced so we have this regulatory gene here sequence or portion of the DNA we have this control region and then we actually have the region of the DNA that's encoded for certain structural proteins so in terms of repression we have the regulatory gene actually produces this repressor protein that then lands on the DNA and essentially stops the transcription from occurring so the transcription starts for instance and with this repressor protein it just stops right there and that protein does not get synthesized and so what protein does not get synthesized is the protein out here so that's one way that we can actually cause repression or stopping the synthesis of a particular protein now in terms of induction of that we can actually have occur in a couple of ways so we can either have a secondary molecule called an inducer that will land on the repressor protein and actually inactivate it so that's one way another way there are other regions on the DNA that are actually set up that if something lands on that DNA that will cause the machinery to start but in this case we actually stop the repression and then we get transcription and translation and then ultimately we get the protein gets produced and in the case of that bacteria we get we get that toxin okay we get that toxin produced by taking away that the function of that repressor protein now let's talk about mutation now mutation is another concept that's common to both eukaryotic and prokaryotic cells and the question again has arisen for quite a long time in genetics in terms of defining what a mutation is so Eric Piarco who is a very famous ecologist defined a mutation as a genetic change to a germ cell now what does a germ cell when a eukaryotic cell a germ cell or in a human host a germ cell is an OVA or a sperm and when we have a genetic change to the to a germ cell that genetic change is passed on to our offspring and so in terms of evolutionary biology that genetic change will either enhance or disrupt a populations fitness to reproduce so this has an evolutionary connotation but you know certainly these have supply to bacteria as well especially in the case of the antibiotic resistance that we discussed in the beginning of the course so only germline mutations are transmitted to offspring Moin curie who's another famous geneticist clarified that terminology now there are also some definitions of genetic mutation in relationship to molecular biology and a genetic mutation is in molecular terms a permanent transmissible change in the nucleotide sequence of a chromosome usually in a single gene that leads to a change or loss of its normal function and again if we have that change occurring in germ cells that change is transmitted to all the cells in the organism now the other concept about mutations that's important is that mutations are rare and also they're random now these mutations can be beneficial neutral or harmful and in the respect that they're random it's it doesn't matter if the mutation is useful or not the fact that it occurs at all it's a random process not all mutations matter some if they're integral to particular functioning of some protein then they do matter but if they can occur in a part of the DNA that's not a gene or not expressed in that particular manner and so in that case not all mutations matter somatic mutations now we were just talking about germline mutations now somatic cell mutations are mutations that occur in cells that are not germ cells they're not reproductive cells and they won't pass on to our offspring so for instance if we stay out too long in the Sun our skin cells may have a mutation that's induced in that particular area that got sunburned but the rest of our body isn't going to have that kind of that mutation just that part of the body that was exposed to too much Sun so the example here the golden color of this Apple was caused by a somatic mutation but the seeds of the apples so if we planted this Apple if we planted the seeds the Apple would not grow into being multicolored it was just that particular Apple had that particular genetic change now these mutations are different again from germline mutations because germline mutations all of the cells in our body will carry those germline mutations so what are the causes of mutation so one cause of mutation is that the DNA does not copy accurately so remember in cell replication if our DNA does not get copied accurately that is considered to be a mutation so in that could be even thought of as being random and sometimes that copy of DNA is not quite perfect and the difference from the original sequence is considered a mutation now external influences can cause mutations like for instance radioactivity also chemicals other environmental exposures heavy metals certain endogenous factors these agents cause DNA to break down various poisons and you know we have these other mechanisms that actually can repair the damage that's caused by these mutations so if the mutation escapes this repairing then that cell especially if it's a germ cell could go on to produce a mutation in the offspring so this then is considered a mutation now there are different kinds of mutations you can have a substitution mutation where one nucleotide base is basically substituted for another and so for instance we could change an A to a J and you might think well you know gosh here what will we have millions of nucleotide bases what's the deal if we change one well in some cases actually that can make a huge difference so one really good example is sickle-cell anemia now sickle cell anemia is a function of the actual shape of the erythrocyte generally the shape of the erythrocyte is round and if you look at it kind of as a cross-section and has an indentation and the shape of that erythrocyte is very specific to being able to carry oxygen well with sickle cell anemia the shape is not quite the same it's actually sickles in shape and what causes that sickle cell anemia is actually one base pair that is wrongfully substituted and that changes actually the amino acid sequence for the hemoglobin molecule and because of that we get a sickling of the erythrocyte and then the oxygen is not carried efficiently and we have sickle cell anemia so there are other examples so we can have like we talked about before we can have a mutation in an important part of the gene or we can have what's called silent mutations that occur in portions of the gene that don't encode for any particular protein so but you can also have a mutation and the stop codon and potentially terminate the synthesis of a protein early and make it incomplete and then potentially lose the entire function of the whatever protein process that occurs so there are other kinds of mutations there's an insertion type of mutation where you get additional base pairs inserted into the genome and you can have a deletion where you get some nucleotide bases deleted and you can see that it makes a difference in terms of the sequence of nucleotide bases and that certainly can go on and change the sequence of the amino acid in the protein so there are other kinds of mutations there's what's called a frameshift mutation and a frameshift can happen for instance if you have one base pair deleted but then when the DNA is replicated or transcribed even it shifts the whole reading over and what was one amino acid sequence now become something different and these are called frame shifts and then it's a great example above in terms of that so it's just like with words if we don't if we omit a letter and you know this particular sentence or part of a sentence then it changes the complete meaning of everything so these codons are not are not parsed correctly and that changes the protein or potentially causes it to lose its fine so how do these principles apply to bacteria so that's what we're going to utilize the last latter half of our lecture today is thinking about how these principles apply to bacteria because the ways in which these bacteria replicate and pass DNA between bacteria either across generations or within the same generation is really what makes bacteria such a challenging organism to fully understand so we can have different kinds of genetic recombination in bacteria we can have vertical gene transfer where just like in eukaryotic cells we take the the parent D cell and then we pass the DNA completely to the daughter cells but then we can have this situation where we can have horizontal gene transfer where the genes within the same generation can transfer so for those of us who have siblings right it would be the same thing if if our siblings or we could exchange DNA with our siblings and end up with all kinds of different characteristics that weren't inherited from our parents so genetic recombination just you know in general in both kinds of cells is when you have crossing over and so you have a donor DNA and you have the recipient chromosome and during meiosis when these genes segregate to the various daughter cells you can have a situation where you get this chromosome breakage and then you can have a piece from one chromosome going to the other and essentially you get an exchange of these pieces of chromosomes so then the new recombined version now has a pink section in the gray chromosome and a gray section in the pink chromosome now this is all very simplified but this is exactly what happens with recombination and you can have these characteristics that wouldn't normally occur except for this process of recombination so a very similar thing can occur with bacterial recombination where we have this we can actually have DNA fragments now we're not even talking about cells we're talking about fragments these DNA fragments can get taken up into this recipient cell and then these DNA fragments can combine with the the chromosomes or the DNA from the bacteria and then now we instead of having this a gene right now we have a little a gene and so that little a is going to produce a different protein than the big a did originally and it's because these DNA fragments migrated from donor cells migrated in to this recipient cell and we saw this recombination now here's another kind of genetic exchange so to speak and this is an actual dramatic one it's called bacterial transformation where we actually have qualities from one bacteria taken up or basically an exchange occurring and the whole bacteria essentially transforms so and this is an example of a particular way of testing how bacterial transformation occurs so we have this condition over here where we take living encapsulated bacteria that is lethal to the mouse so we inject it into the mouse and the mouse dies then we isolate these colonies of these bacteria from this dead mouse and now we take living non encapsulated bacteria which is benign okay and we inject those bacteria into the mouse and the mouse survives and we colonize those bacteria and then what we do is we take this encapsulated bacteria from that we got over here and we heat kill it so we take away the lethal properties we inject it into the mouse and there were no colonies isolated the mouse were to remained healthy now we take living non encapsulated and heat-killed encapsulated so basically we're combining these two here okay we're combining these two types of bacteria and then we're injecting it into the mouse now clearly there was an exchange between these two bacterial types and now we have a lethal the so the lethal form or the lethal property combined with the other benign bacteria and now we have the back now the mouse so so essentially this produced a fatal form of the disease so it's because we had actually a transformation and where we had an exchange and then we had a transformation so another property of bacteria is the is called conjugation and we talked a little bit about this in an earlier lecture where we actually exchange this plasmid and so here we get this F factor plasma that gets replicated and passed over to this other bacteria remember the pill I that's what this structure is here and so we can get a recombination so we can actually exchange the plasmid and we can also down here we can actually incorporate the plasmid into the bacterial chromosome so and this is just another picture of another way that we can have conjugation so we can have replication and transfer of part of the chromosome so we can have a gray the gray part of the chromosome being replicated and then it's actually incorporated into this recombinant bacteria so this is another way that we can interject a chromosome and DNA from one bacteria into another now here's another way that we can have what's called transduction and transduction happens by a bacteriophage now bacteriophage is thought of as a virus to bacteria so basically we have this virus right that infects a bacteria and that phage actually injects its phage DNA into the bacteria and so then the bacteria replicates or actually the phages all replicate within that bacteria and not only does the phage have its own DNA but then the phage also takes up the DNA from the bacteria and then the phage goes to another you know uninfected bacteria and it injects or it carries the bacterial DNA and infects a new host cell and that new host cell now has part of that donor bacterial DNA that came to the bacteria through a bacteriophage so when we get to the virus lectures will see that eukaryotic cells can incorporate viral DNA as well but in this case we're actually transmitting bacterial DNA to other bacterial organisms so this is another way that we can transfer DNA so let's talk a little more about plasmids so what are plasmids they are a self-replicating gene that contains circular excuse-me circular pieces of DNA okay that exists independent of the bacterial chromosome the F so there are several kinds of plasmids the F factor plasmid is a conjugative plasma that carries genes for pill I for the transfer of the plasmid to another cell so and basically the the gene carries its own machinery for transmitting to other cells now other plasmids encode for proteins that enhance the pathogenicity of a bacterium so e coli pathogenic plasmids enhance the toxin production and bacterial attachment so we can have benign ecoli and then we can have a coli that have this pathogenic plasmid and once they get the pathogenic plasmid then they can be disease producing and they can enhance toxin production and bacterial attachment to what to the actual intestinal epithelium now you can have dissemination plasmids that encode for enzymes for catabolism okay so these plasmids actually cause a breakdown of some sort of other kinds of molecules or proteins and then we can have these are factors now these are factors are called ours because they encode for antibiotic resistance so remember the R factor is a type of plasmid that when it gets encoded into the genome of the bacteria can actually cause the bacteria to develop antibiotic resistance so antibiotic so in other words if we have right our antibiotic lecture if we have we're giving antibiotics to our patient and we eradicate all bacteria except for a few and those few happen to have an R factor plasmid we can transmit that R right to bacteria that don't have R and then we can transform those bacteria to actually be now resistant to the antibiotic so this is actually a really important mechanism that we're trying to understand better to deal with this antibiotic resistance so then there's another concept called transposons where we get it's a lot like that recombination example that we saw where we can cut out one gene and we concert' it for another and in this case this is another mechanism where we can introduce resistance so these segments of DNA can move from one region of DNA to another and in order for that to happen we need to have this particular DNA enzyme that can cut the DNA and cause this transposition to occur and this again is another way that we can introduce these traits into bacteria so here we show here's the transpose gene and the ends of it get caught and then they come back together and lo and behold we now introduce this new type of resistance gene and that is a way in which we can introduce antibiotic resistance so we've covered now several ways in which this recombination and exchange of genetic material can occur between bacteria of the same generation and of course the traditional way of passing across or are actually vertically going from one generation to the other so what is the process called where we have a pathogenic and a non-pathogenic bacteria combining to produce a lethal combination remember the mouse example that is called bacterial transformation okay that's important the thing the way you need to study this material is by fully getting an understanding of how they work and then classifying only the several different types of recombination the transformation the different vertical gene exchange horizontal gene exchange and so forth so the next example is what type of mutation do we have where one base exchanges for another remember that was substitution is that important yes it is remember our example sickle cell anemia it's very important but it can be silent - it can be silent or you know or if it's in a sequence that's read you know in terms of replication and translate and transcription and translation then that can certainly change the function of the protein now the third example which type of transfer results in the exchange of genes among cells of the same generation okay so you got to remember there's hor there's vertical okay and vertical pertains to parents and offspring and horizontal right refers to exchange genes from the same generation so that's a way in which you can distinguish between the transfer of genes now as we have discussed in today's lecture there are several ways that you can have this horizontal transmission remember there's plasmids okay and then there's those different types of recombination okay okay and there's the Transpo transposon bosons okay so these are just some of the ways in which bacteria can exchange DNA so that'll that's it for today thank you very much for visiting educator com we appreciate your input and your attention to what we have discussed

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Id: 2ctmJJmLzuU

Length: 39min 58sec (2398 seconds)

Published: Mon Apr 09 2018