Using DNA to Identify People

hi my name is Megan Rocha and I work here on MIT campus at the Broad Institute of MIT and Harvard which is a biomedical research institute today we're gonna do a lesson talking about using DNA to identify people and as you can see on this slide I'm showing you a picture of DNA this is what DNA looks like it's a molecule made up of atoms there's a lot of carbons and hydrogens and nitrogens and oxygens and it forms a three-dimensional structure shown here as a double helix now the backbone of this structure which is the two spirals you can see are always the same along the DNA molecule but the rhymes in the middle can be different and there's actually four different kinds of rungs and we abbreviate them ATG and C so while this is what DNA actually looks like a three-dimensional molecule on this slide you can see how we represent DNA we represent DNA by writing out the sequence as a series of a STIs G's and C's now this slide shows about two thousand letters and your first instinct may be that this looks like a lot of DNA but actually each cell in the human body contains 3 billion of DNA letters that you get from your mom and 3 billion DNA letters that you get from your dad so this is only 2,000 letters imagine the amount of DNA in one of your cells I want to ask you a question about this slide now which is the following let's say that this 2,000 letters of DNA was the sequence of DNA at one particular region of a chromosome in me and this is the letter these are the series of letters in the sequence and the order that you would find them at some position in my DNA if we looked at the same position in the DNA in anyone else on earth how many of these 2,000 letters do you think would be different between me and anyone else on earth I'm going to give you a minute to discuss that and will answer the question when you get back you welcome back so let's say this screen is two thousand letters of DNA in some position in my DNA and the question was how many of these letters of DNA would be different between me and anyone else on earth well the answer is maybe one or two so let me show you an example on this slide we've got our two thousand letters and maybe in me this whole thing is the order of the letters that you would find in the order and the sequence but maybe in someone else in the world this T right here might be a C and maybe down here this a might be a G all the other letters would be the same sequence in the same order and that's because all humans on earth are at least 99.9% genetically identical to every other person on earth so we're really similar now everyone on earth has a different DNA sequence than everyone else on earth except for one exception I can say that I have different DNA than anyone on earth but the one exception would be if I had an identical twin two identical twins have the identical sequence of all three billion letters that they get from their mom and all three billion letters that they get from their dad but everyone other than identical twins have different sequences from each other and so we can use DNA sequence to tell people apart and this is a technique called DNA fingerprinting or genotyping so on my next slide I'm going to show you those two words this is what our lessons going to be dealing with today it's one technique it just has two different names it's a technique to use DNA to identify people the reason it's called DNA fingerprinting is that that's an analogy to actual fingerprinting where every person has a different pattern of bands on their fingertips so every person also has a different DNA sequence so we can basically fingerprint people by using their DNA the other term is genotyping and this is because this technique allows us to see what type of genes people have so I told you that different people have different DNA sequences and let me show you what I mean by that on my next slide before I are you two different people differing from each other first I just want to show you the DNA sequence from one person and I'm showing you two in this format so that you can see how exactly we write out a representation of a three-dimensional structure so here's a sequence of DNA from a person the double helix that you see is the three-dimensional structure of DNA but I can write out one strand and then write out the other strand next to each other using a series of letters and that's what I've done on the right hand side of the slide these letters are a z' T's GS and C's and anytime you see an N one strand you see a T on the other strand anytime you see a G on one strand you see a C on the other strand haze bond with T's and G's bond with C's so this is how we write a DNA sequence now normally actually we don't write it vertically so I'm gonna turn it for you on my next slide so that it's horizontal so here's an example of the DNA sequence from a person I'm gonna call them human number one when I say that people have differences in DNA sequence what I mean is shown on this slide where now I'm showing human number two and there's a position where human number one has a G on the top strand and human number two at that same position has an A on the top strand all the other letters are the same now you would actually have to look through over a thousand letters of DNA to find this difference but when you do find them this is what they look like so we use DNA fingerprinting to look at differences in DNA like this to tell people apart and what I'd like to ask you to do now is to brainstorm uses of this technique so in what situations do you think it would be useful to use the DNA of people to identify them I'll give you a moment to brainstorm and I'll see you when you're done welcome back so the question was what uses would you have for a technique where you could use the DNA of people to identify them remember DNA fingerprinting or genotyping allows us to identify people based on their DNA so what would we actually use this technique for well hopefully you've had a chance to brainstorm and on my next slide I'll show you some of the possibilities that I came up with one situation is paternity testing or trying to determine the parents of a child another situation is forensics so let's say there's a crime scene and a hair or a piece of skin is found at the scene of the crime you could isolate the DNA from from that sample and figure out who the person was who left that sample there and then my last example is one that actually we do here at the Broad Institute all the time so what we do is we look at the patterns of DNA between people and compare them routinely and the reason that we do this is to try to figure out which portions of the DNA are associated with specific diseases so we take thousands of people who have a disease let's say heart disease and thousands of people who don't and we look at the differences in their DNA sequences we look for the differences that always look one way in the people with the disease and always look a different way in people who don't have disease and that's how we find regions of the genome that are associated with different diseases but today in our lesson we're going to focus on the first two uses in paternity testing and forensics now when you're looking at genes that are associated with diseases you often use the kinds of sequence differences that I showed you before so on this slide I'm reminding you that humans can differ by sequence for instance human number one having a G and human number two having a name although these are the differences we use for mapping genetic diseases these are not usually the differences that we use when we're doing paternity testing or forensics so let me show you what those look like on my next slide I'm showing you a different way that people can differ in their DNA before I was showing you a difference in sequence where a letter was different but here the sequence of these two people is the same both people have a repeat in their D that says GT GT GT over and over again the difference is is that human number one has three repeats their DNA says GT three times in a row where human number two has five repeats GT five times in a row so these two people don't differ in sequence they differ in length and on my next slide I'll show you that if you count up the letters of DNA in the top strand of each person's DNA human number ones DNA would be 18 letters long and human number twos DNA would be 22 letters on so if we have a technique that separates DNA by size we can actually tell these people apart we do have a technique that does this and I'll show you that on my next slide this technique is called gel electrophoresis and there's three things that I'd like you to remember about gel electrophoresis the first is the main point which is that it's technique that separates DNA by size the second is how it works so you actually put the DNA of people into a slab of gel that feels a lot like jello or gelatin now this gel is a matrix and what I mean by that is I could turn this room into a matrix if I took a whole bunch of pieces of string and every piece of string I taped one end to that wall and then I stretch each piece of string across in a different pattern and tape the other end to that wall so imagine the room that you're sitting in now full of string that would be a matrix now if I wanted to race two molecules through the matrix let's say I wanted to race a very large molecule and a very small molecule they would move through the matrix with different speeds this is what we do with DNA and the reason we can get DNA to race through the matrix is that DNA is negatively charged so what would you do to the other end of the room if I were a piece of DNA to make me want to go to the other end of the room so I'm gonna have you give you a moment to think about two things first of all what would you do to the other end of the room to make me want to move there and secondly if there were two pieces of DNA here one very large and one very small how might they behave differently as they race towards the other end of the room I'll give you a few minutes to brainstorm and I'll see you when you're done welcome back so if DNA moved to the matrix first of all why would it want to move in the first place well if I'm DNA and I'm negatively charged if you make the other side of the room positive then I'll be attracted to the other side of the room and I'll want to move there now what if there were two molecules of DNA and they both had to move through the matrix but one was very large and one was very small well the small one can move quickly through the matrix because it's small enough to fit in between the holes in between the pieces of string whereas the large molecule would try to move but it would be very large and so it would get tangled up and thus move slower and that's how gel electrophoresis works I'd like to show you how this technique works in my lab but first I just want to introduce you to the principles and then we'll go into lab to take a look at the technique so let's say that you had DNA samples from people and you wanted to separate them by size using a gel what would you do on my next slide I'm gonna show you the first step that you would do what you would do is you would take all the DNA in the sample which remember every human cell has 3 billion letters from their mom 3 billion letters from the dad and we're only usually analyzing a very small amount of that DNA so what we do is we use a technique called PCR where we pick a left and right boundary of the DNA and we copy only the segment in between those boundaries so on my next slide I'll show you what that looks like you pick the rut left and right boundaries and then you make many many copies of just that region of DNA and here I'm showing you only a few copies but in the lab we make millions and billions of copies so now we have a bunch of copies of just the region we want to look at so basically here's how the technique works you isolate the DNA from the person you make many copies of just the region that you're you want to study and then you're gonna run the DNA fuj oh I'd like to show you what this actually looks like in lab so why don't you come with me and we'll go do this technique together in my lab hi welcome to my lab here at the Broad Institute I'm going to be showing you how this process works here in lab and here you can see the equipment that I'll be using to show you how to use gel electrophoresis to identify people based on their DNA so typically we store our DNA samples like this in these tubes and I got these tubes out of the freezer typically we store DNA at minus 20 degrees Celsius which is the temperature of a standard freezer the first thing that I'm going to do is I'm going to take a little sample of those samples of DNA and I'm going to put them into these smaller tubes which we'll use in these smaller tubes to copy the segments of the DNA that were interested in so I'll be using my pipette 'men to transfer for each sample a small amount of the liquid into the small tube the next thing I'm going to do is I'm going to add the reagents or chemicals that I'll be needing for each copying reaction to each one of the tubes this includes all of the necessary chemicals that take you from having a small set amount of DNA to copying it into a large amount of DNA each time I pipe had a different sample I use a different pipette tip so that none of the samples are contaminated with each other so now that I have all my samples in the tubes at this step I'll be adding the mixture of all the chemicals necessary to do the reaction so I have that here on ice in this tube the reason I have everything on ice is because this reaction is sensitive to temperature and so I want to keep all the reactions on ice until I'm ready to start the reactions again I'm changing pipette tips each time so that none of my samples contaminate one another so now that I have all of my DNA samples mixed with the necessary chemicals I'll be putting these smaller tubes into what we call a PCR machine so remember PCR is the name of the technique to copy a segment of DNA to make many copies of it and so this is my PCR machine right here and these small tubes I'll just give them a little mix there and they fit right into these holes inside the tooth inside the Machine so when I put all four of my tubes into the Machine I just close the lid lock it down and then I hit start and the programs gonna start running what the program is actually going to do is that this machine although it's called a PCR machine all it really is is a heating block this machine is a heat block that changes temperature really rapidly so it's gonna change temperatures and each step of the copying reaction happens at a different ideal temperature so as it changes the temperatures then the steps of the copying reaction can occur in the order in which you want them to now remember what this machine is doing this machine is doing the copying reaction that I'm showing on this slide where we make many many copies of just one piece of the DNA I want to give you a moment to think about why is it that this copying reaction is necessary why can we not just take all the DNA from a person and directly analyze it on a gel I'm going to give you a moment to think about why the copying step is necessary you can talk amongst your classmates and then I'll tell you the answer when we get back welcome back the reason we need to copy the DNA is twofold the first is just an amount issue the amount of DNA we can typically isolate from a person simply isn't enough to see it directly on a gel instead we need to copy it so that we have more DNA such that we are able to see it on the gel the second reason is that we only want to look at one segment of the DNA we don't want to look at all the DNA from the person therefore the copying reaction allows us to just select one piece of the DNA from that person make many copies of it and then visualize just that segment on the gel now this process actually takes a few hours and so I set up PCR reactions before we started filming and I have them in this ice bucket here so the next thing I'm going to do when the PCR reactions are done and I have my segment copy is I'm going to transfer again a small amount of that into these tubes right here and these tubes I'm going to mix the DNA with an orange dye this dye is going to allow me to see the DNA and then I'll be loading it onto this gel here so the first thing I'll do is transfer a small amount of orange dye into each of the tubes that I'll be placing the DNA samples into you can see this orange dye in my pipette tip it's just a colored substance that we can see with our eyes DNA is transparent so we can't see it with our eyes okay so now I'm going to be transferring a small amount of each sample into those tubes so here's my PCR reaction I place it in the tube to mix with the orange so that now we can see it okay so now that we have all of our DNA samples to load onto the gel I'm going to load them onto the dough the way that works is that this is your slab of gel right here if you can see from the side the gel is actually coated in two pieces of plastic but the opaque substance in the middle is the gel matrix now this gel right now has a plastic comb in it that's protecting 12 holes in the gel we can load each DNA sample into one of these holes so I'm going to put the gel down onto the lab bench and for each one of my DNA samples I'm going to load them into one hole in the well in the job sorry so I'm loading my DNA samples of the different people individually into each hole in the gel and the orange dye helps me to see that I've actually successfully gotten the DNA into the hole and once I've loaded all the DNA samples I'm actually going to load one other special sample first I'm going to add a little DNA orange dye to it this special sample is not the DNA from a person but rather it's a mixture of samples of DNA that we buy from a company and the different pieces of DNA in this mixture are pieces of DNA whose sizes we know and so we use it like a ruler or a standard to be able to tell the lengths of the pieces of DNA that we don't know in the samples from the people so this will be our ruler or standard and we call it ladder so now what I'm going to do is I'm going to hit this button and that starts the current running through the gel so now the DNA samples are going to move from the holes where we loaded them all the way down to the end of the gel right here until you'll see the orange dye progressing the orange dye remember is not actually the DNA the DNA strands parent and you can't see it at this step but the orange dye allows us to see that the samples are traveling once the orange dye reaches the end of the gel will expose this gel to a certain wavelength of light and that will allow us to see the DNA this takes about half an hour and so I actually started another gel for you before the we started filming this segment and you can see that the orange dye is almost at the end of the gel here so this gel is almost done running and when it's done running it'll beep to tell me it's done and then we're going to take it into the darkroom so that you can see the results so here we are at the darkroom this is where we're going to visualize the gel I've got my gel here and we're going to go in and shine a certain wavelength of light on it so we can see the DNA so here we are in the darkroom and I've got my job I put my gel into this light box right here and then I simply turn on the light and all of the bands of DNA appear so here you can see the results of the job Lane five is not a sample from a person remember that's ladder that we bought from a company so you see a series of bands of DNA each one is a different size and we know the sizes of those DNA so that's our standard Lane six seven eight and nine contain the DNA from four different samples for instance in Lane nine you can see that there's a single band whereas in Lane seven there are multiple bands so what I'd like you to do now is take a moment to discuss why you think it is that some people show one band whereas other people show multiple bands I'd like you to write down for each person how many bands that you see whether those bands are longer or shorter and then also explain why some people show multiple bands and some people show one band we'll discuss the answers to these questions when we come back so when we were in lab you saw a picture of a gel that looks a lot like this slide here as you can see in this slide some people show one band of DNA other people show two bands of DNA why is that the reason is that people who only show one band of DNA actually have double that amount of DNA as you can see in the difference in the intensity of the band remember everyone gets their DNA from their parents and everyone gets half their DNA from their mom and half their DNA from their dad so if a person shows only one band in this gel that means that the DNA they got from their mom was the same length as the DNA they got from their dad so there's two times as much DNA in that band which is why it's darker a person who shows two bands however must have gotten a different length of DNA at that section from one parent than they did from the other and so you see two different lengths of DNA in their lands so that's how you interpret a gel like this in this gel for instance as you can see from the ladder markings on the left remember the ladder is like a ruler for us person number three has about 22 letters long whereas person number one in person number two have DNA that's both 22 letters long and 18 letters long so I just want to show you a quick pictorial view on this next slide to remind you that when you're looking at these gels you're looking at the DNA that comes from the parents and here's a picture of the DNA actually sitting in a human cell the round part is the nucleus and as you can see there are chromosomes in the nucleus and in my picture here I'm only showing three chromosomes a very long one a medium one and a short one in actual human cells there are 23 different chromosomes you would get 23 chromosomes from your mom and 23 chromosomes from your dad so here on this slide I've labeled a section of DNA in the same position of the long chromosome from the father and the long chromosome from a mother and as you can see one section has three repeats of CCT while the other section only has one region of CCT thus this region from the two parents of this person is different sizes so this person would show two bands at this position so now let's go back and look at the gel I was just showing you I've now labeled the lanes a little bit differently I've labeled one person as me another person as the person I refer to as mom and another person as the person I refer to as dad and I have two questions for you using the data in this gel can these people be my parents and my second question is are these people my parents so I want you to discuss as a group whether you think these people can be my parents based on this gel and whether this gel shows you that these people are my parents and we'll discuss the answers when you come back welcome back let's answer the questions that we were just discussing does this style show are these people my parents can these people be my parents well the answer to can these people be my parents is yes the reason that that's true is that as you can see the person I referred to as dad only has one length of DNA the 22 letter long version if this were my dad he would have to give me the 22 letter long version and I have that version I also have a version that's 18 letters long but so does my mother so I could have gotten the 18 letter long version from my mother therefore this gel is consistent with these people being my parents ie these two people each giving me half of their DNA well are these people my parents this gel does not prove that these people are my parents although the gel is consistent with them being my parents the reason the gel doesn't prove they're my parents is that if every person only has one of three possible patterns that they can show on this gel either only 22 letters long only 18 letters long or both 18 and 22 letters long then given that there's 7 billion people on earth you could pick any 2 people at random and they might be consistent with me in this gel however this is actually how we do do paternity testing and forensics so how do we use this technique for paternity testing in forensics if this gel doesn't prove that these people are my parents well the answer is is that you have to remember that we are just looking here at a very small number of letters from a total number of letters of DNA that's 3 billion letters from a mom and 3 billion letters from my dad if we ran another gel and we looked at another segment and these people were consistent with me you would be more convinced that they were my parents if we ran an infinite number of gels looking at many many different parts of the DNA and they always matched me you would be very convinced these people weren't parents so you can actually ask a statistician how many regions of the genome do you have to look at and have these people match me so that you can say that they most likely are my parents given their 7 billion people on earth and statisticians can answer that question and tell us that we have to look at about 20 different sites on the DNA so that's how this technique works and now I'd like to do an example with you first we're going to do an example with paternity testing and then we're going to do an example with forensics of how to use this technique so on my next slide I'm going to show you the introduction to this exercise the first example is that there's been a mix-up in the maternity ward and there's three babies a B and C and three sets of parents 1 2 & 3 and we're gonna match which sets of parents go with which baby so on your worksheet you'll see a similar picture to the one I'm showing on my next slide here's a region of chromosome 15 this region has a repeat of TTA GGA T in the middle and this region can be repeated n number of times so a different number of times in different people we're gonna analyze this region of chromosome 15 in the babies and the parents to see if it can tell us which babies go through which parents on your worksheet you'll also see the following picture this is the result of the DNA analysis of chromosome 15 you can see the ladder markings on the left and you can see the patterns that are given by all the parents and all the babies so what I'd like you to do is answer the questions on your worksheet and those questions are shown here first given the data so far which of the three babies can you conclusively connect to a set of parents second how did you conclude this third why can you not determine at this point all of the babies and all of their parents and finally how do you think you would go about conclusively determining all the matches of the parents to the babies using DNA fingerprinting analysis I'd like you to answer those questions on your worksheet and we'll talk about the incidence when you're done welcome back let's go through the answers to the questions on your worksheet first the baby who you can conclusively connect with a set of parents is baby a who goes with parents at three the way you can conclude this is the twenty five length repeat so if you look at your ladder and you find any equals 25 if you see that band that band is only present in dad number three and in baby a baby a had to have gotten the twenty five repeat length from someone and the only person they could have got it from his dad number three that's how you can conclude that dad number three and therefore also mom number three goes with baby a the other two parents one and two actually cannot be matched at this point with babies B or C and that's because if you look at the types of genes the genotype of parents number one and parents number two and you think about all the different possible genotypes of children that they could have they could actually each have either child B or C so how would we actually go about concluding whether parents wanted to go with baby B and C well remember this is just a small segment of chromosome number 15 these are maybe hundreds or thousands of letters of DNA but we have three billion letters of DNA from our mom and the three billion letters of DNA from our dad so we can go look at another region of another chromosome and see if that one answers our question so now if you look at your worksheet you'll see a picture like this this is a region of chromosome four and in this region they'll sequence CAG is repeated a certain number of times in the middle of the sequence so now I'm going to show you another gel also shown on your worksheet that shows the results of analyzing these same people at a different region a region of chromosome four what I'd like you to do now is analyze this gel and answer the questions you see on your worksheet those questions are shown here given all the data in this problem I want you to match the three sets of parents with the three babies and I also want you to explain how this site on chromosome four allowed you to match parents one and two with babies B and C so you can work on these questions on your work and I'll see you when you've got those answers welcome back let's go over the answers to those questions the first question is to conclusively connect all three sets of parents with all three babies well we already know from chromosome 15 that baby a goes with parents three so what we can find from this gel is that baby B goes with parents one and the baby C goes with parents too so how do we find that if you look at this gel here baby B has two copies of the 80 repeat lengths if you look at parents one and parents to both dad number one and mom number one have that 80 repeat lengths best they could have each given the 80 repeats to Baby B however neither dad number two norm um number to have that 80 repeat lengths so parents number two could not be the parents of baby B if you look at baby C baby C has the repeat length of 70 and the repeat length of 40 if you look at mom number two mom number two is the only parent who has the repeat length of 40 and thus she could have given this 40 repeat to baby C baby sees other repeat is 70 long and dad number two has the 70 repeat lengths so he could have given it to baby C thus in conclusion parents three go with baby a parents one go with Baby B and parents to go with baby C so that's how you match the babies to the sets of parents now this technique of DNA fingerprinting as we've discussed in this example can be used to identify people for paternity testing but the same technique can also be used in forensics analysis and I'd like to show you an example of that now do you remember the gel that we were looking at earlier in the lesson in lab well I'd like to show you that gel again in this gel you can see that lane 5 contains the ladder and lanes 6 7 & 8 contain the DNA of three suspects we'll call them d e and f lane 9 contains DNA found at the crime scene what I'd like you to do is analyze this gel and answer the following question which suspect de or F has DNA that matches the DNA from the crime scene discuss the answer to that question and I'll see you when you're done welcome back so in the gel you just looked at suspects de NF had the following DNA patterns suspect D had one band that was higher up in the gel e had two bands one that was higher and one was lower an F had one band that was lower when you looked at the DNA from the crime scene there was one band that was lower and it was the same length as person F the suspect FS DNA matched the DNA found at the crime scene that's how DNA fingerprinting analysis can be used in forensics during the filming of this lesson I had the opportunity to visit the identification unit at the Cambridge Police Department this is the lab where the Cambridge Police Department can investigate items found at a crime scene for things like fingerprints or for DNA so for instance here you can see that I'm getting my cheek swab this is how the technicians at the identification unit isolate DNA from someone who's potentially a suspect they can also isolate DNA from items found at a crime scene such as a water bottle or a weapon you can watch a full video tour of the identification unit in the video that's found after this lesson I hope that you've enjoyed the lesson today as we learned about DNA fingerprinting analysis how it can be used to identify people based on their DNA and how it works as you saw in my lap thanks for joining me and I hope you enjoyed learning about paternity testing forensics analysis and DNA fingerprinting thanks for your interest in my blossoms lesson on using DNA fingerprinting and paternity testing and forensics analysis the main objectives of this lesson are for students to learn how DNA fingerprinting works to see it actually work as a technique in lab and to be able to analyze the data that is generated by DNA fingerprinting and use it in situations such as paternity testing and forensics analysis there's no background information necessary for this lesson although it's helpful if students already know that DNA is the genetic material and it's made up of a steez GS and C's and it's also helpful if they know that the genetic material of any one person comes from their parents and that 50% of that material comes from the mom and 50% comes from the dad there's no necessary materials for the lesson other than the worksheets that are provided and an ability to project or print out the slides there's nine segments to this lesson and I'll go through each one and what happens in each segment and between segments now in segment number one I introduce the concept of how you look at a DNA sequence a series of a STIs G's and C's and I asked the students that if you were looking at two thousand letters of DNA how many of those letters might be different between any two people on earth I give the students time to brainstorm this number and usually students guess a lot higher than what it actually is which is one or two letters so when I come back in segment two I reveal that only one or two letters out of two thousand would be different if you compared any two humans because any two humans are 99.9% identical then I talked about how the differences in DNA those less than 0.1% of letters can be used to tell people apart and I asked the students to brainstorm uses for a technique where you would be able to tell people apart based on their DNA so after segment two most likely the students may come up with either paternity testing or forensics but in segment three I come back go over those uses but I also talk about how at the Broad Institute where I work we use this technique all the time to find genes that are associated with disease then in segment 3 I introduce how DNA fingerprinting can work to look at the differences in length of DNA between people for instance a length of a repeated sequence between two people as opposed to differences in the actual sequence or what the letters are these are the differences these length differences that are usually used in paternity and forensics analysis so I talked about how you can separate piece of DNA by size in gel electrophoresis and I post two questions to the students about how they would get DNA to move through a matrix given that DNA is negative and also what they think would happen to the speed of DNA if you raced a big piece of DNA versus a small piece of DNA through matrix when I come back in segment four we talked about how you can get DNA through a matrix by making the other end of the matrix positive so that the DNA is attracted to it and how large pieces of DNA moved slowly through the matrix and small pieces moved quickly then I introduced the concept of what this technique would actually look like and then in segment four we actually go into the lab I do a DNA fingerprinting experiment and I show the results of a gel at the end where there are three different DNA samples the DNA samples one has a large piece of DNA one has a small piece of DNA and one has two different pieces of DNA a large and a small so some people show one band in the gel and other people show two bands and I ask the students to write notes about how many bands they saw in each lane whether the bands were longer or shorter in each lane but also why it is that some people show one band and other people show two bits we answer this question in segment number five and the answer is because every person gets half their DNA from their mom and half their DNA from their dad and if the mother and father's DNA is of different lengths then the person will show two bands but if the person inherits the same length from both parents the personal show one band this is usually a very difficult except for the students to grasp and so I go on to show this concept as a picture where I show the nucleus of a human cell I show the maternal and paternal versions of three different chromosomes and label one section so that they could see that that little section might be slightly different lengths because the maternal version and the paternal version had a different number of repeats then I show the students a sample gel with DNA from three people a person I label as me a person I label as the person I referred to as mom and then a person I referred to as dad and I ask the students if the gel shows whether or not these people can be my parents and whether or not these people are my parents in segment six I explained that the two people can be my parents because the gel is consistent with my DNA but that it doesn't prove it and so I talked about how we use many regions of the DNA to actually show whether we think that someone is the parent of a child or we think that someone's DNA has been found at a crime scene then we start the main exercise of the lesson where I give the students of gel showing three babies and three parents and asking them to use the DNA in this gel to figure out which set of parents one two or three goes with which baby a B or C when we come back in segment seven we discover that you can tell that baby a goes with parents three but you actually can't tell which of the other two sets of parents go with the other two babies so then we go on to examine another site on a different chromosome that actually will tell us which parents ate one or two go with which baby beer see the students analyzed that gel from that other region of a different chromosome between segment seven and eight and then an eight we go over the final answers which are the baby a goes with parents three baby B goes with parents one baby C goes with parents - finally I show the students an example of a gel from forensic analysis which is actually the gel I ran in lab in this gel there were three suspects d e and f d had the one band that was higher in the gel he had two bands one I are one lower and F had one band that was lower there is a final lane of the Jo which is the DNA from the crime scene and that Lane has one band that's lower and it matches the DNA from suspect F and so this shows that the DNA found at the scene of the crime matches the DNA of suspect F so in Lane in segment 9 I have asked the student to determine which suspects DNA matches the DNA from the crime scene they figure out that it's suspect F and the lesson ends with a summary of the lesson being about using DNA fingerprinting to tell people apart and how that's used in both paternity testing and forensics analysis I hope that you enjoy using this lesson and thanks so much for being a part of blossoms you

Channel: MIT BLOSSOMS

Views: 71,907

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Id: gT_KQP1JH30

Length: 45min 49sec (2749 seconds)

Published: Mon Dec 21 2015