DNA for Forensic Science

Summary

Highlights

• The basic principle of forensic DNA is comparing crime scene DNA with suspects or database samples. 🕵️‍♀️
• The Innocence Project utilizes DNA testing to overturn wrongful convictions by proving no match exists. 🚔
• A small percentage of DNA (0.1%) varies from person to person, allowing forensic differentiation. 🔍

Key Takeaways

• DNA comparison is key in forensic science, linking crime scene evidence to suspects or databases for solving crimes. 🧬
• Matching DNA evidence can lead to both convictions and exonerations, as illustrated by the Innocence Project. ⚖️
• The vast size of the human genome showcases the complexity and uniqueness of DNA analysis. 🌐

Overview

Chapters

• 00:00 - 00:30: Introduction to DNA for Forensic Science In this chapter titled 'Introduction to DNA for Forensic Science', the focus is on providing a basic overview of DNA's role within forensic science. The video aims to introduce some of the most critical aspects of DNA analysis for those interested in understanding its application in forensics. It clarifies that the content is not an in-depth exploration of DNA's structure or functions but rather an entry-level discussion meant to set the stage for how DNA is utilized in this field. The chapter sets out to explain what DNA is and how it is applied in forensic scenarios.
• 00:30 - 02:00: The Principle of DNA in Forensic Science DNA found at crime scenes can be compared to DNA from victims or suspects. For instance, foreign skin cells under a victim's fingernails may indicate a struggle, suggesting the killer's DNA could be present due to the victim's defensive actions. This comparison is crucial for identifying perpetrators.
• 02:30 - 05:30: The Innocence Project and DNA The chapter discusses the use of DNA evidence in solving crimes, with a focus on its application in the work of The Innocence Project. It highlights how DNA found at a crime scene can be matched either to a suspect or checked against a DNA database, which contains profiles of individuals who have committed certain crimes. This process has been instrumental in solving many cases, even when there is no initial suspect.
• 05:30 - 07:00: Interpreting DNA Matches The chapter discusses the process of finding and interpreting DNA matches in the context of a crime. The key objective is to find a match between DNA from a crime scene and either a known individual in a database or a suspect. The challenge arises when DNA is found at a crime scene with no clear suspect, as illustrated by a scenario involving an unidentified person found in the woods without any personal connections or evidence of what occurred.
• 07:00 - 10:00: Function and Structure of DNA The chapter discusses the challenges associated with identifying unknown DNA samples, particularly in cases where there are no matches in a database. It highlights the limitations of forensic DNA analysis when no comparative samples are available and draws a parallel with fingerprint analysis. The text implies the potential for connecting unknown DNA samples to individuals after subsequent crimes are committed and the perpetrator's DNA is eventually entered into a database.
• 10:00 - 15:00: DNA's Role in Encoding Information The chapter discusses the role of DNA in criminal investigations, specifically comparing fingerprints found at crime scenes to those of suspects. It highlights the use of DNA evidence not only in solving crimes but also in the appeals process after convictions. Additionally, the chapter introduces the Innocence Project, an organization focused on using such evidence to support justice.
• 15:00 - 21:00: Visualizing DNA - Images and Metaphors This chapter discusses the role of the Innocence Project, an organization dedicated to overturning wrongful convictions. The Innocence Project helps individuals who have been convicted of serious crimes, such as murder or rape, without sufficient evidence. People claiming innocence can request the project's help, and the organization works on their behalf to review their cases and seek justice.
• 21:00 - 29:00: Human Genome Size and Comparison The chapter discusses the role of DNA analysis in criminal cases, specifically in the context of the Innocence Project's efforts. The Innocence Project evaluates past criminal cases to determine if DNA analysis was carried out, especially when it seems pertinent to a crime scene investigation. For instance, if DNA evidence like a knife with the perpetrator's blood was never tested, the Innocence Project may fund the necessary DNA testing to compare the evidence and potentially exonerate wrongfully convicted individuals.
• 29:00 - 33:00: Forensic DNA Analysis - Teaser for Next Steps In this chapter titled 'Forensic DNA Analysis - Teaser for Next Steps', the discussion revolves around the implications of advancements in DNA analysis and how it has become a crucial tool for revisiting past convictions. The narrative highlights scenarios where forensic teams examine evidence, such as blood on a knife, to verify matches with suspects who are currently serving prison sentences. The chapter emphasizes the transformative impact of DNA analysis, particularly through organizations like the Innocence Project, in potentially overturning wrongful convictions. The Innocence Project is mentioned as a key player in advocating for justice by using new evidence to challenge previous convictions, thus offering hope for affected individuals. The chapter closes with a call to action, encouraging readers to learn more about these efforts by exploring the Innocence Project's resources to truly grasp the impact and importance of their work.
• 33:00 - 36:00: Summary The chapter discusses the justice system, highlighting both its successes and its failures. It expresses appreciation for situations where the justice system correctly acquits the innocent and convicts the guilty. However, it also acknowledges the system's inherent flaws and the importance of initiatives like the Innocence Project, which work towards exonerating falsely convicted individuals.

DNA for Forensic Science Transcription

• 00:00 - 00:30 hi everybody welcome back today we're going to be looking at an introduction to DNA for forensic science now let me say up front this is not meant to be a comprehensive video on DNA function or structure if you're looking for that go elsewhere all we want here today is a basic introduction of some of the most important details of DNA for somebody who's looking to gain a basic understanding of how DNA analysis is done for forensic science that out of the way let's begin so first question is how is DNA used in forensic science what's the general principle general
• 00:30 - 01:00 principle is that we compare DNA that's left behind at the crime scene to DNA from victims or suspects so for example perhaps we find the skin cells of some other person underneath the fingernails of a dead victim well it wouldn't be unreasonable to conclude that perhaps those skin cells got there because the victim clawed at the face of the perpetrator while the murder was being committed and if that's true then the murderers DNA is underneath the victims fingernails so if we can compare the DNA
• 01:00 - 01:30 under the fingernails to someone we suspect as the murderer and we find a match well then that's a strong piece of evidence linking that suspect to the crime we can also compare the DNA from the crime scene to a DNA database so folks who commit certain crimes will have their DNA profiles added to a database and if even if we don't have a particular suspect we can still analyze the crime scene DNA and check it against a database and many crimes have been solved this way so let's emphasize what
• 01:30 - 02:00 we're looking for here is a match we want a match either between the crime scene DNA and a hit in the database or the crime scene DNA and someone we suspect as being involved in the murder now what if we found DNA at the crime scene but we don't have any idea who to suspect we find some person with no personal connection to any others in the middle of the woods there's no evidence of what happened here but we have some
• 02:00 - 02:30 other person's DNA present where do we start to look and then imagine there's no hits in the database either how would you know whose DNA that is well the short answer is you wouldn't unless you have someone to compare the crime scene DNA you're kind of out of luck now maybe later on that person will kill again and and they'll get caught and their DNA will be entered into the database and you can look it up but unless you have somebody to compare the crime scene DNA to that's it it's a dead end so just like with fingerprints we needed
• 02:30 - 03:00 to compare the fingerprints found at the crime scene to the fingerprints of a suspect same thing here we need to compare the DNA found at the crime scene now someone we suspect is involved in the crime so we can use DNA evidence as part of a criminal investigation we can also use DNA evidence later on after a conviction as part of an appeals process and I want to introduce you to something called the Innocence Project let's take a look so here we are on the website of the Innocence Project the Innocence Project is an organization that likes
• 03:00 - 03:30 their mission is to overturn what they see as faulty convictions in other words cases where a person's been convicted of murder or rape or other serious crimes and they've gone to jail for a very long time but those people who have been falsely accused and and convicted without sufficient evidence and what the Innocence Project does is is it people will send requests to the Innocence Project saying you know I'm innocent of this crime I've been serving for ten years please help me and what the Innocence Project will do is they'll go
• 03:30 - 04:00 in and they'll talk to the people they look at their case records they'll find out was the lawyer representing this person actually doing a good job and then relevant to our work today they'll ask was DNA analysis done and if DNA analysis has not been done and it looks like it would be relevant to the crime say a knife was picked up from the crime scene and it's got the blood of the perpetrator on it if that was never tested for DNA the Innocence Project will pay for DNA testing to compare the
• 04:00 - 04:30 perpetrators blood on the knife to the suspect who's been serving a sentence for 10 years in prison and if they find that in fact there's no match they'll bring that evidence back before the court and they'll try to get the conviction overturned and a person freed so we can take a look at some of the cases that they've worked on here and and by all means google or go to this Earle here at the top go to the Innocence Project read about these cases it's really it's it's stirring stuff and and I think
• 04:30 - 05:00 anybody who if you're watching this you have some interest in the justice system you know it's inspiring to see when the justice system gets it right when it frees people who haven't committed crime when it when it puts people behind bars who deserve to be there but but we ought to keep in mind that we don't always get it right that justice system has flaws and it's good that there are people who are working to exonerate those who have been falsely convicted and one of the key pieces in their work at the Innocence Project is
• 05:00 - 05:30 DNA evidence so it can be used to get a conviction it can also be used to exonerate a person who's been falsely accused all right so that was a little bit of a sidebar but I think it's important to think about these things let's get back to the DNA okay so we saw in the Innocence Project that if the person who was convicted of the crime if their DNA actually doesn't match the DNA that was found at the crime scene that we're pretty sure was that the murderers the rapists or whatever then that's pretty good evidence that they really shouldn't be in prison for this crime and they may be exonerated but we ought
• 05:30 - 06:00 to ask the question what if the crime scene DNA does match the suspects DNA what sort of conclusions can we draw so imagine that a DNA analyst has come to you and the DA analyst has said I've got a perfect match here I've run the sample of the suspect I've run the sample to the crime scene they match up perfectly what are the possibilities here well one possibility is that the suspect actually wasn't there and the DNA analyst made a mistake that's possible isn't it people do make mistakes this is good technology but nobody's perfect people do make mistakes it's possible
• 06:00 - 06:30 the suspect wasn't there but some other DNA ended up there maybe they were framed maybe somehow their tissue got there even though they were never there it's possible it's possible the suspect was there but had nothing to do with the crime in other words perhaps this person walked through the scene got a paper cut left some droplets of blood at the scene of the crime and then left and then later on someone was murdered in that room if that's the case you'll find their DNA there even though they had nothing to do with the crime that's possible the suspect was there but had a
• 06:30 - 07:00 lesser role in the crime than you suspect it maybe they were the getaway driver not the murderer and then of course it's possible that the suspect was there and had exactly the role in the crime that you suspect so it's important as scientists forensic scientists our type of scientist as scientist we must always consider all of the possibilities we cannot simply say ah well the analyst said that there's a match that means that this person is guilty of murder no we have to consider all the possibilities and it may very well be that the most likely possibility is that in fact the person committed the crime
• 07:00 - 07:30 but we always must be open-minded to all the possibilities and give each one a fair shake alright moving on our next question is what is the function of DNA you might be wondering why do we care for the sake of forensic DNA analysis and the reason is without some knowledge of the function of DNA and in particular the structure of DNA it's gonna be very very difficult for you to understand how forensic DNA analysis is carried out and I'm gonna try to keep this to just the minimum of information that's really need really needed for this project
• 07:30 - 08:00 we're undertaking but without further ado let's get into it the function of DNA so the function of DNA DNA is the stuff that encodes the information that tells dog cells how to make a dog that tells Pat cells how to make a cat that tells banana cells how to make a banana tree or Apple cells how to make an apple tree it's in other words the differences between the DNA and different organisms is what accounts for and produces the differences among the different species it's also what accounts for the similarities in closely related individuals here's the the Hemsworth
• 08:00 - 08:30 brothers and you can see there's a close resemblance between well no coincidence they're siblings they share a disproportionate amount of DNA together compared to random strangers in the population so it's it's no surprise that they share the same jaw structure or the same eyes or the same general type of hair and so forth so let's let's dig in a little deeper DNA basics dNA stands for deoxyribonucleic acid it's present in every cell in your body and in every
• 08:30 - 09:00 cell the number of DNA molecules is 46 what do we call DNA molecules we call them chromosomes where's the DNA stored it's stored in the nucleus of each cell and what it does is it stores the information needed to operate each cell in your body it gives the instructions to the liver cells how do you be a liver cell it gives the instruction to the brain cells how do you be a brain cell and so on now a little bit on the structure each DNA molecule or chromosome is made of two very very very very long strands which are intertwined in a sort of
• 09:00 - 09:30 double helix shape those strands are themselves made of smaller subunits which fit together to make the very long strands and those subunits are called nucleotides it takes millions of nucleotides to make a single molecule of DNA like you would find in one of your cells there are four types of nucleotides adenine thymine guanine and cytosine or if we want to be brief just a T G and C the way that DNA stores information is in the sequence of these
• 09:30 - 10:00 nucleotides in much the same way that a book you'd pull off the shelf stores information in the sequence of letters the difference between war and peace' and Harry Potter isn't that the symbols used are different it's just that the number in the arrangement of those symbols the number in the arrangement of the letters is different and hence you've got either a story about a boy wizard or a tome of a piece of Russian literature all right moving forward let's take a look at some images that might help to clarify some of these pieces we've just learned about so here
• 10:00 - 10:30 is an image of onion cells under a microscope that have been treated with a dye that stains DNA purple and you can see these purple blotches these are the nuclei the nucleuses if you like inside the onion cells it's lighting up purple because that's where the DNA is we can zoom in a little further and we catch some cells in the process of division and most of the time you can't see this but when cells are dividing their chromosomes become visible and you can see them in this image as these little sort of wiggly worm like shapes in some
• 10:30 - 11:00 of these cells well we can actually take the chromosomes out of the cells when they're in this form and examine them individually that's called making a karyotype let's take a look at a karyotype now here you have it here are your 46 chromosomes go ahead and count if you like there are 46 of them here what I'd like to do now is show you a chart that organizes them by type so it's actually the case that of your 46 chromosomes each one has a partner or has a pair in other words in your 46
• 11:00 - 11:30 chromosomes there's actually twenty three types of chromosome and two copies of each type so if you look at the die Grahame here we've gotten the upper left chromosome one there's two copies of chromosome one immediately to the right of chromosome one the two copies of chromosome one there's chromosome two and you can see there's two copies of chromosome two look midway down the page there's chromosome 15 you've got two copies of chromosome 15 so you've inherited one of each chromosome from your mother and one
• 11:30 - 12:00 of each chromosome from your father this becomes really important later on when we look at short tandem repeat analysis to understand that in fact you have two copies of each chromosome and each copy contains generally the same information what I mean by that is that your chromosome number one may contain information on how to make the protein hemoglobin in your blood may have genes that specify your height or the type of a muscle tissue that you generate well
• 12:00 - 12:30 your other copy of chromosome 1 also contains information specifying how to make hemoglobin or your height or the muscle tissue that you produce the versions of that information may be a little different in other words one version of chromosome one may have the hemoglobin that works really well and the other copy of chromosome one may have a hemoglobin that doesn't work so great but the general idea is that each chromosome in a pair has the same information as the other chromosome in
• 12:30 - 13:00 the pair so chromosome one whether it came from your mom or your dad contains the same sorts of information information about height information about hemoglobin and so on those are just made up examples but I hope it starts to clarify the point when we look at these chromosomes and we zoom in on them we actually see that these large structures are made of smaller threads and we can keep unwinding these threads and what we eventually end up with as we unravel and unravel and unravel and unravel these threads is the familiar double helix shape this is the so-called
• 13:00 - 13:30 twisted ladder the two strands of DNA which are wrapped around each other the classic double helix let's take a closer look at that classic double helix here's your double helix often what we like to do to further understand the structure of DNA is actually untwist this helix and when we do that we get what we'll call it DNA ladder now this is not how DNA exists in your it's just a way of presenting it to make it a little easier to see the structure and so what you can see here now are that the uprights of the ladder the
• 13:30 - 14:00 vertical pieces are what we call the sugar phosphate backbone but the rungs of the ladder if this were a ladder the places you put your feet those are the really key pieces those are the nucleotides let's take a look at those in even greater detail here now we've taken the DNA ladder and we've added a little more molecular detail these gray circles represent phosphate groups these blue Pentagon's represent deoxyribose sugar and all the labeled shapes in
• 14:00 - 14:30 orange yellow green and purple represent the nitrogenous bases and so these nucleotides altogether represent adenine thymine guanine and cytosine really really important vocabulary moving forward a pair of these we call a base pair okay so a pair of these is called a base pair and you have a grand total of three billion base pairs per cell so what you're looking at on the screen right now is four base pairs you've got
• 14:30 - 15:00 three billion per cell that's a lot of base pairs so a little more about the sequence of the bases the information in your genome is encoded in the sequence of these bases in other words do we see AAT GC c GC c da or do we see GC c ta a a GC ttae CC all right what's the order of the letters if we take all of the DNA in one of your cells and put it all together we call all that DNA all the
• 15:00 - 15:30 information and encodes all the molecule we call that your genome and your genome contains about 3 billion base pairs of nucleotides as a metaphor your genome is like a library a library with a theme and the theme is how to make a human specifically how to make you each molecule of DNA or remember the term chromosome contains the information to determine many different traits and contains an average of about a hundred million base pairs so every molecule 100
• 15:30 - 16:00 million base pairs these are big molecules here's your meta for each chromosome is kind of like a book in the library and the topic is a variety of different things you'll need for making a human specifically for making you you may wish to keep in mind at this point that the chromosomes are not organized by trait in other words it's not like this chromosome tells you how to make blood cells this chromosome tells you how to make liver cells that chromosome tells you how to make brain cells the traits are near as I can tell rather randomly distributed across the chromosomes with the exception of the
• 16:00 - 16:30 sex chromosomes if we can get to later each trait is encoded by what we call it gene this is a length of DNA that's on average about 17,000 base pairs in length as the meta for each gene is like a chapter whose topic is one thing you'll need for making a human specifically you a few more notes genome is really big and it's sometimes difficult to understand quite how big this becomes important in forensic science it gives us a sense of how big the task is of actually comparing one
• 16:30 - 17:00 person's genome to another person's genome so if each gene were really a chapter and it had 17,000 characters it would have about 2,800 words that's about half the size of your average Harry Potter chapter be about 12 pages an above-average reader would take about nine minutes to read that so the genes are there's a good chunk of information there if each chromosome were really a book they would have about 17 million words and would span 67,000 pages that's
• 17:00 - 17:30 about 20 times the length of the entire Harry Potter series from the very first book all the way to the last one if you were a fast reader it would take you a hundred and twenty days eight hours a day to read the whole thing that's pretty big now if the genome were really a library it would have about 500 million words and would span 2 million pages the book would be 200 meters tall or 640 feet tall and would weigh nine thousand kilograms are about 20,000
• 17:30 - 18:00 pounds this is a lot of information if you were to read it eight hours per day it would take you nine and a half years to get through the whole thing what's the takeaway here your genome is really big and contains a lot of information let's take a look at point zero zero two percent of the human genome shall we so here we are I've loaded up the first 60,000 letters of the human genome this is the first part of chromosome 1 if you're curious so here you can see all
• 18:00 - 18:30 those a's t's G's and C's and we'll scroll down through them and you can see that this is really quite a large thing and this whole thing is just point zero zero two percent of your whole genome and we're about halfway through it right now mm-hmm nearly there ah there we are
• 18:30 - 19:00 at the end so what you've just seen scrolling past is point zero zero two percent of your genome zero point zero zero two percent of all the 3 billion base pairs that make up your genetic identity pretty amazing so you've just seen a sneak peak of just a snippet just the tiniest fraction of your genome you've seen it's really really big well how are we going to actually make comparisons from person to person it turns out that of those 3 billion base pairs depending on how you count 99.9
• 19:00 - 19:30 percent of them are identical from person to person in other words if we took your DNA and my DNA we compared them 99% of those letters would be in the same place and they'd be the same letter we look at you know base pair number 38 million and chromosome 1 it's a T in me it's almost certainly a T in you and so on so we're almost identical but if 99 percent of our base pairs are the same that means 0.1 percent are different and unique in fact
• 19:30 - 20:00 no two people have exactly the same DNA with the exception of identical twins forensic DNA analysis uses this point one percent uniqueness to distinguish one person from another so we're nearing the end of today's video but I want to give you a little sneak peek just a taste of what's coming next how DNA analysis really works so the basic idea is this we're going to compare the base sequences that's the AE the T's the G's and the C's that pattern from the crime
• 20:00 - 20:30 scene DNA with the same pattern from the suspects DNA we're gonna look are they the same or not if the base sequence is exactly the same the crime scene DNA and the suspects DNA then that means it's their DNA so what's the process what's the general process okay so first DNA is extracted from collected samples you get your samples for underneath the fingernails you take a blood test of the suspect and so on then we process the DNA samples and the goal there is to make those very few
• 20:30 - 21:00 differences among the samples invisible so that we can actually distinguish two different peoples DNA once we've done that we can compare the DNA samples from the crime scene to the DNA samples from the suspect and we can see if there's a match so next time our goal is to answer this question how do we do this how do we process DNA samples to make those very very small differences visible and then how do we compare it to see whether a suspect was
• 21:00 - 21:30 really there at the crime scene let's summarize DNA analysis compares DNA collected from the crime scene with DNA collected from suspects each person carries an astonishing 3 billion base pairs of DNA in every cell these 3 billion base pairs of a person's genome are distributed across 46 molecules which we call chromosomes the 3 billion base pairs are made up of the nucleotides ATG and C that's adenine thymine guanine and cytosine the sequence of the nucleotides encodes the information of the organism in other words it's the order and the identity of
• 21:30 - 22:00 the letters ATG and C are the molecules that they represent that determines what information is stored in the DNA it's those very few differences in each person's genome the sequence of the letters that forms the basis for forensic DNA analysis and then again to summarize the process forensic DNA analysis consists of DNA extraction processing of samples and then finally comparison of samples next time we'll start taking a closer look at how this work is actually done I hope this was helpful we'll see you next time bye for
• 22:00 - 22:30 now