Forensic Bio

What role does DNA play in Forensic Biology?

- with the concept that DNA holds a genetic code within the sequence of bases along the double helix molecule, forensic scientist are able to read that code where they can now question and determine the source of a given DNA sample -with the right analytical tools, scientist can take advantage of DNA's adaptable stability to mimic the process of DNA replication via PCR where the amplified DNA is will be processes and interpreted accordingly to answer questions and determine the source of DNA

What is STR analysis and why is it important?

-"short tandem repeat" analysis is a method used to compare loci on DNA -Using DNA to distinguish between two individuals is a tricky matter, because close to 99.9 percent of our DNA is the same as everybody else's DNA. ALTHOUGH useful differences in the DNA must be found in the remaining one-tenth of one percent (which is not known to code for anything specific). Because this section of the DNA's precise sequence is not so important, it is quite variable, which makes it possible to use DNA to distinguish between individuals

What are loci in DNA?

-A locus is a physical site or location within a genome (such as a gene or another DNA segment of interest), somewhat like a street address. The plural of locus is loci -Locus is a term that we use to tell us where on a chromosome a specific gene is. So it's really the physical location of a gene on a chromosome. It's a way of defining the gene's neighborhood. If you consider the entire chromosome as a country where the gene is found, and then a region of the chromosome would be the city. The more specific area, or the locus, is this particular neighborhood where the gene is found.

what is the difference between autosomal STR and Y-STR? which provides stronger evidence within a case?

-Autosomal STR's are found on chromosomes that are not involved in determining a person's sex/gender -Y-STR are found on the male's sex determining Y chromosome -profiles that are based on autosomal STR have a greater statistical power since autosomal DNA is randomly exchanged between matched pairs of chromosomes in the process of making egg and sperm cells.... so this explains why no two people on earth are exactly like (unless we are dealing with identical twins) -while profiles based on Y-STR are stasitically weaker because only males have a Y chromosome and all males get theirs from their fathers, so all males in any paternal line have nearly identical Y chromosomes... but can still be used to discriminate individuals in court

How many loci are being analyzed in STR analysis?

-In the United States, 13 autosomal STR loci are now accepted as the system used for forensic purposes...if given a robust crime scene DNA sample, there is a 1 in 1 billion chance that within the 13 loci, a unrelated person from the individual of interest will have the same STR sequences -By contrast, experimental work with a very robust set of 30 Y-STR loci showed a probability of about 1 in 50,000 for a perfect match

Why is STR analysis so reliable?

-Short repeating sequences are sequences that repeat a variable number of times in different individuals - A collection of these can give nearly irrefutable evidence statistically of a person's identity because the likelihood of two unrelated people having the same number of repeated sequences in these regions becomes increasingly small as more regions are analyzed

What is the process of DNA profiling?

-collecting potential DNA from submitted evidence -DNA extraction and or a differential is performed -Quantification -Amplification -Electrophoresis -Interpretation

How is genetic variation helpful in DNA analysis?

-genetic variation is the differences in DNA amongst individuals in a population -the greater variation within a genome leads to greater individualization (meaning less people have the chance of having the same exact orientation of genes, therefore increasing the chances of finding a single individual with an exact match) when certain types of variation has a frequency greater than 1% of the population, it is known as polymorphism Various types of polymorphisms include: -single nucleotide polymorphisms (SNPs) -small-scale insertions/deletions -polymorphic repetitive elements These were known to be helpful in identifying/creating a DNA profile for individuals in samples that were degraded via exposure to outside elements

Why is the chemical stability of DNA so important?

-it is able to analyzed long after it is deposited at a crime scene *although depending on circumstances after deposition at crime scene and or how it is collected and handled by forensic scientists, it can degrade and affect the ability to get usable results*

What are possible sources for DNA?

-saliva -semen -vaginal tract -hair -blood -tissue -bone/teeth etc. -trace DNA (fingerprints)

What methods are used in Forensic Biology?

-serological and biochemical methods -Polymerase Chain reaction (PCR) based methods -Automated electrophoresis platforms -the use of the Combined DNA Index System (CODIS)

What are steps in organic DNA extraction?

1.cellular material is recovered from the substrate (ex: garment or weapon) by scrapping, soaking, swabbing, etc. 2.Tris/HCl buffer containing proteinase K (is an enzyme that breaks open the cell and nuclear membranes,releasing cell contents including the DNA into solution) and detergent such as sodium dodecyl sulfate (SDS) is added to cell pellet 3.phenol/chloroform with an ehtanol ppt. or alternate concentration step successfully seperates nucleic acid from heavier protein molecules, removes proteinaceous waste and inhibitors and results in a purified DNA extract that is amenable for further analysis *the DNA recovered is ds and relatively free of impurities*

What is the process of differential extraction?

1.centrifuge and remove supernatant (the liquid portion sitting above seperated solids) 2. add reducing agent dithiothreitol (DTT) in the prescence of extraction buffer, proteinase K and SDS (this will break the disulfide bonds present in the protein and DNA of the sperm cells)

What are the properties of DNA?

1.small amount of building blocks that are arranged in a way to give the double helix structure its complexity- being able to store a vast amount of information that can define the nature of the biological cell 2.the double helix is stable enough to "open up"-which allows the living cell to use its contained information to go about it life function of transcription or make copies of its self (replication) 3.DNA is also stable enough to be transferred from generation to generation after replication -the possibility of mutation to occur within these daily processes that bring rise to individuality and or something detrimental to ones health such as cancer

What are characteristics of DNA?

1.two polynucleotide strands and their sugar phosphate backbone- alternating five carbon deoxyribose sugars and phosphate groups (provides negative charge which further stabilizes DNA) linked together via phosphodiester bond. This twist the chains in a way where the hydrophobic bases are inside of the structure and protected from aqueous solutions. only certain enzymes and or temperatures are able to break this molecule open 2.complementary nitrogenous base pairing (Chargaff's rule of Adenine always binding with Thymine.. they share 3 hydrogen bonds and Cytosine always binding with Guanine... they share two hydrogen bonds) 3.the physical structure of DNA is double stranded- this is important bc each strand of DNA can serve as a template during DNA replication...each new cell needs to have an exact copy of DNA that was present in the old cell 4. anti parallel orientation (5' phosphate group to 3' OH group strands run opposite of one another)

Where is DNA found and what is the purpose of it? How many chromosomes are there?

DNA is found in the nucleus of the cells. In total, called the genome and contains the genes that encode for proteins that are to be made by the cell to define to cell's type, characteristics, and essentially the entire organism of the cell -DNA sequences can also be regulatory (play a role in expression of certain genes), structural (affect packaging and structure of DNA) -all of these elements are found within 23 separate lengths of DNA also known as chromosomes... humans have 2 sets of these (one from mom and dad) giving a total of 46 chromosomes (some exception may apply where some individuals have more or less...which can lead to developmental disorders)

What is a differential DNA extraction? Why do we need them?

Forensic lab typically employ a differential for the seperation of spermatozoa from other cells such as vaginal epithelia *distinguish between male and female DNA in sexual assault cases* -Strong disuilfide bonds prevent the lysis of the spermatozoa nuclei in the presence of proteinase K and SDS (a detergent), allowing more of the fragile epithelia and other cells to release their DNA which is not what we want

What is Forensic Biology?

Identification of biological fluids such as blood, semen, and saliva, etc. that allows one to create a DNA profile for individualization

What is a phenotype?

The physical appearance of an organism which are the outcome of the interactions within the genotype... this depends on which genes are dominant

What is a genotype? What are the different types?

The set of genes in our DNA that is responsible for a particular trait where the chemical composition within DNA is what gives rise to a phenotype -homozygous recessive (pp) -homzygous dominant (PP) -heterozygous (Pp) -where homozygous dominant and heterozygous will express the same phenotype -possibilty for codominace where both gene variants (alleles) are visible in the phenotype

What is an allele?

different forms/variants of genes that make up a genotype. An individual contains two alleles, one inherited by each parent. -a homozygous for a certain trait is going to have two of the same alleles (pp) and heterozygous is going to have two different alleles (Pp)

What is DNA?

stands for deoxyribonucleic acid. It is a self-replicating material present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information and can be considered "the blueprint of life"

what is the difference between phenotype and genotype?

the genotype is a set of genes that is inherited by the organism's parents and determines the traits are characteristics that will be expressed while the phenotype is the final product of the expression... it is the physical trait or characteristic that we see in a organism

What is organic DNA extraction?

the recovery of DNA from forensic samples involving the use of the enzyme proteinase K, followed by the organic extraction with a solution of phenol and choloform -this method has high quality nucleic acid yield and purification capabilities despite being time consuming, having multiple transfer steps, and uses hazardous materials