Introduction to forensic DNA
- Created by: AAntonianannetti
- Created on: 07-05-19 16:29
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- Forensic DNA Evidence
- DNA Structure
- DNA exists as a double-stranded molecule, that adopts a helical arrangement
- First described by Watson and Crick in 1953
- DNA has a sugar phosphate backbone
- Consists of 5-carbon deoxyribose sugars and phosphate groups
- Linked by a phosphodieser bond
- Consists of 5-carbon deoxyribose sugars and phosphate groups
- There are 4 different nitrogenous bases: A, C, G and T
- A - T and G - C
- Joined by hydrogen bonds
- A - T and G - C
- DNA is made from genes
- Chromosomes
- 46 Chromosomes
- 23 pairs
- Genes are present on the chromosomes
- Sex cells
- Women = **
- Men = XY
- 46 Chromosomes
- DNA exists as a double-stranded molecule, that adopts a helical arrangement
- DNA Replication
- DNA duplicates itself during cell division
- This process is formed at the beginning of every cell division so that when a cell divides each daughter cell will inherit an identical copy of DNA
- Semi-conservative
- Semi discontinuous
- Steps: 1) DNA unwinds the parental double helix. 2) DNA polymerase will add the free DNA nucleotide using complementary base pairing (A-T and C-G) to the end of the primer to allow a new DNA strand to form. 3) The two new strands twist to form a double helix - each identical to the original strand.
- Mitosis
- DNA duplicates itself during cell division
- Genetics
- The aim of genetic analysis for forensic casework is to produce a DNA profile that is highly discriminating
- One that is potentially unique to each individual
- Humans have 99.9% of similar DNA to one another
- It is now possible to analyse any region within the 3.2 billion bases that make up the human genome
- Genetic differenences
- Phenotype
- The observable characteristics of an individual
- Genotype
- The genetic constitution of an individual
- Phenotype
- Human gnome - The total DNA content found within the nucleus of a human cell
- Mendelian Inheritance
- A type of biological inheritance
- Law of segregation
- Each trait from the parents genes (alleles) split and one gene passes from each parent to the offspring.
- Law of independent assortment
- Different genes (alleles) are passed onto the offspring independently
- Exception of genetic linkage
- When genes are very close to each other on the same chromosomesmeaning that they move together in recombination
- Punnet square - used to show the combination of alleles in a gene
- The aim of genetic analysis for forensic casework is to produce a DNA profile that is highly discriminating
- Why is DNA important in forensics?
- Each persons genome contains a large amount of DNA that is potential target for DNA profiling
- Within an individuals DNA, there are detectable patterns that are repeated a number of times showing different characteristics
- Successful DNA analysis produces a profile that has potential to identify a possible source of the evidence left at a crime scene
- DNA is a relatively robust molecule that will usually survive under a range (but not all) environmental conditions
- DNA can be isolated from a wide range of biological samples
- Cheap techniques
- DNA profiles are assembled into databases, which can be searched against by samples found at a crime scene
- Each persons genome contains a large amount of DNA that is potential target for DNA profiling
- DNA Profiling
- The examination of an individuals DNA to produce a unique, unalterable pattern that will be a characteristic of any tissue of body fluid that originates from him/her.
- Examples: Colin Pitchfork, OJ Simpson and Leanne Teirnan
- Short Tandem Repeats (STRs)
- Current technique for DNA profiling uses STRs
- Polymorphisms
- Non-coding DNA that contains repeats of the same nucleotide sequences
- Found at different places or genetic loci in a persons DNA
- Cell division
- Meiosis
- Produces 4 non-identical haploid (sex) cells.
- 1 replication but 2 successive divisions
- Produces 4 non-identical haploid (sex) cells.
- Mitosis
- Produces two identical diploid cells for growth and repair
- No reshuffling of genetic material
- Produces two identical diploid cells for growth and repair
- Meiosis
- Polymerase Chain Reaction (PCR)
- A technique for amplifying DNA
- Uses complimentary primers for specific target DNA sequences
- 1) Denaturation (94 degrees) Breaks hydrogen bonds in between bases to create 2 single stranded molecules
- 2) Annealing (50-65 degrees) Primers anneal to the template DNA and bind to complementary sequences
- 3) Extension (70-80 degrees) Taq polymerase is added and synthesises to a new DNA strand to the complementary template DNA
- 2) Annealing (50-65 degrees) Primers anneal to the template DNA and bind to complementary sequences
- PCR errors
- Taq polymerase does not have the ability to 'proof read' resulting in errors in amplification
- The greater the amplification, the more likely it is that such errors will occur.
- Taq polymerase does not have the ability to 'proof read' resulting in errors in amplification
- A technique for amplifying DNA
- Electrophoresis
- During DNA analysis the individual fragments of DNA can be separated using electrophoresis to produce the distinct DNA fingerprint
- A method of separating molecules by their size through application of an electric field, causing molecules to migrate at a rate and distance dependant on their size.
- An electric current is added
- Negatively charged DNA fragments begin moving through the gel towards the positively charged anode
- The gel acts as a molecular sieve, allowing smaller molecules to travel faster than larger fragments
- Negatively charged DNA fragments begin moving through the gel towards the positively charged anode
- Samples are put into wells at the top of the gel using pipettes
- Fluorescent probes or dyes can be added from visualisation
- Allows fragments to be measured
- An electric current is added
- A method of separating molecules by their size through application of an electric field, causing molecules to migrate at a rate and distance dependant on their size.
- During DNA analysis the individual fragments of DNA can be separated using electrophoresis to produce the distinct DNA fingerprint
- DNA Structure
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