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6.1 Forensic investigations and analysis of DNA


Investigating time of death

ð  Harder after 48 hours

ð  Forensic evidence combined with witness and circumstantial evidence

ð  At death, metabolic reactions slow and then stop

o   Lowering body temperature

§  drops straight after death, plateaus and then drops to room temperature

§  outside the body cools fastest

§  body fat acts as insulation

Rigor mortis

ð  Muscle calls have large stores of ATP and glycogen, so respire anaerobically for a time

o   As they run out of ATP the fibres become contracted ad solid

ð  Starts 2-4 hours after death, needs between 6-8 hours for full effect

ð  Begins in the face and neck, progressing down the body

ð  Depends on levels of ATP at the time of death and level of activity before death

ð  Not permanent

o   Lasts between 36 and 48 hours after death, but can be considerably longer

ð  Muscles soften as enzymes break down tissue

ð  As cells die and gut movement stops, catabolic enzymes break down walls of the gut then surrounding cells

ð  As cells die from lack of oxygen, lysosomes rupture, releasing enzymes to break down cells

Stages of succession

ð  Colonisers

o   Anaerobic bacteria in the gut breed freely

ð  Joined by flies

o   Blowflies

§  Arrive in minutes

§  Attracted to moisture

§  Lay eggs, maggots hatch

§  Feed on tissues and turn into flies

ð  Beetles lay eggs on the carcass

ð  Parasitic wasps lay eggs

ð  Body dries out so cheese flies and coffin flies move in

ð  Strong chewing beetles eat away at muscles and connective tissues

ð  Mites and moths feed on hair until dry bones are left

ð  Speed varies

o   Temperature    

§  Warmer bodies have faster decay rates

o   Exposure

Forensic entomology

ð  Blowflies

o   Lay eggs in corpses

§  In natural openings

o   eggs, maggots and pupae are collects

§  species identified

·         each species have different life cycle lengths

DNA and the genetic code

ð  DNA      

o   Double helix structure

§  Nucleotide bases            

·         Adenine, thymine, cytosine and guanine

o   Unique to every organism

ð  Proteins are formed of amino acids

o   Only 20 naturally occurring

o   Joined together on the ribosomes

ð  Triplet code

o   64 possible combinations

ð  Non-overlapping

ð  Degenerate

Transcription of the DNA molecule

ð  DNA is copied to form mRNA

ð  The breakdown of the hydrogen bonds between bases is catalysed by enzymes called DNA-directed RNA polymerase

ð  Template strand provides the order of sequence of nucleotides in an RNA transcript

ð  RNA polymerase joins many small nucleotide units together to form mRNA


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