SME

• What factors have to be established to estimate TOD?

  Extent of decomposition, stage of succession, forensic entomology, body temperature of the deceased, the degree of muscle contraction
• What colour does the skin appear in the first stage of decomposition?

  Green
• How is TB spread?

  When uninfected people inhale airborne droplets of liquid which are released from the lungs
• What type of organisms carry out decomposition?

  Fungi and bacteria
• What factors effect the rate of decomposition?

  temperature, oxygen availability, exterior pH
• Decomposition would be slower in anaerobic (without oxygen) conditions and at lower temperatures
• Succession refers to the change in types of organisms found in a habitat over time
• Stages of Succession above ground:

  • Bacteria will be found in and on the dead body immediately after TOD
  • As tissue decomposition sets in it creates ideal conditions for flies to lay eggs and their larvae to hatch
  • As more soft tissue is consumed by the fly larvae it creates favourable conditions for beetles to establish 
  • When tissue dries out over time flies will leave the body as they prefer a moisture-rich environment
  • Beetles, however, can decompose dry tissue so they will remain on the body
  • Once all tissues have been decomposed most organisms will leave the body
• Factors that might affect the progression of insect life cycles:

  • Drugs present in the body
  • Humidity of the surroundings
  • Oxygen availability
  • Temperature
• Once a person dies, metabolic reactions will eventually end as there is no more heat produced so the body temperature drops until it reaches the temperature of the surrounding environment.
• Body temperature decreases by 1.5-2 degrees per hour, providing a method of determining TOD
• Conditions affecting the rate at which body heat is lost:

  • Air temperature
  • Surface Area : Volume
  • Presence of clothing / type of clothing
• Muscles in the body begin to contract about 4-6 hours after TOD, leading to rigor mortis
• Rigor mortis is muscle rigidity that develops after death when joints become fixed in a position.

  Small muscles go into rigor mortis first, then the larger muscles
• Hot environmental temperatures means rigor mortis sets in quicker and for a shorter period
• Rigor mortis:

  • no more oxygen reaches the muscle cells after death, meaning they will start to respire anaerobically, producing lactic acid
  • the accumulation of lactic acid decreases pH in the muscle cells, denaturing ATP producing enzymes
  • without ATP the myosin heads cannot be released from actin filaments, locking the muscles in a contracted state - leads to the stiffness
• • As rigor mortis begins in the smaller muscles of the head and end in the larger muscles in the lower body, forensic experts can determine TOD by the progress of rigor mortis in the body.
  • RM would have taken place in every muscle between 12 and 18 hours after death, but will wear off again after 24-36 hours from TOD
• • Muscles contract due to the action of myosin and actin filaments (protein filaments)
  • ATP is required to allow the myosin heads to detach from the binding sites on actin
• The microorganisms involved in decomposition produce CO2 and methane which are released into the atmosphere
• What can DNA profiling be used to determine?

  Paternity/maternity testing, ancestry tests, determining evolutionary relationships between species
• DNA profiles can be created using the following steps

  • Isolating a sample of DNA e.g. from saliva, skin, hair, or blood
  • Producing more copies of the DNA fragments in the sample using the polymerase chain reaction (PCR)
  • Carrying out gel electrophoresis on the DNA produced by PCR
  • Analysing the resulting pattern of DNA fragments
• What is the polymerase chain reaction (PCR)?

  An in vitro method of DNA replication which produces many copies of a piece of DNA
• What is PCR used for?

  • It is used to produce large quantities of specific fragments of DNA or RNA from very small quantities
  • By using PCR scientists can produce billions of identical copies of the DNA or RNA sample within a few hours
  • In each PCR cycle the DNA is doubled, so in a standard run of 20 cycles a million DNA molecules are produced.
• What does a PCR reaction require?

  DNA or RNA to be amplified, DNA polymerase, Primers, Free nucleotides, Buffer solution
• What are the 3 stages of the PCR reaction?

  Denaturation: Double-stranded DNA is heated to 95 degrees which breaks the hydrogen bonds that hold the 2 DNA strands together

  Annealing: Temperature is decreased to 50-60 degrees so that primers can anneal to the ends of the single-strands of DNA

  Elongation: Temperature is increased to 72 degrees, the optimum temp for DNA polymerase to make complementary strands of DNA to produce new identical double-stranded DNA molecules
• Why is a buffer solution used in PCR?

  To ensure the optimum pH for the reactions to occur in
• What are the free nucleotides in PCR used for?

  DNA synthesis (the construction of a new DNA strand0
• What are primers?

  Short sequences of single-stranded DNA that have base sequences complementary to the 3' end of the DNA/RNA being copied; they identify where the DNA polymerase enzyme needs to bind
• Each PCR cycle doubles the amount of DNA
• After PCR, the DNA is treated with restriction enzymes and a fluorescent tag in preparation for gel electrophoresis;

  • Restriction enzymes break up the DNA into fragments of different length
  • Fluorescent tags enable the DNA fragments to be seen under UV light
• DNA profiling can also be useful in selective/captive breeding programmes of animals or in the cultivation of plants
• In selective breeding programmes, DNA profiles of the animals can be compared to determine which are genetically the most different from each other; this allows the breeding of individuals that are not closely related, creates a greater gene pool and prevents inbreeding depression.
• How do prokaryotic cells differ from eukaryotic cells?

  Prokaryotic cells have; no membrane-bound organelles, 70S ribosomes (smaller than eukaryotic), no nucleus, peptidoglycan cell wall, single circular bacterial chromosome, plasmids, capsules, flagellum, pili
• What is the role of the flagella in prokaryotes?

  Its long tail-like structure rotates, enabling the prokaryote to move
• What are the pili?

  Thread-like structures on the surface of some bacteria that enable the bacteria to attach to other cells or surfaces. They are involved in gene transfer during sexual reproduction.
• What features are always present in a prokaryote?

  cell wall, cell surface membrane, cytoplasm, circular DNA, ribosomes
• What features are sometimes present in a prokaryote?

  flagellum, capsule, mesosomes, plasmid, pili
• Viruses have; a nucleic acid core (can be RNA or DNA), a protein coat (capsid), lipid envelope, attachment protein
• How is TB transmitted?

  TB is transmitted by droplet infection (carried in droplets of mucus and saliva, released into air when a person talks, coughs or sneezes)
• What are infectious diseases caused by?

  Pathogens