Edexcel Categories Archives: Topic 6: Immunity, Infection and Forensics



DNA is the molecule that carries instructions for our development. The genetic code is the sequence of nucleotides and amino acids in a polypeptide chain.

DNA is a polynucleotide made up of nucleotides containing deoxyribose (a sugar), a phosphate and a base. DNA is a double-helix structure, with hydrogen bonds between the bases, joined by complementary base pairing.

  • The genetic code is non-overlapping – codons do not overlap so a single point mutation would only affect one amino acid rather than 3 if it were overlapping
  • The code is degenerate/redundant – contains more information than it needs as it is the first 2 nucleotides which are responsible for determining the amino acid – if the final base in a triplet is changed by a mutation, this could still produce the same amino acid without affecting the polypeptide made. So a degenerate code helps protect organisms from point mutation.


Protein synthesis:

  • Transcription and transferring the genetic code into a complementary mRNA strand
  • Translation and transferring the mRNA into a sequence of amino acids in the polypeptide chain


DNA à pre-mRNA à mRNA à polypeptide à protein



Occurs in the nucleus

  • Hydrogen bonds between the bases are broken, separating two DNA strands, this is catalysed by DNA helicase
  • RNA polymerase attaches to one of the DNA strands at a start codon
  • This strand is called the template strand and is transcribed to give a complementary single strand of mRNA. This is brought about by the DNA polymerase
  • Nucleotides join by complementary base pairing
  • When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from DNA
  • This newly synthesised mRNA strand has the same sequence as the non-template
  • The DNA strand with the same sequence as the mRNA is the sense strand. The strand which acts as the template is the antisense strand
  • The mRNA then leaves the nucleus through membrane pores into the cytoplasm for translation


Post-transcriptional changes in mRNA:

mRNA is modified before translation

  • Extrons – Contain coding information
  • Introns – Non-coding information (nonsense sections) – pre-mRNA contains both introns and extrons

Changes take place in pre-mRNA before it leaves the nucleus and attaches to a ribosome. RNA splicing occurs – introns are removed and extrons are rejoined to form a single continuous coding strand of mRNA. This is carried out large enzymes called spliceosomes.

Sometimes extrons are removed too so the code on the final mRNA is different from the code on the DNA which was transcribed.

So strands of mRNA from the same bit of DNA may not be the same so will code for polypeptides containing a slightly different sequence of amino acids.



Occurs on ribosomes in the cytoplasm

  • The mRNA enters the cytoplasm and attaches to the ribosomes
  • The ribosome starts reading the mRNA at the start codon
  • As each codon is read, the tRNA with the specific anticodon picks up the specific amino acid from the cytoplasm and carries it to the ribosome
  • The tRNA lines up its anticodon alongside a complementary codon in the mRNA. There will always be two tRNA molecules held on the ribosome
  • Hydrogen bonds between the two bind the tRNA to the ribosome while amino acids are joined by peptide bonds, catalysed by an enzyme
  • Once the peptide bond is formed, the tRNA leaves the ribosome
  • The ribosome moves along the mRNA molecule until it reaches a stop codon, leaving a completed polypeptide chain
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Antibiotics are used to fight bacterial infections.

There are 2 main types:

  • Bactericidal: Antibiotic that kills bacteria
  • Bacteriostatic: Inhibits growth of bacteria


When bacteria are no longer affected by an antibiotic they are resistant.

Resistance happens because:

  • Mutations
  • Pathogens have evolved to evade immune systems – struggle is the evolutionary race


Hospital Acquired Infections

They try to combat antibiotic resistance by:

  • Only using antibiotics when needed and ensure course of treatment is completed (reduced selection pressure on organisms and destroys all bacteria causing infection)
  • Isolating patients with resistant diseases (prevents transmission)
  • Good hygiene
  • Screening of patients entering a hospital (detection and treatment)


Investigating bacteria and antibiotics:


  • A sterile nutrient agar plate is seeded with bacteria
  • Antibiotic applied to sterile paper discs, lay on agar using sterile forceps
  • Seal petri dish but not completely
  • Incubate at 30oC for around 24 hours
  • Look for inhibition zones around the antibiotic discs (clear zones). Bigger areas indicate a better antibiotic against this species
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Immunity is the ability of microorganisms to resist infection by protecting against disease causing microorganisms. When the body has enough antibodies (or can produce quickly enough) to fight infection.

  • Active Natural: Exposed to foreign antigen by getting the disease – immune system is activated – body produces memory cells, making the body immune to the same disease in the future
  • Active artificial: Vaccination – Injection of dead and weakened disease organisms, toxins or antigen fragments means the body is exposed to the antigen and produces memory cells – develops immunological memory – if exposed to same disease again, antibodies are quickly released
  • Passive natural: A mother’s antibodies across the placenta or through breast milk – these antibodies protect the baby against any pathogens the mother has encountered – short-term as antibodies are broken down within a few days
  • Passive artificial: Antibodies are formed in one individual, injected with antibodies that provide immediate protection against the invading pathogen they are specific for – then gradually broke down and not replaced


Active = body produces its own antibodies – catching a disease – long-term

Passive = body receives ready made antibodies – vaccination – short-term



Substance that stimulates immunity without getting the disease – it is non virulent. Antigens come from weakened, dead or fragmented pathogens. This stimulates memory cells to develop – ready to destroy the real pathogen if encountered.

Vaccinations need to be affordable, available, have few side effects and be able to provide herd immunity.

Types of vaccination: pathogenic, antigens, harmless toxins, live MOs

Vaccinations don’t eliminate disease if:

  • Immune system is defective
  • Disease develops before immunity is built
  • Mutations arise (antigenic variability)
  • If the pathogen hides/conceals itself
  • May varieties of a pathogen
  • Individuals don’t receive vaccination

Advantages of vaccination:

  • Protects against disease
  • Herd immunity
  • Most common vaccinations are relatively cheap

Disadvantages of vaccination:

  • Some may suffer from allergic reactions
  • Minority of children become severely ill/death
  • Linked to a rise in asthma and allergies

Ring vaccination: used for new cases of disease. Vaccinates people in immediate vicinity of the disease – e.g. surrounding people, can control livestock disease

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Relies on lymphocytes which respond to foreign antigens on the surface of bacteria and viruses

Can take two forms – Humoral response (B cells) and cell mediated response (T cells)


Humoral response

Has two main stages

  • Pathogen with antigens on surface is engulfed by macrophage
  • Macrophage presents antigens via MHC’s on its surface and becomes an APC
  • APC binds to T helper cells with complementary CD4 receptors
  • T helper cell is activated and divides into T memory and T helper cells. T memory cells remain in the body and become activated if the pathogen is encountered again
  • Effector stage: B cell binds to the complementary pathogen and engulfs it by endocytosis
  • B cell becomes an APC
  • Activated T helper cell with a complementary receptor binds to APC
  • Cytokines are released which stimulate the B cell to divide to form clones of B effector cells and B memory cells
  • B effector cells differentiate into plasma cells which secrete antibodies and only last for a few days
  • B Memory cells are long lasting and are involved in the secondary immune response


Cell mediated response

  • Pathogen infects host cells and presents the antigens on MHC to become an APC
  • T killer cell with complementary receptor binds to APC
  • Cytokines from T helper cells stimulate the differentiation
  • T killer cell divides to form clones of active and memory T killer cells
  • Active T killer cells bind to infected cells presenting antigens on MHCs
  • T killer cell releases chemicals causing pores in the infected cell, causing lysis
  • Cell becomes permeable so water and ions enter the cell, it swells, bursts and dies


B cells

Lymphocytes involved in the humoral response. They make specific antibodies against antigens, perform the role of APCs and develop into B memory cells. Each has a unique receptor protein on its surface. They originate in bone marrow.

  • B effector cells – differentiate to produce plasma cells which release antibodies into the blood and lymph – immediate response – involved in the primary immune response
  • B memory cells – remain in the body for months or years, enabling an individual to respond quickly to the same antigen. They are specific to the antigen encountered during the primary immune response – involved in the secondary immune response


T cells

T cells are involved in cell mediated immunity. They have T cell receptors on their surface. T cells originate and mature in the thymus.

  • T helper cell – when activated, these stimulate B cells and aid T killer cells to divide and become cells to produce antibodies
  • T killer cell – Destroy cells with foreign antigens on their surface
  • T memory cell – A long lived T cell that has receptors for an antigen due to its encounter with a prior infection of vaccination

T cells

Killer cellsHelper cells
Search for infected cellsUndergo clonal selection
Attach to cellsClonal expansion
Secrete toxic substances into themSecrete cytokines
Kill pathogen insideB cells divide and fire antibodies



B cellsT cells
Produced and mature in bone marrowProduced in bone marrow but mature in thymus gland
Involved in humoral response (involving antibodies)Involved in cell mediated response (involving body cells)
Produces antibodiesDoes not produce antibodies
Responds to foreign cells outside body cellsResponds to foreign material inside body cells
Responds to bacteria and virusesResponds to own cells affected by virus, cancer or response to a transplanted tissue



  • Cytokines – chemicals that T helper cells release that stimulate division and differentiation of B cells
  • Antibodies – Y shaped protein molecules that belong to a class known as immunoglobulins. Antibodies bind to antigens and act as labels, allowing phagocytosis to recognise and destroy the cell
  • Antigen – any substance foreign to the body recognised as non-self that causes an immune response and is capable of binding with an antibody or T cell. They produce antibodies
  • Lysis – disintegration of cell membrane
  • APC (Antigen Presenting Cell) – displays foreign antigens with MHC on their surfaces for T cells to recognise


The primary immune response is slow because there are few cells which can make the antibody needed to fight the pathogen. There is time needed for macrophages to present antigens, B cells to attach and divide, and plasma cells to differentiate and produce antibodies.


Secondary immune response

Involves memory cells

If infected by the same pathogen again, the immune system responds faster.

There is a greater production of antibodies and it lasts longer.

B memory cells differentiate immediately to produce plasma cells to release antibodies.

T memory cells remember the specific antigen and will recognise it second time round.

Faster because b cells already have complementary antigen receptors, memory cells can divide rapidly into plasma cells and so there is quicker clonal expansion.

The invading pathogens are often destroyed so rapidly that the person is unaware of any symptoms – they are said to be immune


Ways pathogens enter the body:

  • Vector (organism that transmits infection) – prevent by blood clotting, skin and sebum
  • Formites (objects carrying pathogens) – prevent by skin, skin flora and vomiting
  • Direct contact – prevent by lysozymes, defensive secretions, mucus and skin flora
  • Inhalation (e.g. TB) – prevent by mucus, vomiting lysozymes and cilia
  • Ingestion (contaminated food) – prevent by vomiting, mucus and saliva
  • Inoculation (break in the skin e.g. HIV) – prevent by skin, antibodies, immune response and blood clotting
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At the site of infection, damaged white blood cells and mast cells release histamines that cause arterioles to dilate and capillaries to become more permeable. Blood flow to the area increases and plasma, white blood cells and antibodies leak out into the tissue. This causes oodema (swelling) and often pain. This enables white blood cell to attack. It also causes heat and redness. The locally raised temperature reduces the effectiveness of pathogen reproduction in the area.

Fever – causes the hypothalamus to reset to a higher body temperature which will combat infection – reduce the ability of many pathogens to reproduce effectively and also the specific immune response works better at higher temperatures.


Lysosome action:

An enzyme found in tears, sweat, saliva and nasal secretions, it destroys bacteria by breaking down the bacterial cell walls – causing lysis. Lysozymes protect the body from harmful bacteria in the air and food.



A chemical released from cells which inhibits/stops protein synthesis in viruses – prevents the virus from multiplying.



White blood cells engulf, digest and destroy bacteria and foreign material which is too large to diffuse in the form of vesicles released from the cell-surface membrane.

These phagocytes include neutrophils (most abundant, first to arrive, leave blood capillaries by squeezing between capillary walls, manufactured in bone marrow, short lived), lymphocytes and monocytes which become macrophages (circulate in blood for a few days before they move into the tissue, manufactures in bone marrow, settle in lymph nodes).

  • Phagocytes attach themselves to the surface of the pathogen by recognising their antigens
  • Pathogen engulfed by phagocytes by vesicles (cytoplasm moves around the pathogen) – forms a phagosome
  • Enzymes in lysosomes join the phagosome and release their contents (fusion)
  • The enzymes digest and break down the pathogen
  • Discharge of waste materials/indigestible material
  • The phagocyte presents the pathogen’s antigens. It sticks the antigens on the surface to activate other immune system cells


The immune system is the specific response of the body to invasion by pathogens. It enables the body to recognise anything that is non-self and remove it from the body efficiently.

The immune system has 4 key characteristics:

  • It can distinguish ‘self’ from ‘non-self’
  • It is specific – responds to specific foreign cells
  • It is diverse – recognise ~10 million different antigens
  • It has immunological memory – once responded to a pathogen, you can respond rapidly if you meet it again
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Pathogens enter through areas not covered by skin – nose, mouth, eyes, gas exchange surfaces, gastrointestinal tract and genital tract and through wounds.

  • Eyes, nose, mouth: Tears contain the enzyme lysozyme which helps to digest microbes – breaks down bateria cell walls
  • Skin: A tough, physical barrier containing keratin which is strong and impermeable – hard to penetrate
  • Skin flora: The skin has its own microbes which out-compete pathogens – reduce colonisation by other bacteria. Sebum is an oily fluid made by the skin and can also kill microbes
  • Earwax: Bacteriocidal
  • Respiratory tract: Contain mucus and cilia which traps bacteria, preventing the entry of pathogens into the lungs – transported up the trachea to be swallowed into the stomach
  • Vaginal secretions: Acidic
  • Stomach: Contains hydrochloric acid which protects against microbes and kills bacteria – the low pH denatures enzymes of most pathogens
  • Blood clotting: Seals wounds to prevent entry of pathogens
  • Intestines: Harmless bacteria out-compete pathogens



Non specific:

  • Do not distinguish between pathogens but respond to all in the same way
  • Act immediately as a barrier or through phagocytosis


  • Distinguish between different pathogens
  • Less rapid but long lasting
  • Involve lymphocytes – cell-mediated (T cells) or humoral (B cells)
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Infectious diseases are mostly caused by bacteria and viruses.


  • Useful bacteria are found on the skin and in the digestive tract
  • Prokaryotic cellular structure, single celled organelles
  • Prokaryotic cells store carbon compounds in the form of glycogen and lipids.
  • Ribosomes – site of protein synthesis, smaller than in eukaryotic cells, occur free in the cytoplasm
  • DNA is circular not chromosome form, attached to cell membrane at least at one point
  • Plasmid – small circles of DNA containing non-essential genes. Can be exchanged between different bacterial cells
  • Cell membrane – made of phospholipids and proteins
  • Mesosome – tightly-folded (infolded) region of the cell membrane containing proteins required for respiration and photosynthesis
  • Cell wall – made of protein/peptidoglycan, does not contain celluluse
  • Have cell surface membrane, cytoplasm, cell wall, ribosomes, plasmids, sometimes mesosomes, flagellum and pili
  • Pili – protein tubes that allow bacteria to attach to surfaces
  • Flagellum – used for cell movement
  • Capsule – a mucus layer for protection and allow bacteria to form colonies
  • They can live independently

Gram stain – used to colour the bacterial cell wall for identification:

Gram positive bacteria – have a thick cell wall and stain purple. Teichoic acid bind to crystal violet.

Gram negative bacteria – have a thin cell wall with an outer lipid layer and stain pink. Don’t have teichoic acid, crystal violet is decolourised, replaced by safranine


Bacterial reproduction

Binary fission – replication of DNA, replication of plasmids, cytoplasm and cell wall splits into two

Generation time = time between divisions of bacteria

Transformation – many can acquire new genes by taking up DNA molecules from their surroundings

Transduction – bacterial DNA is moved between bacteria by a virus

Conjugation – some bacteria can transfer their chromosome to a recipient bacteria



  • Smallest living organism
  • No cell wall, cell surface membrane, organelles or cytoplasm
  • Obligate parasites – cannot reproduce without a host. Invade other cells and take over their biochemistry to make more viruses
  • Nucleic acid core (DNA or RNA), enclosed in a protein coat (capsid) – made of protein subunits called capsomeres
  • Some have lipid rich covering around capsid called the envelope which is usually formed from host cell membrane and may have spikes to help recognise and attach to the host cell. They contain glycoproteins from the virus – these are antigens, recognised by the immune system as non self
  • Retroviruses are viruses with RNA as their genetic material
  • Slime layer (capsule) – polysaccharide layer outside of the cell wall. Used for sticking cells together, food reserve, protection
  • Pathogenic bacteria – cause infectious diseases e.g. TB, cholera
  • Wide range of shapes and sizes
  • After reproducing inside the host cells, new virus particles may bud from the cell surface or burst out of the cell – killing the cell – lysis



  • Caused by the bacterium mycobacterium Tuberculosis
  • Caused by inhalation – droplet infection. Close contact, poor health, poor diet and overcrowded living conditions increase the risk
  • Treatment: prolonged drug treatment (3-9 months), at least 3 antibiotics used. Bacteria destroyed before they cause damage. Rest, healthy diet, observation therapy
  • Prevention: improve living standards, treat cattle diseases, protective clothing
  • There are two phases of the disease:
  • Primary infection – may have no symptoms. immune system responds through an inflammatory response. In a healthy person macrophages engulf the bacteria but tubercules form. TB can survive inside macrophages, resisting killing mechanisms by having a thick waxy cell wall which makes them difficult to break and hides their antigens from other macrophages. They can be dormant for years and suppress T cells, reducing antibody production and attach killer cells.
  • Secondary infection – If the immune system can’t contain the diseases or an infection may break out if the immune system is weak (old age, malnutrition or poor living conditions)


  • Affects the lungs – Tuburcules – a mass of tissue formed in the lung as a result of inflammatory response
  • infection occurs through inhaling contaminated air or drinking infected milk
  • primary infection – early stage of infection
  • development of tubercle
  • bacteria are destroyed by WBC and the tissue heals
  • some bacteria produce thick waxy layer around them and survive
  • they remain dormant but if the person’s immune system is weakened these bacteria become active and divide
  • this leads to active TB – lung tissue is slowly destroyed by the bacteria causing symptoms of: breathing problems, coughing up blood in sputum, suppression of immunity, weight loss, poor appetite, fever and sweats
  • the TB bacteria also targets cells of the immune system so the patient cannot fight other infections
  • HIV causes AIDS which directly targets white blood cells, reducing a patient’s ability to fight infection
  • The initial symptoms include fever, general weakness and severe coughing, caused by inflammation of the lungs. As TB progresses, it damages the lungs and can lead to respiratory failure and death. TB can spread from the lungs to other parts of the body and can lead to organ failure.


90% of those infected with M. tuberculosis never develop TB because:

  • They are immune/resistant/vaccinated
  • Have antibodies/memory cells
  • Bacteria destroyed before they cause damage



  • Causes AIDS
  • Symptoms of aids – fever, diarrhoea, weight loss, secondary infections such as TB/pneumonia , fatigue,
  • 3 – 12 months after infection, HIV antibodies appear in the blood so the person is HIV positive
  • AIDS usually develops after 8 – 10 years
  • Transmission – sexual contact, infected blood (drug users sharing needles/repeated use of needles) or from mother to foetus (early stages of pregnancy, during birth or breastfeeding)
  • Prevention – condoms, clean needles and awareness programmes


HIV replication:

  • HIV attaches to surface of macrophage (host cell)
  • Gp120 protein (virus surface) and CD4 protein (host cell membrane)
  • The enzyme reverse transcriptase makes a DNA copy of the viral RNA
  • DNA copy is replicated by the same enzyme
  • This new DNA moves into the nucleus and integrated into the host cell chromosome by the enzyme integrase
  • Using the host cell machinery, mRNA is synthesised from the new proviral RNA
  • Viral mRNA is translated to make viral enzymes and structural proteins
  • Viral RNA genome is also made from proviral DNA
  • The viral genome and structural proteins assemble to form the basic structure of the virus
  • These move out of the host cell by exocytosis, taking part of the cell membrane with it (lipid layer around the virus)
  • Eventually the gene that codes for gp120 protein is mutated
  • The new protein attaches to a different CD4 protein present in T cells
  • The same cycle repeats in T cells
  • But as the virus leaves the T cell, it destroys the cell membrane killing the host cell – this reduces the number of T cells in the body reducing immunity


Difficult to create a vaccine for AIDS because:

  • virus mutates rapidly
  • therefore antigens on the viral surface continually change
  • working on animals to develop vaccines not possible because HIV only infects humans
  • the virus hides itself for years inside macrophages, therefore most may not work properly


  1. Explain why unbroken skin is an effective barrier against HIV infection:
  • keratin/protein in skin surface/epidermis
  • forms a hard/impenetrable/physical barrier


  1. Explain the change in numbers of CD4 T-lymphocytes during the first 6 weeks after infection with HIV:
  • glycoprotein/gp120 on virus
  • binds with receptors/CD4
  • on surface membrane of lymphocytes
  • viral RNA enters the lymphocyte
  • viral RNA used to produce viral DNA in lymphocyte
  • by action of reverse transcriptase
  • formation of new viruses
  • lymphocyte destroyed when new viruses bud out of/leave cell
  • T killer cells/lymphocytes destroy T helper cells/lymphocytes


  1. Suggest one effect that this change would have on one other component of the infected blood:
  • B cells not activated/are inhibited
  • Fewer antibodies/T killer cells increase


Evolutionary race – TB and HIV

The bacterium which causes TB and the virus which causes AIDS have evolved features to help them evade the immune system.

  • TB – a thick waxy coat is produced which protects it from the enzymes of the macrophages
  • TB – when engulfed by phagocytes, they produce substances that prevent the lysosome fusing with the vacuole – aren’t broken down and so can multiply undetected in phagocytes
  • Disrupts antigen presentation in infected cells – preventing immune system from recognising and killing infected phagocytes
  • HIV – the protein coat is constantly changing – the immune system cannot target and destroy it
  • HIV – reduces the number of immune cells in the body – reduces the change of HIV being detected
  • HIV – high rate of mutation in the genes which code for antigen proteins – change the structure of antigens – antigenic variation. The memory cells won’t recognise other strains with different antigens – has to produce a primary response for each new strain


The carbohydrate present in storage granules is glycogen.

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Investigating time of death:

As soon as a person dies, chemical changes occur in order, so can be used to estimate the time of death.


Rigor mortis/degree of muscle contraction:

·         Begins when muscle cells become deprived of oxygen

·         Respiration is anaerobic, causing the build up of lactic acid in muscles

·         The pH decreases due to the lactic acid, inhibiting enzymes to produce ATP

·         ATP is required to keep muscles relaxed, so no ATP means the bonds become fixed, the muscles contract so the body stiffens

·         Smaller muscles contract first

·         Begins about 2 – 4 hours after death, full effect is about 6 – 8 hours. It passes at about 36 – 48 hours after death

·         The bodies stages in rigor mortis can give a rough outline of time of death


Body temperature:

·         Metabolic reactions slow down so the core body temperature cools slowly from around 37oC to room temperature over time.

·         If the temperature of the room is known, it is possible to create a cooling curve so discover the time of death

·         The body reaches room temperature after about 18 hours

·         Body temperature can be influenced by – body size, body position, clothing, air movement, humidity


Extent of decomposition

After death, tissues break down due to the action of enzymes.

Autolysis occurs – the loss of oxygen in tissues favours the growth of anaerobic bacteria.

Bodies usually follow a standard pattern of decay. Enzymes in the gut start to break down the wall of the gut and the surrounding area. As cells die, they release enzymes which help break down tissues.

The signs of decomposition such as decolouration of the skin (putrefication) and gas formation, combined with environmental conditions allow time of death to be estimated.

The warmer the environment, the faster the decay

Injuries also allow bacteria to enter, aiding decomposition

·         After a few hours – cells and tissues are broken down by enzymes and bacteria present before death – turns skin green

·         Few days-weeks – microorganisms decompose tissues and organs – produces gasses which cause bloating. Skin blisters and falls off

·         Few weeks – tissues liquefy and seep into the area around the body

·         Few months – years – skeleton remains

·         Decades – centuries – skeleton disintegrates – nothing left



Forensic entomology:

2 main ways – succession and life cycles of insects


As a body decays, the populations of insects on it change – there is a succession in species. The community of species present when the body is found allows the stage of succession to be determined and the time of death estimated.

·         A dead body is a newly exposed habitat

·         Anaerobic bacteria thrive in the no oxygen and acidic (lactic acid) conditions

·         Certain flies, such as blowflies arrive, they are attracted to the moisture, smell and open wounds. They lay eggs on the carcass

·         The eggs hatch, maggots eat the skin and tissue of the body, this liquidises certain parts which the adult flies feed on

·         Beetles are attracted, they lay eggs and the grub that hatch eat the maggots

·         Parasitic wasps lay eggs in the beetle and fly larvae

·         Eventually the body dries out and species such as cheese and coffin flies are abundant

·         Dehydration continues, maggots can’t survive. Beetles with strong mandibles, such as carcass beetles move in and eat the remaining muscles and connecting tissues

·         Finally mites and moth larvae digest the hair

Forensic entomologists see what species are living on the body, therefore know how long down the line of succession of insects the body is – using this they can estimate the time of death.

The season, weather, size and location of the body will influence the type and number of species present.


Insect lifecycles:

·         Insects go from eggs, which hatch to larvae, turn into a pupa and back into an adult

·         When a forensic entomologist finds a body, they collect the eggs and larvae and pupa and let them grow into adults

·         From the stage the insect was the found and the fact that insects have different life cycles for each stage, the entomologist can tell how long the insects have been there. This linked with succession can give a more accurate time of death

·         Egg (1 day) à larva (9 days) à pupa ( 6 – 12 days)

Using forensic entomology, the date of death can be confirmed to a few days or theorised to a few months, but is mostly used for bodies 4 – 14 days old.


Genetic identification

Polymerase Chain Reaction (PCR):

Allows small samples of DNA to be amplified for use during DNA profiling, meaning that forensics only need small DNA samples such as a strand of hair.

A cycle of temperature changes results in huge numbers of DNA fragments being produced.

1)      Place a mixture of enzymes, primers, DNA and reactants in a vial

2)      Place vial in a PCR machine/thermal cylinder

3)      Heat to 90-95oC for about 30 seconds, this separates strands of DNA

4)      Heat to 50 – 60oC for about 20 seconds, this binds/anneals primers (short DNA sequences complementary to DNA adjacent to STR) to DNA strands. The DNA primers are marked with a fluorescent tag.

5)      Heat to 75oC for at least a minute, DNA builds complementary DNA strand

6)      Repeat steps 3 – 5 as much as wanted

Each PCR cycle doubles the amount of DNA present

DNA profiling/DNA fingerprinting:

·         To identify genetic information

·         Everyone’s DNA is unique because of the variety of the DNA sections not used to code for proteins (introns)

·         Look for short repeated sequences in introns (short tandem repeats)

·         Mini satellites contain 20 – 50 base pairs

·         Micro satellites contain 2 – 4 base pairs

·         Can be several hundred copies of STR at a single locul – people vary

·         The STR at many loci build up a unique pattern for that individual

Double stranded DNA and restriction enzymes.

1)      DNA is cut into fragments of double stranded DNA using restriction enzymes (restriction endonucleases). They carry a negative charge

2)      Fragments of double stranded DNA are loaded into the walls of an agarose gel in a tank – gel electrophoresis and is submerged in a buffer solution

3)      Negatively charged DNA moves towards the positive electrode. These fragments separate into individual bands

4)      Fragments move at different rates according to their size and charge. Small fragments with fewer satellite repeats travel faster (pass through gel quicker) and end up closer to the electrode after a set time

5)      DNA is transferred to the nylon membrane using southern blotting

6)      The membrane is washed with a buffer solution and labelled DNA probe which binds to the repeated sequence producing visible bands. Single stranded DNA probe binds to fragments with a complimentary sequence

7)      Shown by X ray film or UV light

8)      Only those that bound to the probe show up – this resulting image is called a DNA fingerprint

9)      The DNA profile is then compared to a reference, e.g. a suspect or relative



Fingerprints can be taken, which are unique to an individual.

Fingerprints are taken using fine aluminium powders.

Once obtained, the main type of fingerprint can be classified – an arch, tented arch, whorl or a loop. Arch patterns are rare, the loop is most common.


Dental records

Can be used if someone has no fingerprints on file or on the body.

Teeth and fillings decay slowly, they are resistant to burning. A forensic dentist makes a chart of the teeth, including dental work, fillings and missing teeth. This is compared to dental records. The forensic dentist may also look at the development of the teeth to determine the age.


Summary of forensics

·         When: rigor mortis, decomposition, forensic entomology, temperature

·         Who: personal ID, finger prints, dental records, DNA profiling

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