Download Topic 6 Checkpoint Answers File

Checkpoint answers for topic 6
Q6.1 Draw up a flow chart that outlines the steps required in DNA
amplification and separation to produce a DNA profile.
DNA sample obtained
DNA fragments created either using restriction enzymes or the polymerase
chain reaction. The fragments contain short tandem repeated sequences.
Restriction enzymes cut the DNA at specific base sequences on either side of
the STR sequence. PCR fragments are created by the binding of primers
(DNA complementary to the DNA adjacent to the STR sequence); the action
of DNA polymerase and a cycle of temperature changes results in the copying
of the STR sequence.
Fragments of DNA separated according to their size using gel
electrophoresis. Smaller fragment pass more quickly through the gel towards
the positive electrode.
If restriction enzymes have been used to create the fragments these double
stranded fragments are digested to form single strands and transferred to a
nitrocellulose or nylon membrane by Southern blotting. The membrane is
incubated with a radioactive or fluorescent probe, the probes DNA sequence
attaches to the DNA fragment. The probes position on the membrane can be
visualised using either an X-ray film or ultraviolet light. If PCR has been used
to create the fragments, the primers have fluorescent tags which can be
detected using a laser scanner, this is usually highly automated.
Q6.2 Produce a bullet-point summary of methods that can be used to
determine time of death.

Body temperature: human core body temperature is normally in the
range 36.2–37.6 °C, but almost as soon as a person dies their body
starts to cool. Using a cooling curve the temperature of the body can
be used to estimate the time of death. This assumes that the person’s
temperature was normal, 37 °C, at the time of death. The estimate
may be somewhat imprecise due to the many factors that can affect
post-mortem cooling, such as body size, body position, clothing, air
movement and humidity.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 1 of 8

Rigor mortis: after death, muscles usually totally relax and then
stiffen; this stiffening is known as rigor mortis. There is a progression in
the development of rigor mortis, with smaller muscles stiffening before
larger ones. The rigor mortis eventually passes in the same order in
which it developed as the muscles start to break down. Most bodies will
have complete rigor mortis between 6 and 9 hours after death and the
degree of rigor mortis gives an estimate of the length of time since
death. However, environmental factors can affect how quickly rigor
mortis develops and so the estimate is not likely to be very precise.

Decomposition: the degree of decomposition due to the breakdown of
tissues by autolysis can be used to estimate time of death. There is a
sequence of characteristic changes that the body goes through during
decomposition. The rate of these changes will be affected by the
external conditions of the body but if these are known it is possible to
estimate for how long the body has been decomposing.

Forensic entomology: the presence of insects allows a forensic
entomologist to make a more precise estimate of how much time has
elapsed since death. If maggots are found on the body, their stage of
development gives an estimate of how long ago the person may have
died assuming that the eggs were laid on the body soon after death. If
there is more than one species of insect present on the body it is
possible to determine the stage of succession that the body has
reached. The number of species present increases as succession
progresses. The length of each stage in the succession depends on
the condition of the body, which in turn depends on environmental
conditions.
Q6.3 Draw up a table of comparison to allow you to distinguish between the
structure of bacteria and that of viruses.
Feature
Bacteria
Viruses
Type of cell
Prokaryotic cell
Size
0.5–5 µm in diameter
Structure
Contain cytoplasm, a
long circular strand of
DNA, plasmids, cell
surface membrane, free
ribosomes, and have a
rigid cell wall containing
polysaccharide murein.
May have mesosomes
(infoldings of the cell
Viruses are small
organic particles with a
much simpler structure
than bacteria. No
proper cell structure.
20–400 nm in length; a
wide range of sizes
and shapes.
A strand of nucleic acid
(RNA or DNA) enclosed
within a protein coat.
May have an outer
envelope taken from the
host cell surface
membrane but
containing glycoproteins
from the virus.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 2 of 8
Reproduction
surface membrane
where respiration
occurs), flagella and pili.
Some have a capsule
outside the
cell wall.
Reproduce asexually by
binary fission; after
replication of the DNA
they divide into identical
daughter cells. No
sexual reproduction
occurs in animals, but
cell-to-cell contact
through conjugation.
Enter the cells of the
organisms they infect
(the host) and use the
host’s metabolic
systems to make more
viruses. The virus’s
genetic material is
replicated and then the
protein coat
synthesised. Newly
formed virus particles
are released by budding
from the cell surface or
lysis of the cell.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 3 of 8
Q6.4 Produce a flowchart showing the sequence of events that occurs in the
non-specific response at the site of a cut.
Damaged basophil white blood cells and mast
cells release histamine.
Arterioles in the area dilate and venules
constrict due to the presence of histamine.
Blood flow in the capillaries at the infected site increases. The site of the cut
becomes red. It also becomes warm due to increased metabolic activity.
Capillaries become leaky as cells in the
capillary walls separate slightly.
Plasma fluid, white blood cells and antibodies leak from the
blood into the tissue causing oedema (swelling).
Neutrophils, a type of white blood cell, engulf bacteria. Each neutrophil
engulfs between 5 and 20 bacteria before becoming inactive and dying.
Macrophages, another type of white blood cell, engulf
bacteria, debris from damaged cells and foreign matter.
Pus collects at the site of the cut; this thick white liquid
is made up of dead white blood cells.
The pus gradually breaks down and is
absorbed into the surrounding tissues.
If viruses have infected cells at the side of the cut, the cells produce the
protein interferon. This diffuses to the surrounding cells where it prevents
viruses from multiplying by inhibiting viral protein synthesis.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 4 of 8
Q6.5 Sketch a diagram or use downloaded figures from the mediabank book artwork to show
how Figures 6.36, 6.38A, 6.38B and 6.39 are interconnected.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 5 of 8
Q6.6 Produce a flowchart which summarises the course of disease for TB and for AIDS.
TB
Tuberculosis (TB) is a contagious disease caused by
the bacterium Mycobacterium tuberculosis. Respiratory
or pulmonary TB is the most common form.
Infection may occur when M. tuberculosis bacteria are
inhaled and lodge in the lungs. Here they start to
multiply.
The first phase (primary infection) can last for several
months; it may have no symptoms.
An inflammatory response by the host’s immune
system occurs. Macrophages engulf the bacteria.
Anaerobic tissue masses known as a granuloma or
tubercules form in response to infection. They contain
dead bacteria and macrophages at their centre.
After 3–8 weeks, the infection is controlled and the
infected region of the lung heals.
Some M. tuberculosis bacteria may survive inside macrophages. The
bacteria have very thick waxy cell walls, making destruction inside the
macrophages very difficult. The bacteria can lie dormant for years.
The second phase (active tuberculosis) occurs if there are too many
bacteria for the immune response to deal with, or if an old infection breaks
out because the immune system is not working properly.
The bacteria multiply rapidly and destroy the lung
tissue, creating holes or cavities.
The lung damage will eventually kill the sufferer if they
are not treated with an appropriate antibiotic.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 6 of 8
AIDS
AIDS, acquired immune deficiency syndrome, is caused by infection with the human immunodeficiency virus,
HIV.
HIV infection occurs when the body fluid (blood, vaginal secretions and semen, but not saliva
or urine) of an infected person is transferred directly into the body of an uninfected person.
This can occur
through
unprotected sex.
This can occur when
sharing needles,
whether used illegally
or legally.
This can occur with
direct blood-to-blood
transfer through cuts
and grazes.
This can occur from
mother to child across
the placenta or in
breast milk.
HIV invades T helper cells and macrophages. The HIV gp120 molecules attach to their CD4 receptors allowing
the virus envelope to fuse with the host cell surface membrane, enabling the viral RNA to enter the cell.
Once inside, the virus uses reverse transcriptase to produce DNA from its RNA. The DNA is integrated into the
host’s DNA by another HIV enzyme, integrase. The viral DNA is transcribed and translated to produce new
viral proteins and assemble new viruses.
The new virus particles bud out of the T cell, taking some of the surface membrane with them as their
envelope, and killing the cell as they leave.
When a person is first infected by HIV, there is an acute phase of infection. There is rapid replication of the
virus and loss of T helper cells.
As the number of viruses increases, the number of host T helper cells decreases. Macrophages, B cells and
T killer cells are not activated and the infected person’s immune system becomes deficient.
The infected person may experience symptoms such as fever, sweats, headache, sore throat and swollen
lymph nodes, or they may have no symptoms.
The virus continues to reproduce rapidly, but the numbers are kept in check by the immune system.
T killer cells recognise the infected T helper cells and destroy them.
There may be no symptoms during this chronic phase, but there can be an increasing tendency to suffer
various infections which are slow to go away. Dormant diseases such as TB and shingles can reactivate.
The chronic phase can last for years, especially if combined with drug treatment.
An increased number of viruses in circulation (viral load) and a declining number of T helper cells indicate
the onset of AIDS, the disease phase.
The weakened immune system makes the patient more prone to opportunistic infections such as
pneumonia and TB. There may also be significant weight loss, dementia (memory and intellect loss) and the
cancer Kaposi’s sarcoma. AIDS is usually fatal.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 7 of 8
Q6.7 Write a definition for each of the four types of immunity: passive natural immunity,
active natural immunity, active artificial immunity and passive artificial immunity.
Passive natural immunity occurs when antibodies pass from a mother to baby either
across the placenta before birth, or via breast milk after birth. This helps protect the baby
for a short time.
Active immunity occurs when the body makes its own antibodies in response to an
antigen. Active natural immunity develops following an infection. The ‘specific immune
response’ to the foreign antigens produces a supply of antibodies and B memory and T
memory cells that will respond quickly if the body is reinfected with the same pathogen.
Active artificial immunity develops following immunisation. Antigens in the vaccine
trigger a ‘specific immune response’ by the body’s B and T cells and antibodies are
produced. Vaccines usually contain weakened (attenuated) forms of the virus or
bacterium. These stimulate the immune system to produce the antibodies but do not cause
the disease. For some infections dead pathogens or antigen fragments
may be used to trigger the response.
Passive artificial immunity develops when a person is given ready-made antibodies in a
medical procedure. This method provides immediate protection in emergency situations
but is short lived. The protein antibodies are broken down after a few days or weeks.
Salters-Nuffield Advanced Biology, Pearson Education Ltd 2009. ©University of York Science Education Group.
This sheet may have been altered from the original.
Page 8 of 8