Topic 6, SNAB pastpaper questions and markscheme

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Topic 6: Immunity, Infections and Forensics
DNA Profiling
Polymerase is the.... enzyme used in the Polymerase Chain Reaction to amplify DNA in a small
sample of blood taken from a crime scene.
Gel Electrophoresis is the .....process used to separate DNA fragments to create a DNA profile
Describe how gel electrophoresis can be used to analyse DNA. (3)
*First, a DNA sample will be collected from blood, saliva or semen etc; *these small samples of DNA
can then be amplified by PCR. DNA profiling then takes place which uses *restriction enzymes to
break the DNA and then *uses electro potential difference, with the DNA in a gel, to draw the bands
apart. *The DNA is stained so it can be seen and will *show up as bands/bars. *The number of bands
that match indicates the similarity of the DNA.
Name substances X, Y and Z:
Substance X ...........DNA Primers
Substance Y......... (mono)nucleotides
Substance Z ..........DNA Strands
What are the temperatures for?
T1: Heated to 90­95 °C
T2: Cooled to 55­60 °C
T3: Heated to 75 °C
Using DNA profiling explain how a suspect is found guilty. (5)
A DNA match is needed, this means that *all of the bands in the sample are the same as the ones
shown in the evidence sample. *DNA profiling assumes every individual's DNA is unique/different;
*apart from identical twins. *DNA profiling analyses the introns/noncoding blocks/STR parts of DNA
as the *non-coding areas are hypervariable because *there are a large number of
introns/non-coding blocks and so there can be *many combinations of STRs at each locus.
Suggest how DNA profiling could be useful to scientists who examine fossils of animals and
plants. (2)
*Comparisons could be made between DNA from fossils and other organisms *to find genetic
relationships/how closely related they are. It may also be *used in taxonomy/classification *to
understand evolutionary lines/to determine a common ancestor.

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Explain how the results of DNA profiling of tissue samples from the two sub-species could be
used to provide evidence that they share common ancestry. (3)
DNA profiling will *produce bands that will have spread to *certain positions. *Common/similar bands
will contain similar DNA fragments; *the more similar these patterns, the closer the
relationship/more likely the sub-species will have a recent common ancestor. *There will still be very
few differences between the DNA of sub-species.
Suggest how DNA analysis could give further evidence for evolution.…read more

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Describe the role of microorganisms in the recycling of the carbon from compounds in a dead
animal. (3)
Decomposition/putrefaction occurs by microorganisms, they may digest the carbon in the animal and
then release the carbon into the atmosphere by *respiration where the *carbon dioxide is used for
photosynthesis. *Methane is released in anaerobic conditions and is *available as fuel.
Suggest three factors that could influence the rate at which a body cools after death.…read more

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The polypeptide chain would be synthesised on the *ribosomes attached to the membrane of the
rough endoplasmic reticulum. *The ribosomes consist of rRNA and are a protein component of two
sub-units/ a large and small sub-unit.
Explain why a molecule of DNA can be described as a double-stranded polynucleotide. (3)
*It is double-stranded because it is made of two strands *joined by hydrogen bonds between the
bases and it is *a polynucleotide of many nucleotides that are *linked by phosphodiester bonds.…read more

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RNA is synthesised).*Post-transcriptional modification of mRNA then occurs where
introns (non-coding regions of DNA) are removed by splicing using the enzyme spliceosome, leaving
only exons behind in the mRNA to join back together. After modification the *mRNA moves from
nucleus to the cytoplasm through a pore in the nuclear membrane, so that *translation may occur on
the ribosomes found attached to the rER (ribosomes aid codon-anticodon interaction).…read more

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When MRSA enters the blood it can stimulate the production of several different clones of
plasma cells. These produce a variety of antibodies (polyclonal antibodies). Suggest an
explanation for this. (4)
Because the *bacterium is made of many different polymers/chemicals *which can act as different
antigens, *individual B-lymphocytes will recognise specific antigens/antibodies are specific and so
only certain *B-lymphocytes are activated by T-lymphocytes. These cells of *B-lymphocytes will then
divide by mitosis *to form genetically identical plasma cells that secrete specific antibodies.…read more

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An antibiotic is used to control/kill/prevent reproduction of bacteria. *Bacteriostatic antibiotics will
prevent the reproduction/division/multiplication/growth of bacteria whereas *bactericidal
antibiotics will destroy/kill the bacteria.
Suggest why antibiotics may be used as part of the treatment for influenza. (2)
Because *influenza may allow the development of other diseases e.g. opportunistic infections and
*the antibiotics will kill/inhibit growth of these bacteria.
Suggest why health authorities in the USA are encouraging the reduction in the number of
prescriptions of antibiotics.…read more

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A placebo has no effect on the bacteria *so there will be more bacteria, and *thus more
lymphocytes due to *clonal expansion.
Vaccinations and prevention.
Suggest how
scientists may be able to develop a means of producing active immunity to HIV infection using
synthetic HIV antigens. (5)
*This will be a method of *artificial (active) immunity, perhaps by using a *vaccine/vaccination
*containing the synthetic molecule/(synthetic) antigen/(synthetic) glycoprotein.
*A specific/humoral immune response to the synthetic antigen will be stimulated, i.e.…read more

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State how the genetic material in HIV differs from the genetic material in the Mycobacterium
tuberculosis that causes TB. (2)
*RNA is found in HIV/ virus and DNA in the bacterium/TB, *the nucleic acid in the bacterium is circular
whereas it is linear in HIV. *There are also plasmids in bacterium and no plasmids in HIV.
One of the ways in which HIV may enter the blood is through the use of infected needles.
Explain why unbroken skin is an effective barrier against HIV infection.…read more

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Because *HIV has many varieties of strains/antigens/protein coats, *some of these strains are/will
become resistant to a specific/particular drug and so *would survive if only one/the same drug was
used. *A mixture of drugs has more chance of getting rid of all/ more strains.
*A concoction of drugs are used together because viruses have a rapid rate of mutation and a *rapid
rate of multiplication.
Suggest why data about HIV infections are often estimates.…read more


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