NATIONAL QUALIFICATIONS CURRICULUM SUPPORT
Biology
Unit 2, Part 3: Metabolism in
Microorganisms
Teacher’s Notes
[HIGHER]
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Contents
(b)
Genetic control of metabolism
(i)
Genetic variation
4
4
(b)
Genetic control of metabolism
(ii) Recombinant DNA technology
7
7
(c)
Ethical considerations in the use of microorganisms
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Investigating metabolism in microorganisms
(b) Genetic control of metabolism
(i)
Genetic variation
Links to prior/prerequisite knowledge
SCN 3-14b
SCN 4-14c
Students have already studied mutations in Unit 1 (see Section 3, The Genome)
and so should be aware that physical changes to the DNA of a cell or a change in
the number of chromosomes can arise naturally. They should also be familiar
with the concept of improvement through mutat ion since polyploidy crops are
discussed in Unit 1.
New content areas
 Examples of mutagenic agents and their effect on genetic material.
 Transfer of DNA between bacteria, uptake of DNA by bacteria from their
environment.
 Production of new genotypes by sexual reproduction between existing strains
of fungi and yeast.
Background information
 Mutations may arise naturally by physical change s to the DNA of a cell or a
change in the number of copies of an entire gene or chromosome. When such
a change in genotype produces a change in phenotype, the organism affected
is called a mutant. In natural conditions, mutations arise spontaneously and at
random.
 While they occur rarely, the frequency of mutation can be increased by
exposure to mutagenic agents such as mustard gas and various types of
radiation. Exposure to natural mutagens such as ultraviolet (UV) light, to
industrial or environmental mutagens such as benzene or asbestos can all
cause mutations. For geneticists, the study of mutagenesis is important
because mutants reveal the genetic mechanisms underlying heredity and gene
expression.
 Genetic transformation is the uptake of DNA from the environment. The cell
is genetically altered as a result of direct uptake, incorporation and expression
of DNA from its surroundings. Transformation occurs most commonly in
bacteria and in some species occurs naturally. Bacteria capable of being
transformed are said to be competent. Transformation is thought to be a
significant cause of increased drug resistance when one bacterial cell acquires
resistance and quickly transfers the resistance genes to many other cells. The
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main method of cell-to-cell transfer is conjugation. Bacterial conjugation is
the transfer of genetic material between bacterial cells by direct cell -to-cell
contact or by a bridge-like connection between two cells. During conjugation
the donor cell provides a genetic element that is most often a plasmid. The
plasmid transferred is often beneficial to the recipient. Benefits may include
antibiotic resistance.
 A fungus is a member of a large group of eukaryotic organisms that includes
microorganisms such as yeasts and moulds. Although many species of fungi
and yeasts reproduce asexually, many also carry out sexual reproduction and
therefore have the ability to increase genetic variation. During this process,
meiosis forms genetically varied spores that are then released from the fungi
by specialised mechanical or physiological mechanisms.
Identification of key concepts
 Mutations are rare but their incidence can be increased by exposure to
mutagenic agents.
 Mutagenic agents can be chemical, such as asbestos or mustard gas, physical ,
such as several forms of radiation, or biological, such as bacteria phage.
 Mutations may be of benefit to the species or may introdu ce characteristics of
commercial value.
 Recombinant DNA technology allows deliberate alteration of a genome. This
may involve the addition, modification or deletion of one or more genes in a
cell. As a result the cell may receive an additional property, fo r example the
ability to make a new protein.
 Some species of bacteria are able to carry out transformation that involves
uptake of DNA from their environment or another cell. The most common
form of transformation is called conjugation and involves direct contact
between two cells. DNA can then be transferred from a donor cell to a
recipient cell.
 Many species of fungi are able to reproduce sexually and therefore increase
variation. This is most commonly achieved by the production of genetically
varied spores that are dispersed from the fungi and fuse with other sexual
spores.
Identification of particular areas of difficulty
The idea of sexual spores produced by fungi is probably a new concept and many
available sources of information on this topic are very advanced and involve
challenging vocabulary.
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Links to websites, animations, PowerPoints, audio or video files etc
http://www.bbc.co.uk/learningzone/clips/mutations-and-geneticdiseases/10653.html
http://www.microbiologyonline.org.uk/about -microbiology/introducingmicrobes/fungi
http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/animation6.html
http://www.microbeworld.org/index.php?option=com_content&view=article&id=
123&Itemid=118
http://www.bbc.co.uk/learningzone/clips/genetic-engineering-and-insulinproduction/4200.html
Co-operative Learning Activities 3 and 4
Other useful information to stimulate interest
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(b) Genetic control of metabolism
(ii)
Recombinant DNA technology
Links to prior/prerequisite knowledge
 Refer to Organisation of DNA in prokaryotes and eukaryotes in Unit 1 to
remind students about the existence of circular chromosomes.
 Students will probably be aware of the use of genetic engineering for the
production of insulin, factor VIII and human growth hormone, and should
therefore be familiar with the role of plasmids and the process at a basic
level.
New content areas
 Use of recombinant DNA technology to create enzymes, genetically modified
foods and pharmaceuticals and its future role in gene therapy.
 Examples of species commonly used for genetic engineering, such as E. coli,
and reasons for their suitability.
 When the gene for a protein is cloned, it is placed on a plasmid adjacent to a
region where the expression of genes can be controlled easily.
 Structure of plasmids/vectors.
 Use of endonucleases and ligase during the process of genetic engineering .
 Yeast as an alternative to bacteria.
Background information
 The field of genetic engineering involves the isolation, manipulation and
expression of genetic material. Genetic engineering is a rapidly growing
technology and it is thought that it will have profound effects on our
everyday lives. Some examples of how it may affect us are:
- In the field of medicine it may improve the diagnosis and cure of
hereditary defects and disease.
- It is being used for the development of new drugs and vaccines for use by
humans and animals.
- In agriculture it is being used to improve food production.
- It is being used to monitor and reduce environmental pollution.
 The process commonly utilises bacterial cells and their plasmids. Foreign
genes can be inserted into isolated plasmids , which are returned to the
bacterial cells. The cells reproduce, cloning the recombinant DNA as the
cells replicate their plasmids. Under suitable conditions , the bacterial culture
will produce the protein encoded by the foreign gene.
 Genetic engineering requires three biological ‘tools’.
1.
Enzymes to cut DNA: The first step in many genetic engineering
processes is the isolation of DNA from cells. When purified DNA has
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been obtained it is cut into smaller fragments using restriction
endonucleases. These enzymes were discovered in the late 1960s and
work by cutting up DNA. They recognise and cut short specific
sequences (between four and eight base pairs) within DNA. One of the
most commonly used restriction enzymes is called EcoR1. It recognises
the following six-base pair DNA sequence:
5′ GAATTC 3′
3′ CTTAAG 5′
EcoR1 then cuts the DNA sequence as follows:
5′ G
3′ CTTAA
AATTC 3′
G 5′
When EcoR1 cuts DNA it produces two double -stranded fragments, but
the cuts do not occur at the same position. Instead the cut is staggered
by four nucleotides, so that the DNA fragments have single -stranded
overhangs (known as sticky ends). If another piece of DNA is cut with
the same enzyme and so has the same sticky ends, the pieces of DNA
can be joined together by base pairing between the sticky ends. Genes
of interest and suitable vectors are treated with the same endonucleases
to create complementary sticky ends, which are then combined using
DNA ligase to form recombinant DNA.
2.
A vector or transfer agent such as a plasmid : Cloning vectors can be
manipulated so that they have the following characteristics:
(a) They can be cut with restriction enzymes and foreign DNA
sequences (cut with the same restriction enzymes) can be inserted
into them using an enzyme called DNA ligase.
(b) Antibiotic resistance marker genes can be added to them. These
genes code for proteins that break down antibiotics. If a cloning
vector is inserted into a microorganism, the microorganism gains
the antibiotic resistance gene and so is able to grow in the
presence of this antibiotic. The microorganism becomes resistant
to the antibiotic and can be easily identified.
(c) Some cloning vectors contain part of the lac operon. This is used
to control the expression of the foreign DNA sequences. The
foreign DNA is transcribed and translated only when the lac
operon is switched on.
After a foreign sequence of DNA has been inserted into a cloning
vector using DNA ligase, the cloning vector is mixed with the
microorganism into which it is to be transformed. Some of the
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microorganisms will take up the cloning vector, some will not. To
separate the transformed microorganism from those that are not, it is
grown in media containing the antibiotic to which the transformed
microorganism has acquired resistance. The transformed
microorganism has the cloning vector that has the antibiotic resistance
gene, so it is able to grow in the presence of the antibiotic.
Any microorganism that does not possess the cloning vector is unable
to grow in this medium.
The transformed microorganism is isolated from the medium and
transferred to another medium where it is allowed to reproduce and
grow in large quantities. Each new microorganism that is produced is
genetically identical to the original transformed microorganism. Each
genetically identical microorganism is called a clone. The process of
producing lots of genetically identical microorganisms is known as
cloning.
3.
An appropriate host cell for the recombinant DNA : Transformation is
the name used to describe the process when a foreign sequence of DNA
(such as a gene or cDNA) is introduced into microorganisms such as
bacteria and yeast. Two microorganisms that are commonly used in
transformations are the bacterium E. coli and the yeast S. cerevisiae.
Both microorganisms are single-celled organisms that have fast
reproduction rates and thus are quick growing. This makes them ideal
for large-scale production in industrial fermenters.
E. coli: This is a prokaryote that is often used as a recipient for foreign
DNA. Large sequences of foreign DNA can be inserted into E. coli
using a plasmid. The DNA is transcribed and translated, and it is
possible for the protein coded for by the foreign DNA to account for
60% of the total protein produced by the bacterial cell. E. coli is
relatively easy to transform. While there are many advantages of using
E. coli, there are some disadvantages – mainly due to the fact that it is
a prokaryote and the foreign protein produced may originally have
come from a eukaryote.
There are some disadvantages of E. coli. The foreign protein produced
is not always secreted easily from E. coli. This may be due to E. coli
not being able to carry out modifications to the protein after it is made,
for example addition of sugar groups. If the protein is not secreted by
the bacterium, it causes problems for the biotechnologist as E. coli
must be harvested, the bacterial cells broken open (lysed) and the
protein purified. This increases the production costs. E. coli does not
always fold the foreign protein into its natural three-dimensional shape.
This causes the protein to be inactive.
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S. cerevisiae: This is a eukaryote (yeast) that can be used instead of E.
coli as the recipient for foreign DNA. Since it is eukaryotic, it can fold
proteins into their three-dimensional shape, which allows the proteins
to be active. Foreign proteins made by S. cerevisiae are secreted from
the cell because S. cerevisiae can carry out post-translational
modifications (eg it can add sugar groups to proteins), which allows the
proteins to cross the cell wall. Thus proteins secreted by S. cerevisiae
can be extracted from the culture medium.
Identification of key concepts
 Recombinant DNA technology allows the transfer of plan t or animal gene
sequences to microorganisms to produce plant or animal proteins.
 Useful genes that that remove inhibitory controls or amplify specific
metabolic steps in a pathway can be introduced to increase yield.
 Restriction endonucleases cut target sequences of DNA , leaving sticky ends.
Treatment of vectors with the same restriction endonucl ease forms
complementary sticky ends.
 Ligase combines complementary sticky ends and seals foreign DNA into the
plasmid.
 Suitable microorganisms for transformation include bacteria such as E. coli
and yeast such as S. cerevisiae.
Identification of particular areas of difficulty
Clear visual aids should be used to illustrate the action of endonucleases and the
production of sticky ends as students may find the concept difficult to
understand. The web link below provides a narrated animation sequence.
Links to websites, animations, PowerPoints, audio or video files etc
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter16/animations. html#
(whole series of biotechnology animations).
Co-operative Learning Activities 1 and 5
Other useful information to stimulate interest
http://www.sciencedaily.com/news/plants_animals/genetically_modified/
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(c)
Ethical considerations in the use of microorganisms
Links to prior/prerequisite knowledge
Students will have already discussed the ethics of stem cell research and
sources of stem cells (refer to 2 (ii) Research and therapeutic value of stem
cells) and may have carried out the suggested case study related to this
outcome.
New content areas
 Consideration of the hazards involved in genetic engineering processes.
 The policies and practices in place to control risks associated with genetic
engineering.
Background information
 The earliest concerns around genetic engineering were that genetic
manipulations could create hazardous new pathogens, which might escape
from the laboratory. This led to the introductio n of formal guidelines
administered by agencies such as the Food and Drug Administration (FDA)
in the US and the Medicines and Healthcare products Regulatory Agency
(MHRA) in the UK. Today governments throughout the world grapple with
how to promote the potential benefits of genetic engineering while ensuring
that its products are safe.
 With new medical products the main cause for concern is the potential for
harmful side effects. Hundreds of new genetically engineered vaccines,
diagnostic kits and drugs await government approval. Before considered for
general marketing, each substance must pass exhaustive tests in laboratory
animals and humans.
 In the case of environmental problems, such as oil spills or chemical wastes
that threaten our soil, water and air, genetically engineered organisms may
be part of the solution, but their own impact on the environment must be
considered before they are widely used.
 There have been concerns that genetically engineered crop plants could
potentially become ‘superweeds’ if they have been engineered to have
resistance to herbicides, disease or pests and escape into the wild to overrun
native species.
 Concerns over genetically modified foods have been high profile in the
media and the issues raised have included:
- the appearance of new allergens
- increased antibiotic resistance
- the creation of brand new disease-causing organisms, made up of genetic
material from many different species.
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 Ethical considerations as well as concerns about potential environmental and
health hazards will probably slow the application of genetically engineered
products. There is always a danger that too much regulation will stifle
potential benefits, but the nature of the work clearly requires caution. The
challenge seems to lie in striking a safe but productive balance.
Identification of key concepts
 What are the risks associated with genetic engineering?
 How are these risks managed?
 What ethical issues have been raised?
Identification of particular areas of difficulty
 While it is important that students are able to form their own opinions on this
topic, they must also be able to appreciate the debate as a whole and
communicate the issues objectively.
Links to sources of further information
http://www.beep.ac.uk/content/index.php
Links to websites, animations, PowerPoints, audio or video files etc
http://www.who.int/foodsafety/publications/biotech/ 20questions/en/index.html
http://www.mhra.gov.uk/index.htm
http://www.fda.gov/
http://www.geneticallymodifiedfoods.co.uk/
Other useful information to stimulate interest
http://www.hse.gov.uk/biosafety/gmo/index.htm
http://www.geneticallymodifiedfoods.co.uk/
DVD: Science in Focus, The Virtual Body: Genetic Engineering. Available to
buy at www.channel4.com/learning or to view on Teacher’s TV
(http://www.teachers.tv/series/science-in-focus-the-virtual-body)
Co-operative Learning Activity 2.
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