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endotoxins
produced by gram negative bacteria
lipopolysaccharide embedded in the cell surface membrane
component embedded in the outer phospholipid of the cell membrane is toxic
which bacteria produce endotoxins?
salmonella
s. entera invades cells lining the intestinal wall. upon lysis, endotoxins are released which cause inflammation and results in diarrhoea
s. typhi invades the body via the lymphatic system once ingested and invades body cells once spread around the body via the lymph - when endotoxins are released typhoid fever is caused
which bacteria produces exotoxins?
staphylococcus
haemolysins: polypeptides that become integrated in cell membranes of host cells, creating pores which cause the cells to lose water and ions
superantigens: polypeptides that stimulate immune cells and cause massive release of cytokines into the blood, which can cause toxic shock syndrome - high fever, low blood pressure, coma, multiple organ failure
what is a granuloma?
the macrophages that have engulfed tuberculosis bacteria become surrounded by other cells in the immune system
how does a chronic infection of tuberculosis occur?
occurs when the recipient has a weak immune system
bacteria secretes hydrolytic enzymes into the host’s cells and digests them
energy source is cholesterol - component of cell membrane
cavities appear in lung tissue, blood vessels are broken down, and fluid collects as bacteria digest cells
what is found in a granuloma?
t-cells: immune cells produce signals that stimulate bacterial killing by macrophages
a TB -infected macrophage
‘caseous necrosis’: characteristic crumbly core material containing dead cells
how does tuberculosis affect a healthy recipient?
growth of bacteria is restricted in the lungs
bacteria is engulfed by macrophages in the alveoli and bronchioles
granuloma is formed
macrophages kill the bacteria but few survive as a latent infection that could result in infection years later
what is Ug99?
variety of stem rust fungus discovered in uganda in 1999
devastating crops of wheat in east africa, the middle east, and asia
has the potential to cause worldwide food scarcity
computer model predicts it will reach india, one of the world’s largest producers of wheat
85% of the worlds population depend on wheat as their only sources of energy
60% of the worlds population depend on wheat as their main source of dietary protein
hyphae
stem rust fungus spores germinate and produce threadlike structures
mycelium
a mass of fungal hyphae
how does stem rust fungus digest host cells?
secretes digestive enzymes from its hyphae onto the material on which it is growing
enzymes digest chemicals in the stem and the fungus absorbs the products of this digestion
how does stem rust fungus infection damage the host plant?
weakens the stem, often causing the plant to fall over
makes mechanical harvesting impossible
uses nutrients that would otherwise be stored in the plant’s seeds
reduces the harvest
breaks the outer epidermis of the stem
increases the rate of water loss from the plant
makes the plant more susceptible to infection by other plant pathogens
how can fungicides be used to control stem rust fungus spread?
kill stem rust fungus
usually expensive - reduces their availability to poor farmers in africa and asia
can damage delicate ecosystems
associations between plant roots and fungi are vital to the roots’ efficient absorption of inorganic ions from soil
why is it difficult to control the spread of stem rust fungus?
stem rust fungus depends on the barberry plant to complete its life cycle
north america has an ongoing barberry eradication programme, but spores are carried to north america by the wind from southern american states and mexico
spore dispersal occurs over very large distances
how can gene manipulation control stem rust fungus spreading?
using genetic engineering, scientists have produced cereals resistant to some strains of stem rust fungus - identifying a number of genes that confer resistance to stem rust fungus
have only been used in australia - worldwide effectiveness is unknown
what does the influenza virus look like?
eight short strands of RNA - segmented genome
surrounded by protein capsid
envelope with outer protein layer and inner lipid layer
glycoproteins project through the envelope and cover the surface of the virus
H glycoproteins help the virus particle enter a cell of the host
N glycoproteins allow newly formed virus cells to escape from a cell of the host
what happens during influenza virus infection?
influenza enters lungs through inhaled air droplets
enters epithelial cells lining the bronchus and bronchioles by endocytosis
replication of the virus occurs in the host cells
toxins are released as the host cell lysis - brings about many of the symptoms of influenza, can cause secondary infection
why is influenza having great antigenic variability an issue?
humans have little to no resistance to these new antigens - influenza can easily cause an epidemic
immune systems have not encountered them before, meaning no memory cells
antigenic variability
variation in the chemical nature of the same type of antigen resulting from frequent gene mutations and, in eukaryotes, different splicing of pre-mRNA molecules transcribed from the same gene
what are the three pre-requisites for a flu pandemic?
a novel virus strain, unfamiliar to human immune systems, must reach human hosts from its point of origin
the virus must be able to replicate in humans and cause disease
the virus must be efficiently transmitted between humans
examples of influenza pandemics
spanish flu 1918-19
asian flu 1957-58
hong kong flu 1968-69
how is malaria transmitted?
transmission of plasmodium from an infected person to another person is by the anopheles mosquito
female anopheles takes a blood meal from a human, injecting saliva containing coagulants
if the female is infected with plasmodium, the saliva contains the sporozoites of plasmodium, causing infection of the human
how does plasmodium infect a human?
sporozoites infect cells in the liver and rapidly divide to form thousands of daughter cells called merozoites
infected liver cells lose and their merozoites are released into the blood
merozoites enter red blood cells, where they digest the haemoglobin as a food source
each merozoite undergoes several cell cycles to produce 8-32 new red blood cells
the red blood cell bursts releasing the new merozoites, which infect other red blood cells
each red blood cell that bursts produces toxins produced during the breakdown of its contents by the merozoites, which cause the symptoms of malaria - raised body temperature, intense fever symptoms, and a swollen spleen
endemic
a disease that occurs frequently, or is constantly present at a low level of infection, in particular areas or among people of a particular population
what are the issues with draining wetland areas where mosquitoes breed?
not possible to drain large lakes, which are also breeding grounds for mosquitoes
some people earn their living, or derive their main food source, from wetlands
successful drainage might involve inter-governmental co-operation, which is not always possible
what are the issues with spraying areas where malaria is endemic with insecticide to kill mosquitoes?
the areas are vast, much larger than areas treated in europe, and would often involve co-operation between different governments or areas occupied by warring groups
insecticide would damage other wildlife, including beneficial insects
what are the issues with covering beds with fine-mesh nets, coated with insecticide?
cheap and effective, as most mosquitoes bite during the night
however mosquitoes that bite during the day are emerging in many areas where malaria is endemic
what are the issues with releasing sterilised male mosquitoes that could mate with females but not result in viable eggs being laid?
since releasing more insects in order to reduce the number of insects is counter-intuitive, local groups of people are reluctant to accept its use
large-scale, and effective, education programmes are needed to convince people to accept such programmes
what are the issues with vaccinating against malaria?
a vaccine would have to be suitable for use with babies and young children, since malaria is most severe during the first years of life
pharmaceutical companies need to charge for their products
countries in which malaria is endemic are often poor and could not afford country-wide vaccination programmes
plasmodium shows great antigenic variability, making the production of an effective vaccine difficult
t killer cells
destroy body cells infected by viruses
t helper cells
release cytokines that stimulate production of b cells
memory cells
remain in the body and bring about the secondary response
how are the three types of t cell produced in the cell mediated response?
t cells bind to antigen presenting cell
activated t helper cell is formed that releases cytokines
release of cytokines stimulates the activated t cell to divide repeatedly to form a clone
active artificial immunity
antibodies are made following administration of a vaccine
b cells, t cells, and memory cells are made
passive artificial immunity
antibodies are administered by injection (immunisation)
active natural immunity
production of antibodies following infection by, and recovery from, a disease
b cells, t cells, and memory cells are made
passive natural immunity
mother’s antibodies cross placenta to fetus or ingested by baby in mother’s breast milk, especially colostrum
what is the longevity of active immunity?
long-lasting since memory cells are maintained throughout life
what is the longevity of passive immunity?
fades with time, since the recipient has not made memory cells and the antibodies received are themselves treated as non-self antigens and destroyed by recipient’s active immunity
what can the active agent in a vaccine be?
the intact pathogen that has been inactivated (stop it causing infection) or attenuated (reducing ability to cause infection)
purified components of the pathogen that have been found to have antigenic properties but do not cause disease
toxoids - modified toxins - to develop active immunity against toxin-based diseass
genetically engineered dna - which can be desgined to stimulate particular target cells of the immune system
what do vaccines do?
cause the recipient’s immune system to make antibodies against the pathogen without suffering an infection, and then to retain the appropriate memory cells
herd immunity
provided a large enough proportion of a population are immune to a particular contagious disease, the likelihood of the causative pathogen being transmitted to a member of the population who is not immune is negligible
what do the complement proteins on phagocytes (macrophages and neutrophils) do?
attract more phagocytes to the site of infection
binding to, and forming pores in, the surface membranes of foreign cells, leading to the lysis of these cells
binding to surface membranes of foreign cells, aiding the attachment of a phagocyte to a foreign cell
proteins are called opsinins - cause opsonisation
what do b lymphocytes do?
secrete antibodies and produce the humoral immune response
what do t lymphocytes do?
attack infected cells, producing the cell-mediated response, and assist b cells
how are t cells produced?
produced by multipotent stem cells in bone marrow
leave the bone marrow and complete their maturation in the thymus gland
how are b cells produced?
produced by multipotent stem cells in bone marrow
complete their maturation in the bone marrow they were formed in
major histocompatability complex proteins
glycoproteins found on the cell surface membrane of a cell that are unique to that individual
identify cells
what is the first step of the humoral immune response?
a pathogen enters the body; its surface has non-self antigens on it
collides with a b cell randomly which has an antigen receptor on its surface that is complementary to a non-self antigen on the pathogen
other pathogen cells are attacked by non-specific macrophages
what is the second step of the humoral immune response?
the b cell engulfs the antigen and digests it
the b cell then displays fragments of the antigen on its cell surface membrane, bound to its own mhc proteins
what is the third stage of the humoral immune response?
the macrophage digests antigen-carrying pathogen and displays fragments of antigen bound to the mhc proteins on its cell membrane
what is the fourth stage of the humoral immune response?
antigen-presenting macrophage comes into contact with a t cell that has antigen-receptor protein complementary to one of the pathogen’s antigens now displayed on the macrophage
the two cells bind
the t cell is activated, which is now called an activated t helper cell
what is the fifth stage of the humoral immune response?
an activated t helper cell now binds to a b cell displaying the same antigen on its cell membrane, which activates the b cell
what is the sixth stage of the humoral immune response?
stimulated by secretion of cytokines from activated t helper cell, the activated b cell divides by mitosis and forms plasma cells
antibody is mass-produced and exported from plasma cell by exocytosis
what is the seventh stage of the humoral immune response?
a few activated b and cell cells remain in the body as memory cells
form the basis of natural immunity
epigenetic modification
stable, long-term changes in the ability of a cell to transcribe its genes
changes are inherited but they are not caused by changes to the base sequence of the dna
what do promoters do?
short base sequences that lie closed to their target genes
initiate transcription by enabling rna polymerase to bind to the gene they regulate
what do enhancers do?
short base sequences that lie some distance from their target genes
stimulate promoters causing an increase in the rate of transcription of the genes they regulate
what do regulatory sequences do?
stimulate the transcription of genes
transcription factor
a protein, or assembly of several proteins, that regulate the production of mrna
specific transcription factor binds to a promoter region upstream of its target gene and either promotes or inhibits the binding of rna polymerase to the target gene
isoforms
different arrangements of exons in mature messenger rna transcribed from a singe eukaryotic gene
different isoforms result in different polypeptides being produced from one gene
what does alternative splicing do?
alternative splicing of pre-mrna produced from a single gene can result in the production of completely different polypeptides
how is mature mrna formed?
exons and introns are coped into the base sequence of an rna molecule - pre-mrna
molecule is spliced; the introns are removed and the exons are rejoined, to form mature mrna
mrna migrates into the cytoplasm where is base sequence is translated by ribosomes
how does miRNA destroy mrna?
miRNA moleule binds to a protein to form a RISC
the miRNA within the RISC will bind to mRNA in the cytoplasm
the two RNA molecules bind by the formation of hydrogen bonds between base pairs, but the pairs are not fully complementary, forming ‘bulges’ which prevent mRNA being transcribed
how does siRNA destroy mrna?
bind with a protein complex to form a RISC
proteins within the RISC unwind the rna and remain bound to one of the strands
this single-stranded, antisense rna, bound within the RISC, that binds to the target mRNA
bound mRNA is hydrolysed by yet another RNA hydrolase
what does dna methylation do?
methylated dna prevents the activation of rna polymerase
target genes of these promoters are silenced
what happens when histones are acetylated?
the binding of the histones changes and they become more loosely packed
loosening of the histone frees the gene, so that the transcription factors and RNA polymerase can now gain access to it
how are induced pluripotent stem cells produced?
scientists identified genes which were vital to embryonic stem cell function
these genes were transferred into mouse fibroblasts using retroviruses as vectors
devised a technique which enabled them to find which cells had become pluripotent
found four genes which induced change from adult fibroblast to pluripotent stem cell
how can ips cells be used in medical research?
produce new types of cell that directly replace those affected by disease
generate new types of cell that can be used in the laboratory to test the effectiveness and safety of new drugs, reducing need for animal and human experimentation
why must ips cells be used with caution?
scientists do not know how to control the differentiation of pluripotent stem cells
two genes that produce ips cells are oncogenes
conversion of adult cells to ips cells is currently slow and the success rate is very low
growing cells for therapies will require specialist systems and research centres, so access to therapies may be limited to areas with suitable facilities
the safety aspects of introducing pluripotent stem cells into recipients must be trialled
clinical trials take a long time and are expensive
what are the reagents of PCR?
primers: short lengths of single-stranded DNA that are complementary to the base sequence of part of the 3’ ends of the strands of target DNA to be copied
free DNA nucleotides, each with an adenine, cytosine, guanine, or thymine base
thermostable DNA polymerase
what are the steps of PCR?
separate chains of double stranded DNA with known end sequences by heating to 95oC
make primer that matches the end sequences of the DNA fragment to be copied. cool to 45oC to allow the primer to stick each strand
add heat tolerant taq polymerase and nucleotides, and heat to 75oC
what can recombinant dna do in genetically modified soya beans to increase the crop yield?
resistance to glyphosate, a commonly used weed killer that would also affect the soya plant
resistance to pests, including fungi, nemotade worms, and insects
tolerance to drought and to soil salinity
how can recombinant dna change biochemical components of soya beans?
the balance of fatty acids, increasing the percentage of oleic acid and reducing the percentage of linoleic acid
during cooking, linoleic acid is more easily oxidised to produce trans fats that oleic acids - trans fats are harmful to human health
production of active pharmacological ingredients - molecular pharming
how have bacteria been used to insert recombinant dna into soya plants?
when the transgenic bacteria infects the plant it transfers its own dna into the cells of the infected plant, creating tumours
transgenic bacteria infect soya tissue cultures, resulting in soya plants in which all the cells contain the desired genes
what is a ‘knockout’ mouse?
a laboratory mouse in which scientists have inactivated one of its genes
how are ‘knockout’ mice created?
add a piece of foreign dna that disrupts transcription of a gene into the genome of a mice
scientists begin with embryonic stem cells from an early mouse embryo
using a virus the scientists insert the artificial gene into the stem cells
these cells are cultured in a laboratory before injecting them into new mouse embryos, which are transplanted into the uterus of a female mouse
some tissues of the pups will have the inactivated gene - crossbred to produce mice which are homozygous for the target gene (all tissues have the targeted gene inactivated)
how can ‘knockout’ mice be used in research?
research scientists can investigate the function of the gene they knockout
mice and humans have many genes in common - understand the function of human genes
heart disease, cancer, and parkinson’s disease research
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