Locating genes, genetic screening and counselling

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  • Created by: Hindleyc
  • Created on: 20-04-19 10:03
What do many human diseases have
a genetic origin
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what are these often the result of
a gene mutation
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What has recombinant DNA technology enabled us to do
diagnose and treat many of these genetic disorders
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In doing so what Is it often necessary to know
exactly where a particular DNA (gene) is located
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how do we achieve this
we use labelled DNA probs and DNA hybridisation
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What is a DNA probe
short single-stranded length of DNA that has some sort of label attached that makes it easily identifiable
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What are the 2 most commonly used probes
Radioactively labelled and fluorescently labelled probes
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What are the radioactively labelled probes made up of and how is it identified
nucleotides with the isotope 32p. probe is identified using an X-ray film that is exposed by radioactivity
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What do the fluorescently labelled emit
light (fluoresce) under certain conditions eg when the probe has bound to the target DNA sequence
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The DNA probe is made that
has base sequences that are complementary to part of the base sequence of the DNA that makes up the allele of the gene that we want to find
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What is the double stranded DNA that is being tested treated to do
to separate into 2 strands
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What are the separated DNA strands mixed with
The probe
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what does this do
binds to the complementary base sequence on 1 of the strands- known as DNA hybridisation
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How can the site at which the probe binds be identified
by the radioactivity or fluorescence that the probe emits
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before we can make a specific probe what do we need to know
the base sequence in the particular allele that we are trying to locate
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What is used to sequence the exact order of bases in a length of DNA
number of different methods
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When does DNA hybridisation take place
when a section of DNA or RNA is combined with a single-stranded section of DNA which has complementary bases
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Before hybridisation can take place what must happen
the 2 strands of the DNA molecule must be separated
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how is this achieved
by heating DNA until its double strand separates into its 2 complementary single strands (denaturation)
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When cooled what happens
the complementary bases on each strand recombine (anneal) with each other to reform the original double strand
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give sufficient time what will all strands in a mixture of DNA do
pair up with their partners
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if however other complementary sections of DNA are present in the mixture as the DNA cools what are these just as likely to
anneal with one of the separated DNA strands as the 2 strands are with one another
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Using DNA probes and DNA hybridisation it is possible to
locate a specific allele of a gene eg determine whether someone possesses a mutant allele that causes a particular genetic disorder
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how
first we must determine the sequence of nucleotide bases of the mutant allele we are trying to locate
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how can this be achieved
using DNA sequencing techniques
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however what do we now have and so what can we do to obtain the sequence
we now have extensive genetic libraries that store the base sequence of most genetic diseases and so we can simply refer to these to obtain the sequence
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So what is then produced
fragment of DNA that has a sequence of bases that are complementary to the mutant allele we are trying to locate
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How do we produce multiple copies of our DNA probe
formed using the PCR
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How is the DNA probe then made
by attaching a marker eg a fluorescent dye to the DNA fragment
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what is the DNA from the person suspected of having the mutant allele we want to locate
heated to separate into its 2 strands
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What are the separated strands then
cooled in mixture containing many of our DNA probes
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If the DNA contains the mutant allele what is one of the probes likely to do
bind to it because the probe has base sequences that are exactly complementary to those on the mutant allele
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THe DNA is then
washed clean of any unattached probes
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What will the remaining hybridised DNA now be
fluorescently labelled with the dye attached to the probe
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how is the dye detected
by shining a light onto the fragments causing the dye to fluoresce which can be seen using a special microscope
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What can the labelled DNA fragments labelled with probe be
distinguished from the rest of the DNA fragments
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if complementary fragments not present
dna probes won't fluoresce
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What are many genetic disorders the result of
gene mutation
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How may gene mutations arise
if 1 or more nucleotide bases in DNA are changed in any 1 of a variety of ways
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If mutation occurs in the dominant allele who will have the genetic disorder
all individuals
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If the allele is recessive who will it only be apparent in
those individuals who have 2 recessive alleles, that is, who are homozygous recessive
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Individuals that are heterozygous will
not display symptoms of the disease but will carry one copy of the mutant allele
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What are these individuals known as and what do they have the capacity to do
Carriers,, to pass the disease to their offspring if the other parent is also heterozygous or homozygous recessive
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What is it important to screen
individuals who may have a mutant allele
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what do such individuals often have
a family history of disease
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What can screening determine
the probabilities of a couple having offspring with a genetic disorder
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As a result what can potential parents who are at risk obtain
advice from a genetic counsellor about the implications of having children based on their family history and results of genetic screening
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What is it possible to fix 100s of in an array (pattern) on a glass slide
different DNA probes
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by adding a sample of DNA to the array, what will happen
any complementary DNA sequences in the donor DNA will bind to one or more probes
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in this way what is possible
to test simultaneously for many different genetic disorders by detecting fluorescence that occurs where binding has taken place
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What is another area where genetic screening can be valuable
detection of oncogenes which are responsible for cancer- cancers may develop as a result of mutations in the TS genes inhibiting cell division
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What must be present to inactivate the TS gene and to initiate the development of a tumour
mutations of both alleles
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What do some people inherit and what does this mean
1 mutated TS gene therefore these individuals are at a greater risk of developing cancer
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If a mutated gene is detected by genetic screening, what cab individuals who are at a greater risk then make
informed decisions about their lifestyle and future treatments eg choose to give up smoking, lose weight, eat more healthily and avoid mutagens as far as possible
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What can they also do
check themselves more regularly for early signs of cancer which can lead to early diagnosis and a better chance of successful treatment
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What may they choose to undergo
some form of surgery or other treatment
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What is one of the advantages of genetic screening
personalised medicine
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What does it allow doctors to provide
advice and health care based on an individuals genotype
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What can some peoples genes that
a particular drug may be either more or less effective in treating a condition
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so by genetically screening patients what can doctors and pharmacists determine
more exactly the dose of a drug which will produce the desired outcome
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What can this save and why
money that would otherwise be wasted on overprescribing the drug
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In some cases what do it avoid
medication that could cause harm or avoids rising false hopes
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example
painkillers as to function effectively need a specific enzyme to activate them and half have genes that alter function of it so screening allows dosage to be adjusted to compensate for the way in which the genes affect the individuals metabolism ofit
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what does it ensure
their use is both safe and effective
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other eg
Vit E can reduce risk of cardiovascular disease for people with diabetes among certain genotypes but it can inc risk for those with a diff genotype so advantageous to screen a person who has diabetes before advising to take vitamin E supplements
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What does genetic screening go hand in hand with
genetic counselling
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why
expert advice provided by a counsellor helps individuals to understand the results and implications of the screening and so make appropriate decisions
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What is genetic councilling
like a special form of social work where advice and information are given that enable people to make personal decisions about themselves or their offspring
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What is one important aspect of genetic counselling
to research the family history of an inherited disease and to advise parents on the likelihood of it raising in their children
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what can GC make aware of
if no family history for man assume doesn't carry allele for disease and none of them develop it although may be carriers whereas if has F.H of it could carry it so make aware 1 in 4 chance affected 2 in 4 chance carriers
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What can the counsellor also inform the couple of
the effects of the disease eg sickle cell anaemia and its emotional, psychological medical and social and economic consequences
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On the basis of this advice what can the couple then choose
whether or not the have children
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What can counselling also make them aware of
any further medical tests that might give a more accurate prediction of whether their children will have the condition eg IVF with screening of embryos
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What is GC closely linked to
genetic screening
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what do the screening results provide
the genetic counsellor with a basis for informed discussion
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eg in that cases of cancer screening what can it help detect
Oncogene mutations which can determine the type of cancer that the patient has and hence the most effective drug or radiotherapy to use, gene changes that predict which patients are more likely to benefit from certain treatments
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What will they then have
and have the best chance of survival eg drug Herceptin most effective at treating certain types of breast cancer
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what else can it detect
a single cancer cell among millions of normal cells thus identifying patients at risk of relapse from certain forms of leukaemia
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what can this information help a counsellor
to discuss with the patient the best course of treatment and their prospects of survival
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