genetic fingerprinting

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Emily

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principles of genetic fingerprinting:
not all of an organism's genome codes for proteins
some of the genome consists of variable number tandem repeats (VNTRs) - base sequences that don't code for proteins and repeat next to each other over and over (sometimes thousands of times)
the number of times these sequences are repeated differs from person to person
so the length of these sequences in nucleotides differs too
the repeated sequences occur in lots of places in the genome
the number of times a sequence is repeated (and so the number of nucleotides) at different places in the genome can be compared between individuals - genetic fingerprinting
the probability of two individuals having the same genetic fingerprint is very low
bc the chance of 2 individual's having the same number of VNTRs at each place they're found in DNA is very low
step 1: - PCR is used to make DNA fragments
a sample of DNA is obtained e.g. from a person's blood, saliva etc
PCR is used to make copies of the areas of DNA that contain the VNTRs
primers are used that bind to either side of these repeats and so the whole repeat is amplified
different primers used for each position under investigation
end up with DNA fragments where the length (in nucleotides) corresponds to the no. of repeats the person has at each specific position
e.g. one person may have 80 nucleotides, another 120
step 1:
a fluorescent tag is added to all the DNA fragments (usually to the primer)
so they can be viewed under UV light
a locus (loci) is the fixed position of a gene on a chromosome
step 2: - separation of the DNA fragments by gel electrophoresis
to separate the DNA fragments, DNA mixture is placed into a well in a slab of gel and covered in a buffer solution that conducts electricity
an electrical current is passed through the gel - DNA fragments ae neg charged so the move towards the positive electrode at far end of the gel
shorter DNA fragments move faster and travel further through the gel
so DNA fragments separate according to length - produces a pattern of bands
fragments move through the gel in order of length, so longer fragments stay towards to top (-ve) end and shorter fragments move further down towards the +ve end
positive electrode = anode
negative electrode = cathode
step 3 - analysis of the genetic fingerprints
after the gel has been running long enough, equip turned off and gel placed under UV light
under the UV light the DNA fragments can be seen as bands - bands make up the genetic fingerprint
DNA ladder may have been added to gel - a mixture of DNA fragments of known length - allows you to work out the length of the other bands on the gel
2 genetic fingerprints can be compared e.g. if both have a band at same location on gel - means have the same no. of nucleotides and so same no. of VNTRs at that place - match
gels are also used to separate RNA by length or proteins according to size (can be run vertically in slightly different equipment)
uses of genetic fingerprinting:
determining genetic relationships
determining genetic vulnerability within a population
in forensic science
for medical diagnosis
in animal and plant breeding
determining genetic relationships:
we inherit VNTR base sequences from parents
roughly half the sequences come from each parent
more bands on a genetic fingerprint that match - more closely related (genetically similar) 2 people are
e.g. paternity tests are used to determine the biological father of a child by comparing genetic fingerprints
if lots of bands match - person is most probably the child's father
higher no. of places in the genome compared - the more accurate the test result
genetic fingerprinting can also be use to look at much wider ranging genetic relationships e.g. to see if a population of black bears found in Virginia is descended from a population in Canada or Alaska
idea is still the same - more bands the populations have in common - more closely related they are
sometimes might be interested in tracing only the male or female line of descent
to look at the female line - need to look at DNA in the mitochondria
bc in humans and most other organisms mitochondrial DNA (mtDNA) is only inherited from mum
if after male side - need to look at Y chromosomes, only men have a Y chromosome
roughly half the bands will match in a paternity test - inherit half our DNA from mum and half from dad
comparing mtDNA to see how closely related species are is used a lot in phylogenic (study of the evolution of organisms)
2: determining genetic variability within a population:
the greater the no. of bands that do not match on a genetic fingerprint, the more genetically different individuals are
means you can compare the no. of repeats at several places in the genome for a population to find out how genetically varied that population is
e.g. the more the no. of repeats varies at several places, the greater the genetic variability within a pop
3: - in forensic science:
forensic scientists use genetic fingerprinting to compare samples of DNA collected from crime scenes (e.g. DNA from blood, semen, skin cells, saliva, hair etc) to sample of DNA from possible suspects - could link them to crime scenes
DNA is isolated from all the collected samples (from crime scene and from suspects)
each sample is replicated using PCR
the PCR products are run on an electrophoresis gel and genetic fingerprints produced are compared to see if any match - if match - links a person to a crime scene
PCR is used to amplify the areas of DNA that contain the repeated sequences, so enough is produced for them to be seen on the gel
in fingerprint analysis in the UK - results from 10 different loci are analysed - chances of 2 fingerprints matching by chance is at least 1 in a million
4: for medical diagnosis:
in medical diagnosis, a genetic fingerprint can refer to a unique pattern of several alleles
can be used to diagnose genetic disorders and cancer
useful when the specific mutation isn't known or where several mutations could have caused the disorder
bc it identifies a broader, altered genetic pattern
e.g. preimplantation genetic haplotyping (PGH) screens embryos created by IVF for genetic disorders before they are implanted into the uterus
faulty regions for the parent's DNA are used to produce genetic fingerprints, which are compared to the genetic fingerprint of the embryo
if the fingerprints match - the embryo has inherited the disorder and so is not implanted
can be used to screen for CF, Huntington's
e.g. genetic fingerprinting can be used to diagnose sarcomas (types of tumour)
conventional methods of identifying a tumour e.g. biopsies only show the physical difference between tumours
now the genetic fingerprint of a known sarcoma (e.g. the different mutated alleles) can be compared to the genetic fingerprint of a patient's tumour
if there is a match (i.e. the mutated alleles are the same) the sarcoma can be specifically diagnosed and the treatment can be targeted to that specific type
the type of genetic fingerprinting used in medical diagnosis is slightly different to the normal
genetic disorders and cancer are both caused by mutations in DNA
a specific mutation can be found using gene probes and sequencing
5: in animal and plant breeding:
genetic fingerprinting can be used on animals and plants to prevent inbreeding - decreases the gene pool (no. of different alleles in a population
inbreeding can lead to an increased risk of genetic disorders, leading to health, productivity and reproductive problems
since genetic fingerprinting can be used to identify how closely related 2 individuals are it can be used to identify the least related individuals in a population so that we can breed them together
can also be used by animal breeders to prove pedigree (who an animal's parents and descendants are)
animals with a good pedigree will sell for more money
e.g. the offspring of Crufts or Grand National winners can sell for a lot of money if you can prove their pedigree