Biotechnologyx

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23 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Biotechnology

Two decades of DNA fingerprints



Sir Alec's breakthrough has transformed crime investigations

Scientists in Leicester are marking the 20th anniversary of the invention of
genetic fingerprinting.

The breakthrough was made accidentally by Professor Sir Alec
Jeffreys

at the city's
university on 10 September 1984.

Since then, the technique has been used to trap criminals, identify victims of war,
settle paternity disputes, and prove the claims of clones like Dolly.

It has also led to a national database in the UK of 2.5 million genetic profiles,
mainly from convicted criminals but also from unsolved casework.

It is a development Sir Alec has some qualms about, and he opposes the practice,
approved by a court in 2002, of retaining DNA samples from suspects who are
acquitted.

"My view is, that is discriminatory," he said. "It works on a premise that the suspect
population, even if innocent, is more likely to offend in the future
."

Two decades of DNA fingerprints



He
would prefer to see a database that included all individuals, with
strict guidelines on what information could be stored. The professor
would not allow sensitive personal details such as a person's medical
history or ethnic origin to be mined from the data.

'Fantasy' vision

Sir Alec refers to the time when his lab stumbled across the
technique as a "eureka moment".

He and colleagues had been studying genetic variation and how it
might be used to track hereditary disease through families.





The double
-
stranded DNA molecule lies at the heart of nearly all our cells

Chemical components called bases
-

adenine (A), thymine (T), cytosine(C) and guanine (G)
-

spell out a
profile unique to the individual

DNA fingerprinting looks for patterns in what appear to be random repeats in this code

Enzymes chop up a sample of DNA, with the fragments sorted according to size

Further processing with X
-
ray film produces a characteristic barcode

If two barcodes from two different samples match, they probably come from the same source

"That magic moment was on a Monday morning 20 years ago, when I pulled that grubby bit of X
-
ray film
out of the developing tank and saw these fuzzy but extraordinarily variable patterns of DNA," he
explained to the BBC.

"The penny dropped pretty well immediately; we could see the potential for individual identification."

Within a year, it had been used in a Leicestershire double
-
rape
-
and
-
murder case, to prove one man
could not have committed the crimes and to confirm another had.

Indeed, it is in the area of criminal investigation that DNA fingerprinting has had its most profound
impact.

Sir Alec recalls: "Not so long ago I was very kindly invited down to the Old Bailey, to have lunch with
some of the judges there and sat opposite a judge who was very excited because he was trying a case at
the Old Bailey that didn't have any DNA evidence."

Since the DNA profiling discovery in 1984, Sir Alec has won world acclaim for his work.

In April this year, he won the Louis
-
Jeantet

Prize for Medicine, which is awarded to scientists who are
distinguished for the highest quality of biomedical research in Europe.

"If you had told me that 20 years later this technology would directly touch the lives of 10 million people
worldwide, I would have thought 'fantasy, no way'; I am amazed."



Profiling Techniques


DNA Profiling / Fingerprinting:

DNA is cut, by special enzymes, at specific base
sequences which will indefinitely vary from person to person.


These cut strands are then run through electrophoresis gel and can be compared
with the DNA from other sources. If they match they are the same DNA.


Genetic probes


Genetic probe/gene probe:

a fragment of DNA or RNA labelled with radioactive
isotopes / fluorescent markers to aid in the identification of specific sequence of
bases.


The fragment is added to the denatured DNA and if it combines then it will be
identifiable in the chromosome later.


Polymerase chain reaction (PCR)


Segments of DNA are artificially multiplied using DNA polymerase (generally
Taq

polymerase from the
Thermus

aquaticus

bacteria) and primers (which get process
started)


DNA is denatured (by heat) so it splits


Then the primers are added which provides
a starting
point for the process (on
both strands)



Polymerase causes reproduction of strand


The process continues on the products of the 1
st

PCR (more primer and
polymerase are added each time) but the strand becomes more precise.


Genetic Engineering (Recombinant DNA Technology)


Introduction of foreign or modified DNA into an organism



Transgenic organisms:

genome altered by additional genes


Generally the purpose in not to make copies of a gene but to give the vector the
gene to produce something else (
eg
. Insulin or
hGH
)



Plasmids
: are circular double stranded units of cytoplasmic DNA (can replicate in
a cell
independantly

of chromosomal DNA)



Restriction enzyme
: enzyme which cuts strands of DNA at specific nucleotide
sequences (usually with ‘sticky ends’)



Insulin


Initially obtained from pancreas of pigs and cattle


hGH


Initially obtained from anterior pituitary gland of deceased people.


Factor VIII


Haemophilia is caused by a disorder in a blood clotting protein known as factor VIII


Factor VIII was originally obtained from the plasma of donors but as a result, infections of
HIV and
Hrp

C were abundant.


Recombinant Factor 8 has solved many problems.


Vaccines


E.g

Hep B vaccine



Gene Therapy


Replacement of a faulty gene with a healthy one



E.g.
Cystic fibrosis:

-
Affects mainly lungs and pancreas causing excess mucus production which may
trap bacteria in the lungs causing infection.


lung damage, decreased life
expectancy

-
Also affects secretion of digestive enzymes

-
Mutations in CFTR gene (Cystic Fibrosis
transmembrane

regulator) cause Cystic
fibrosis

-
By adding normal copies of gene the mutations don’t occur. (unfortunately
when new cells die they are replaced by
fautly

ones
-

approx

every 120 days)

-
Delivered by fat capsules, nose drops, synthetic vectors, possibly aerosol.



E.g.
Huntingtons

disease

-
Mutated protein (called
huntingtin
) results in brain nerve cells being damaged

-
Results in flailing limbs and
eventual dementia

-
Possible gene therapy to introduce a corrective gene which boosts a natural
shield against effects of defective
huntingtin
.

Cell replacement therapy


The process of replacing defective cells with new stem cells


Tissue Engineering


Process of restoring healthy organs by replacing defective tissues


The tissue is grown from stem cells which attach to a ‘scaffold’ (natural or artificial
biodegradable,
poreous
, structure which will allow tissue to form around it)