Human Biology (DNA)


  • Created by: kez
  • Created on: 16-03-13 00:39

DNA profiling (aka DNA fingerprinting)

- Everyone's DNA is unique, except for identical twins. Half inherited from mother/half from father.

- DNA profiling can be used for establishing an individual's identity (e.g. for crime scenes, finding criminals, identifying corpses) or determining their parentage. Also used in anthropological research and for detecting genetic variations and/or mutations that may play a role in the development or progression of a disease.

- A DNA profile can be created from a tiny sample of DNA. A profile can be established even from very old or damaged DNA. From a single sample many copies can be produced using a technique called polymerase chain reaction.

- Many differences in the DNA of different people; not practical to look at all of them. Usually ten key differences are studied. These DNA sequences are known to vary widely between individuals.

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- A DNA profile usually examines ten repeat base sequences, known as short tandem repeats (STRs) aka microsatellites. These DNA sequences occur on different chromosomes.

STRs are short sequences of DNA, normally of length 2-5 base pairs, that are repeated numerous times in a head-tail manner, i.e. the 16 bp sequence of "gatagatagatagata" would represent 4 head-tail copies of the tetramer "gata".

- STRs occur in non-coding sections of the DNA - for example you might find the sequence CCTG repeated several times in one particular section of DNA on one chromosome.

- A person might inherit six repeats of their mother's chromosome and eight repeats on the matching chromosome from their father.

- When the ten STRs are analysed it is highly unlikely that anyone else will share the same pattern of repeats.

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- The number of repeats determine the length of the DNA within an STR.

- The DNA can be chopped up and the STRs isolated using special enzymes.

- The pieces of DNA are sorted according to size using electrophoresis.

Gel electrophoresis:
- The samples to be tested are injected into small wells in a sheet of porous, jelly-like material.

- An electric current is passed through the gel and the fragments are drawn towards the positive end, with small fragments travelling faster than the larger ones.

The smaller fragments travel through the gel faster (and therefore further) than the heavier ones.

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DNA sequencing

-  DNA sequencing is the determination of the exact order of the base pairs in a segment of DNA.            

- DNA sequence is the process of determining the precise order of nucleotides within a DNA molecule. It includes any method or technology that is used to determine the order of the four bases—adenine, guanine, cytosine, and thymine—in a strand of DNA. The advent of rapid DNA sequencing methods has greatly accelerated biological and medical research and discovery. Knowledge of DNA sequences has become indispensable for basic biological research, and in numerous applied fields such as diagnostic, biotechnology, forensic biology, and biological systematics.

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Polymerase Chain Reaction

The polymerase chain reaction (PCR) is like a biological photocopier. It is used for creating multiple copies of a specific section of DNA from a sample (DNA amplification). PCR is useful when only small amounts of DNA are available for analysis. The three steps in PCR are :

  • denaturing
    Denaturing involves heating the DNA to  separates the two strands.
  • hybridisation: 
    - Primers
    (short synthetic DNA fragments) are added to the DNA.-
    - The primers bind to complimentary base sequences on the separated DNA strands.-
    - The primers act as starting points for the replication of new DNA molecules.
  • synthesis 
    - DNA polymerase is added.
    - Starting at the primer, the DNA polymerase reads the DNA code and builds a complementary strand of DNA.
    - Each cycle takes 3-5 minutes. 
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Recombinant DNA

- Recombinant DNA technology, also known as genetic transformation, is the technology used in genetic engineering.  

- Recombinant DNA is made by recombining fragments of DNA from different sources.

- Restriction enzymes cut base sequences at specific points leaving sticky ends.

- DNA ligase is an enzyme that acts like DNA glue. It can paste matching pieces of DNA back into the squence.  

* Plasmids are small circular pieces of DNA that occur in bacteria and protozoa. They are not in a chromosome but can replicate independently in a host cell.

- Plasmids can be cut at specific points using restriction enzymes. The cut plasmids are mixed with DNA ligase to form recombinant DNA. Plasmid reintroduced into E. coli. E. coli with tetracycline and kanamycin resistance.

- Recombinant DNA technology is used for making a range of drugs (e.g. interferon), hormones (e.g. insulin & growth hormone), and vaccines. - Many new strains of plants have been created using recombinant DNA technology. 

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Gene therapy

Gene therapy is an experimental technique that uses genes to treat or prevent disease. In the future, this technique may allow doctors to treat a disorder by inserting a gene into a patient’s cells instead of using drugs or surgery. Researchers are testing several approaches to gene therapy, including:

  • Replacing a mutated gene that causes disease with a healthy copy of the gene.

  • Inactivating, or “knocking out,” a mutated gene that is functioning improperly.

  • Introducing a new gene into the body to help fight a disease.

Although gene therapy is a promising treatment option for a number of diseases (including inherited disorders, some types of cancer, and certain viral infections), the technique remains risky and is still under study to make sure that it will be safe and effective. Gene therapy is currently only being tested for the treatment of diseases that have no other cures. It has long been anticipated that cystic fibrosis will be one of the first diseases to be treated by gene therapy. However, since the first clinical trials in the early 1990s numerous problems have been encountered.

It is expected that a clinically effective treatment will be available in the next 10 years.

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