Genes, which are carried on chromosomes, are the basic physical and functional units of heredity. Genes are specific sequences of bases that encode instructions on how to make proteins. Although genes get a lot of attention, it’s the proteins that perform most life functions and even make up the majority of cellular structures. When genes are altered so that the encoded proteins are unable to carry out their normal functions, genetic disorders can result.

Gene therapy is a technique for correcting defective genes responsible for disease development. Researchers may use one of several approaches for correcting faulty genes:

• A normal gene may be inserted into a nonspecific location within the genome to replace a nonfunctional gene. This approach is most common.
• An abnormal gene could be swapped for a normal gene through homologous recombination.
• The abnormal gene could be repaired through selective reverse mutation, which returns the gene to its normal function.
• The regulation (the degree to which a gene is turned on or off) of a particular gene could be altered.

Working Of Gene Therapy

For correcting the defective genes responsible for disease by replacing an healthy one with that. There are so many methods to replace the defective genes. Currently, the most common vector is a virus that has been genetically altered to carry normal human DNA.

Some of the different types of viruses used as gene therapy vectors:

• Retroviruses – A class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. Human immunodeficiency virus (HIV) is a retrovirus.
• Adenoviruses – A class of viruses with double-stranded DNA genomes that cause respiratory, intestinal, and eye infections in humans. The virus that causes the common cold is an adenovirus.
• Adeno-associated viruses – A class of small, single-stranded DNA viruses that can insert their genetic material at a specific site on chromosome 19.
• Herpes simplex viruses – A class of double-stranded DNA viruses that infect a particular cell type, neurons. Herpes simplex virus type 1 is a common human pathogen that causes cold sores.

Besides virus-mediated gene-delivery systems, there are several non viral options for gene delivery. The simplest method is the direct introduction of therapeutic DNA into target cells. This approach is limited in its application because it can be used only with certain tissues and requires large amounts of DNA.

Another nonviral approach involves the creation of an artificial lipid sphere with an aqueous core. This liposome, which carries the therapeutic DNA, is capable of passing the DNA through the target cell’s membrane.

Researchers also are experimenting with introducing a 47th (artificial human) chromosome into target cells. This chromosome would exist autonomously alongside the standard 46 –not affecting their workings or causing any mutations. It would be a large vector capable of carrying substantial amounts of genetic code, and scientists anticipate that, because of its construction and autonomy, the body’s immune systems would not attack it. A problem with this potential method is the difficulty in delivering such a large molecule to the nucleus of a target cell.

This researches begun formally in 1990, the U.S. Human Genome Project was a 13-year effort coordinated by the U.S. Department of Energy and the National Institutes of Health. The project originally was planned to last 15 years, but rapid technological advances accelerated the completion date to 2003. Project goals were to

• identify all the approximately 20,000-25,000 genes in human DNA,
• determine the sequences of the 3 billion chemical base pairs that make up human DNA,
• store this information in databases,
• improve tools for data analysis,
• transfer related technologies to the private sector, and
• address the ethical, legal, and social issues (ELSI) that may arise from the project.

What’s a genome?

• A genome is all the DNA in an organism, including its genes. Genes carry information for making all the proteins required by all organisms. These proteins determine, among other things, how the organism looks, how well its body metabolizes food or fights infection, and sometimes even how it behaves.
• DNA is made up of four similar chemicals called nucleotides (adenine(A),Guanine(G),Thymine(T) and Cytosine(C) )are the basic and functional units of DNA that are repeated millions or billions of times throughout a genome. The human genome, for example, has 3 billion pairs of bases.

More information just visit

http://www.ornl.gov/sci/techresources/Human_Genome/project/info.shtml

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