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DNA

Since the time Gregor Mendel began studying about inheritance in garden plants some 150 years back, researchers have worked to learn more about the language of life – how characteristics pass from one generation to another. Researchers began to understand DNA from the 1800s when they stated that all living beings, whether plants, humans, animals, or bacteria, comprised cells that have the same basic components.

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Living organism are made up of cells, i.e. cells are the basic units of life. For example, each of us is made up of billions of this basic unit. If one closely inspects the structure of the cell, one is likely to find various smaller bodies or organelles like mitochondria that generates the energy required to perform all life processes (‘the powerhouse’), chloroplast (only in green plants and responsible for their coloration), the central core – ‘the nucleus, to name a few. The nucleus harbours the blueprint of life and the genetic material – DNA or deoxyribonucleic acid – and is the control centre of any cell. The genetic material or the blueprint is contained in all the cells that make up an organism and is transmitted from one generation to another. A child inherits half of the genetic material from each of his/her parents.

The chemical structure of everyone's DNA is the same. Structurally, DNA is a double helix: two strands of genetic material spiraled around each other. Each strand contains a sequence of bases, also called nucleotides. A base is one of four chemicals: adenine, guanine, cytosine, and thymine. The two strands of DNA are connected at each base. Each base will only bond with one other base, as follows: Adenine (A) will only bond with thymine (T), and guanine (G) will only bond with cytosine (C). If one strand of DNA looks like A-A-C-T-G-A-T-A-G-G-T-C-T-A-,the DNA strand bound to it will look like T-T-G-A-C-T-A-T-C-C-A-G-A-T-C.

Together, the section of DNA would be represented as given in Figure

T-T-G-A-C-T-A-T-C-C-A-G-A-T-C

A-A-C-T-G-A-T-A-G-G-T-C-T-A-G

The length of the DNA strand varies from organism to organism but within individuals of a particular species it is nearly constant. For example, a certain virus may have only 50 000 (5 x 104) bases constituting the genetic material whereas a human cell contains nearly 3.2 billion (3.2 x 109) bases in each of the cells (except the germ line cells). The amount and sequence in all the cells of an organism is identical. The DNA is for most part of the time present as condensed body called chromosomes (coloured body) except when it is replicating or dividing. A piece of a chromosome that dictates a particular trait, for example, eye and skin colour in humans, is called a gene. In any cell, the DNA can be classified into two categories – the sequence that codes for traits or genes and the sequence that has no apparent function or the non-coding DNA. The coding sequence (genes) in humans constitutes only five per cent of the total DNA and is identical in all humans. The non-coding sequence, which is nearly 95% in humans, varies from one individual to another, and forms the basis of DNA fingerprinting.

DNA fingerprinting

The only difference between two individuals is the order of the base pairs. Each individual has a different sequence of DNA, specially in the non-coding region. Using these sequences, every person could be identified solely by the sequence of their base pairs. However, because the entire DNA is so huge, the task would be time-consuming and nearly impossible. Instead, scientists are able to use a shorter method.

The steps involved in DNA fingerprinting can be summarized as follows.

bullet.gif (62 bytes) Isolating the DNA in question from the rest of the cellular material in the nucleus.
bullet.gif (62 bytes) Cutting the DNA into several pieces of different sizes.
bullet.gif (62 bytes) Sorting the DNA pieces by size. The process by which the size separation, or ‘size fractionation’, is done is called gel electrophoresis.

This is the basic concept behind fingerprinting technique.

DNA fingerprinting in plants

The concept of DNA fingerprinting can also be extended to plants and many institutions in the country are doing it today. TERI has successfully generated fingerprints of various medicinal plants such as neem, ashwagandha, and amla with the objective of determining their identity. With the help of fingerprints one can find out the genetic diversity in India. This knowledge has profound implications. Based on the extent of genetic diversity, one can establish the centre of origin of a particular plant species. And having done that we are better equipped to prevent bio-piracy or the theft of our genetic resources.

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