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3. The Role of DNA and Genetic Mutations

At the top of the diagram is the original molecule. In the middle of the diagram the molecule is shown with its two strands unwinding. In the bottom portion, each of the strands is shown with a newly synthesized complementary strand, connected by the process of pairing.

The Molecule of Life!

DNA is the genetic blueprint of life. Within each cell of every organism there are a number of DNA molecules which contain all the information required to create and make an organism function. DNA consists of 2 strands and each strand is made of a sequence of 4 kinds of chemicals called nucleotides or bases. These bases are called Adenine (A); Guanine (G); Cytosine (C) and Thymine (T).

The two strands of DNA are joined together to form a ‘spiral ladder’ which is more formally known as a ‘double helix’. Each strand of DNA is an exact opposite of the other – wherever there is an A on one strand there is a T on the other, and wherever there is a G on one there is a C on the other. The links, or steps, of the ladder are made by the pairingof these complementary bases. (See the diagram).

To compare DNA between species, scientists must work out the sequence of bases along sections of the DNA molecule. This is done by artificially separating and copying the DNA from the region of interest from each species, doubling the amount of DNA at each copying cycle. The technique used to accomplish this is known as the polymerase chain reaction (PCR) is based on several unique properties of DNA.

DNA is a special molecule because it can make copies of itself. However, occasionally mistakes are made in coping the molecule and these errors are referred to as mutations. There are several different kinds of mutation that are known.

A particularly easy mutation to understand is a change in a single nucleotide (base, code letter). For example an A may be substituted for a G and in many cases these mutations don’t seem to have a noticeable effect on the organism, because some changes merely produce a different “codon” (three nucleotide sequence that tells which amino acid to use in making proteins) that codes for the SAME amino acid. Over a very long time and through many generations of organisms, mutations accumulate. Different species tend to have different and unique sets of mutations and so, these mutations can be used to measure how long ago two species diverged from one another. Species with similar DNA codes are more closely related than those with dissimilar DNA. For example, the DNA of humans is very similar to that of chimpanzees but quite different to that of a horse and very different from a gum tree DNA.

Genetic Mutations

To understand the the numerous kinds of genetic mutations that do occur we suggest that you go to the following website, as the information is clearly presented there.

Kinds of mutations

Sources and Further Reading:

http://www.dnalc.org - Gene Almanac

The Genetic Code (the Codons). This shows which triplets of DNA letters (codons) indicate which amino acids are to be inserted at a location. But, demonstrating that science progresses by testing ideas and discarding false ideas, this research shows that sometimes a "silent" mutation, a mutation that makes a codon for the same amino acid that was at a location before, can cause changes in the protein that is made! This overthrows a long-held but false idea that if two codons direct that the same amino acid shall be used at a location, then the earlier and the mutated proteins will fold up to be identical.

PBS site “Cracking the Code of Life”